Spin-Dependent Tunneling in Magnetic Junctions

Transcription

1 2 CHAPTER ONE 2 Spin-Dependent Tunneling in Magnetic Junctions H.J.M. Swagten * Contents 1. Introduction From GMR to tunnel magnetoresistance Elementary model for tunnel magnetoresistance Beyond the elementary approach Scope of this review Basis Phenomena in MTJs Basic magneto-transport properties Oxidation methods for Al 2 O 3 barriers Towards optimized barriers Tunneling Spin Polarization How to measure spin polarization? Data on tunneling spin polarization Ingredients of tunneling spin polarization Crucial Experiments on Spin-Dependent Tunneling The relevance of interfaces: using nonmagnetic dusting layers Quantum-well oscillations in MTJs Role of the ferromagnetic electrode for TMR Towards infinite TMR with half-metallic electrodes Role of the barrier for TMR Coherent tunneling in MgO junctions Outlook Acknowledgements References Abstract 38 This chapter reviews the physics of spin-dependent tunneling in magnetic tunnel junctions, i.e. ferromagnetic layers separated by an ultrathin, insulating barrier. In magnetic junctions the tunneling current between the ferromagnetic electrodes depends strongly on an external magnetic field, facilitating a wealth of applications in the field of magnetic * Eindhoven University of Technology, Department of Applied Physics, COBRA Research Institute and center for NanoMaterials (cnm), P.O. box 513, 5600 MB Eindhoven, The Netherlands h.j.m.swagten@tue.nl Handbook of Magnetic Materials, edited by K.H.J. Buschow 2007 Elsevier B.V Volume 17 ISSN DOI /S (07) All rights reserved. 46 1

2 2 H.J.M. Swagten 1 media and storage. After a short introduction on the background and elementary principles 1 2 of magnetoresistance and tunneling spin polarization in magnetic tunnel junctions, 2 3 the basic magnetic and transport phenomena are discussed emphasizing the critical role 3 4 of the preparation and properties of (mostly Al 2 O 3 ) tunneling barriers. Next, key ingredients 4 5 to understand tunneling spin polarization are introduced in relation to experiments 5 6 using superconducting probe layers. This is followed by discussing a number of crucial 6 7 results directly addressing the physics of spin tunneling, including the role of the polarization 7 8 of the ferromagnetic electrodes, the interfaces between barrier and electrodes 8 9 and quantum-well formation, and the successful use of alternative crystalline barriers 9 10 such as SrTiO 3 and MgO Key Words: magnetic tunnel junctions, magnetoresistance, spin polarization, spin tunneling, spintronics Introduction This review is focusing on the fundamental aspects of magnetic tunnel junctions or shortly MTJs. It will cover the preparation and experimental aspects of MTJs, and most of the crucial experiments that were performed to unravel their basic physics. In the last section, new promising directions for further research will be reviewed. In this introductory section the following subjects will be covered: 22 the breakthrough towards magnetoresistance in layered magnetic structures, more specifically in metallic multilayers and subsequently in magnetic tunnel junctions phenomenology of magnetoresistance in MTJs using the Julliere model, including the concept of so-called tunneling spin polarization the shortcomings of elementary models via an introduction to some crucial experimental observations and advanced theoretical approaches. It should be noted that several other reviews exist also partially covering the physics and applications of spin-polarized tunneling in tunnel junctions; see Meservey and Tedrow (1994), Moodera et al. (1999a, 2000), Moodera and Mathon (1999), Dennis et al. (2002), Ziese (2002), Maekawa et al. (2002), Miyazaki (2002), Tsymbal et al. (2003), Zhang and Butler (2003), Zutic et al. (2004), Shi (2005), and LeClair et al. (2005). In most cases, however, the focus is different as compared to the present paper, and some recent developments in this rapidly evolving field may not be included. To assist the reader, the last part of this introduction will briefly explain the scope of the present review. 1.1 From GMR to tunnel magnetoresistance Magnetic tunnel junctions are within the florishing field of magnetoelectronics or spin electronics, shortly spintronics. In this area, nanostructured magnetic materials are used for functional devices explicitly exploiting both charge and spin in electron transport, the so-called spin-polarized transport. As we will see later on, magnetic 44

3 Spin-Dependent Tunneling in Magnetic Junctions 3 junctions are offering several unique opportunities for studying new, sometimes unexpected effects in physics, and, furthermore, they have opened up a number of new research directions within spintronics. Apart from that, magnetic junctions are superb materials for exploring novel device options, such as improved readhead sensors, magnetic memories or magnetic biosensors. Before a more detailed insight in the principles of magnetic tunneling will be given, it is instructive to first shortly review the field of spin-polarized transport and the ongoing increasing role of tunneling transport. In the mid-eighties the first crucial steps are made towards the exploitation of magnetic nanostructures for new electrical effects. These breakthroughs were strongly stimulated by the progress in ultra-high vacuum deposition and characterization techniques, enabling full control of layer-by-layer growth of metallic magnetic (multi-)layers. One of the first intriguing observations by Carcia et al. (1985) isthepresenceofperpendicular magnetic anisotropy in ultrathin magnetic (multi-)- layers due to strong magnetic surface anisotropies, see, e.g., also Parkin (1994) and Johnson et al. (1996). Due to perpendicular anisotropy, the magnetization can be pointing out of the plane of a magnetic thin film, a novel way of engineering the direction of magnetization in ferromagnetic films. To illustrate the technological relevance, this phenomenon is now used in magnetic media to increase the data density as compared to in-plane magnetized (longitudinal) magnetic disks. The subsequent discovery of magnetic interaction across ultrathin nonmagnetic spacers has been critically important for the field of spin-polarized transport. It is shown by Grünberg et al. (1986) that this so-called interlayer coupling may favor an antiparallel, in-plane alignment of two neighboring magnetic layers separated by only a few atomic planes of a nonmagnetic element. It is now well accepted that the driving mechanism for the interaction is spin-dependent electron reflection and transmission at the interfaces between the magnetic and nonmagnetic layers (for a review, see Bürgler et al., 1999). The first observation of remarkable, unexpected electrical effects in these magnetic nanostructures is independently reported by the research groups of Fert and Grünberg (Baibich et al., 1988; Binasch et al., 1989). They have demonstrated that the resistance of a multilayered stack of magnetic layers separated by nonmagnetic spacers strongly depends on the mutual orientation of the layer magnetization. Due to the presence of antiferromagnetic coupling, the magnetization of these layers can be engineered between parallel and anti-parallel via an externally applied magnetic field. The enormous magnitude of the magnetoresistance at room temperature explains the term giant magnetoresistance or GMR used since then. The observation of GMR has initiated an intensive research effort. Fundamentally, the physics of the underlying spin-polarized transport is studied extensively using magnetic engineering tools, novel material combinations, and a variety of theoretical approaches (Coehoorn, 2003). Along with the fundamental interest, the application potential of this effect has been immediately recognized by the magnetic recording industries. As a well-known achievement in this area, the concerted scientific and industrial effort led to the introduction of a GMR read head already in 1997, just nine years after the pioneering, curiosity-driven experiments. A similar strong interplay between scientific discovery and subsequent device im

4 4 H.J.M. Swagten 19 Figure 1.1 The development of room-temperature magnetoresistance in layered magnetic structures. Giant magnetoresistance (GMR) data are restricted to spin valves, where the active 20 part is consisting of two ferromagnetic layers separated by a metallic spacer. The data for tunnel magnetoresistance (TMR) are shown since 1995 for tunneling across Al 2 O 3 barriers, as well 22 as for MgO, showing a huge rise of TMR in recent years. Note that only a limited number of the available data have been collected in the graph just to give a representative illustration of the developments. 24 plementation can be observed in the field of magnetic tunnel junctions (MTJs). Although junctions were already studied for a long time (e.g. in the case of one superconducting and one metallic electrode), especially in the beginning of the nineties an increasing number of contributions are devoted to full magnetic junctions with two ferromagnetic electrodes. Although these experiments are certainly inspired by the original work of Julliere (1975) and Maekawa and Gafvert (1982) on Fe-Ge-Co and Ni-NiO-Ni(Co,Fe), respectively, the booming interest for GMR in metallic systems has also fuelled the renewed interest. For some of these pioneering experiments on MTJs in the beginning of the nineties, see Miyazaki et al. (1991), Nowak and Raułuszkiewicz (1992), Suezawa et al. (1992), Yaoi et al. (1993), and Plaskett et al. (1994). The final breakthrough in this field takes place in 1995 when unprecedented large magnetoresistance effects are discovered at room temperature. Moodera et al. (1995) as well as Miyazaki and Tezuka (1995a) are the first to show that a system of two magnetic layers separated by a very thin nonmagnetic oxide layer displays a huge tunnel magnetoresistance or TMR effect, substantially larger than GMR in a similar system with a metal spacer (for a review on exchange-biased spinvalves, see, e.g., Coehoorn, 2003). To illustrate the order of magnitude of GMR versus TMR, Fig. 1.1 shows the chronology of these developments. It is clear from the graph that the TMR data on Al 2 O 3 -based MTJs have shown a steady increase and are always well above GMR data. In more recent years, the use of MgO as a barrier (as well as other oxide and ferromagnetic material combinations) have un- 44

5 Spin-Dependent Tunneling in Magnetic Junctions 5 15 Figure 1.2 The magnetoresistance (a), expressed as V /I, as a function of external magnetic field H at low temperature (T = 10 K) of an evaporated 80 Å Co/14 Å 15 Al + oxidation/150 Å NiFe junction; see the schematics in (b). The arrows indicate the direction of magnetization of the two ferromagnetic electrodes. Antiparallel alignment between the 18 layers is facilitated by different coercivities of the Co and NiFe layer; see also section From Moodera (1997). 19 doubtedly demonstrated the record-high magnitude of TMR effects. In comparing these data, one should realize that the physics behind the magnetoresistance in tunnel junctions is completely different from that in all-metallic GMR structures, since quantum-mechanical tunneling is now the fundamental process governing the electrical transport. We will return to that in section 1.2. InFig. 1.2 an experimental example of tunnel magnetoresistance is shown from the group of Moodera, using two magnetic layers of different coercivity separated by a thin alumina barrier. It clearly demonstrates a large resistance change when the two magnetic layers are switched from a parallel to an anti-parallel orientation by an external magnetic field. The magnetoresistance in MTJ s can be exploited in a novel solid-state memory. It consists of (sub)micron-sized tunneling elements connected via word and bit lines in a two-dimensional architecture, a similar layout as in macroscopic ferrite core memories invented in the fifties; see Livingston (1997) and references therein. The fact that the electrical current flows perpendicular to the layers in an MTJ (due to the quantum-mechanical tunneling process across the insulator) rather than in the plane of the layers (as in GMR) allows for an efficient use of word and bit lines addressing individual bits. This, together with the huge magnetoresistances of MTJs paved the way to a fast implementation in memory applications. In fact, new nonvolatile solid-state memories based on magnetic tunnel junctions have entered the market in the beginning of the new millennium. In Fig. 1.3a a schematics is shown of one bit cell within a so-called magnetic random access memory or MRAM. It is shown how to use the magnetoresistance effect (as displayed in Fig. 1.2) tostore information in a solid-state device. In this system one of the layers, the reference layer, is always pointing in one direction (in Fig. 1.3b to the right), which means that the applied magnetic fields created by the orthogonal word and bit line should never exceed its coercivity. On the other hand, the softer magnetic layer is used to 22 44

6 6 H.J.M. Swagten Figure 1.3 (a) Schematics of a magnetic tunnel junction incorporated in a single cell of a magnetic random access memory (MRAM). Orthogonal word and bit lines create a magnetic 17 field that is able to set the free layer magnetization direction of the MTJ. Semiconductor (transistor) elements are used as a switch for read-out. In (b) the memory function of an MTJ is illustrated by the magnetoresistance of a Co/Al 2 O 3 /NiFe junction (see Fig. 1.2 for the full 19 curve of a similar MTJ). The arrows indicate the direction of the in-plane magnetization. To write a 0 or 1, a magnetic field is applied by the word/bit line that is just large enough to switch the softest (storage) magnetic layer, but small enough not to switch the (magnetically 22 harder) reference layer. To read a bit, the resistance is measured at zero magnetic field. 22 actually store the information, and is switched by a small magnetic field to create a zero-field state with low or high resistivity, corresponding to a logical 0 or 1. The reader is referred to Tehrani et al. (2000, 2003), de Boeck et al. (2002), Parkin et al. (2003), DeBrosse et al. (2004), Shi (2005), and references therein, for papers on MRAM technology. Although the magnetoresistance effects in MTJs have been reproducibly reported by many groups, and applications are being developed since then, the fundamental issues in explaining the observed effects are far from fully understood, and need a careful introduction. In the following, it is explained how the existence of TMR can be predicted in the most elementary phenomenological model capturing some of the basic fundamental properties of these devices. This will serve as a starting point for a further exploration of the underlying physics, which is addressed later on in the review. 1.2 Elementary model for tunnel magnetoresistance In elementary textbooks on quantum mechanics, the tunneling current through a potential barrier is extensively treated, illustrating the finite probability for an electron to tunnel through energetically forbidden barriers. Within the Wentzel- Kramers-Brillouin (WKB) approximation, which is valid for potentials U varying slowly on the scale of the electron wavelength, the transmission probability across 44

7 Spin-Dependent Tunneling in Magnetic Junctions 7 Figure 1.4 The wave function in a metal-oxide-metal tunnel structure schematically shows the concept of quantum-mechanical tunneling for electrons with an energy close to the Fermi energy E F. The barrier height at the interface between metal and oxide is given by φ. A nonzero 13 tunneling current is flowing when a bias voltage V is applied between the metallic electrodes The grey areas in the metal regions represents the occupied density-of-states; in the barrier the energy gap of the insulator is indicated in white. 15 a potential barrier is in one dimension proportional to: ( t ) T(E) exp 2 2m e [U(x) E]/ h 2 dx with E the electron energy, m e the electron mass, and x the direction perpendicular to the barrier plane. This equation directly shows the well-known exponential dependence of tunnel transmission on the thickness t and energy barrier U(x) E. Note that the electron momentum in the plane of the layers is assumed to be absent, i.e., k = 0. In fact, when electrons are impinging the barrier under an off-normal angle (k 0), the tunneling probability rapidly decreases with increasing k since in that case the term 2m[U(x) E]/ h 2 in the exponent of the transmission should be replaced by 2m[U(x) E]/ h 2 + k 2. In an experimental situation, this tunneling process can be measured in a metaloxide-metal structure, a trilayered structure of two metals or electrodes separated by an insulating spacer. The thickness of the spacer is in the order of just 1 nanometer, a few atomic distances, otherwise the exponentially decaying tunneling current (proportional to the transmission in Eq. (1)) becomes immeasurably small. The metal-oxide-metal junction is drawn in Fig. 1.4 where the potential of the barrier U(x) is assumed to be constant across the barrier and located at an energy φ above the Fermi energy E F of the metals. Without a voltage difference between the metals layers, the Fermi levels will be equal on either side of the barrier, and the tunnel current is zero. When a finite bias voltage V is applied, the Fermi level is lowered at the right-hand side of the barrier, and electrons are now able to elastically tunnel from filled electron states (left) towards unoccupied states in the second (right) electrode. Note that in this case the electrode at right is at a higher electrical potential as compared to the left electrode, yielding a net electrical current from right to left. As a result, the amount of current will be proportional to the product of the available, occupied electron states on the left, and the number of empty states at the right electrode, multiplied by the barrier transmission probability. Therefore, the (1)

8 8 H.J.M. Swagten tunneling current is directly proportional to the density-of-states of each electrode (at a specific energy E) multiplied by the Fermi Dirac factors f(e)and 1 f(e) to account for the amount of occupied and unoccupied electron states, respectively. To analytically calculate the net tunneling current in the metal-oxide-metal structure, we first write the current due to electrons tunneling from left to right assuming an elastic (energy-conserving) electron tunneling process from occupied states on the left to empty states at the right (see the figure): I L R (E) N L (E ev )f (E ev )T (E, V, φ, t)n R (E)[1 f(e)]. As indicated by Eq. (1), the transmission T(E, V, φ, t) depends on the electron energy and barrier thickness and potential, but it is also affected by the bias voltage V that effectively reduces the barrier height φ. For the opposite current we write a similar equation, by which the total current I is obtained by integrating I L R I R L over all energies: I N L (E ev )T (E, V, φ, t)n R (E)[f(E ev ) f(e)] de For small voltages ev φ only the electrons at (or close to) the Fermi level E F contribute to the tunneling current, by which the transmission no longer depends on energy E. Moreover, in this limit also the density-of-states factors are in principle independent of E, which reduces the current to: I N L (E F )N R (E F )T (φ, t) [f(e ev ) f(e)] de For low enough temperature (k B T ev ) the integral over the Fermi functions simply yields ev, by which we end up with a transparant expression for the tunnel conductance: G di/dv N L (E F )N R (E F )T (φ, t). It shows that in this simple model the tunnel conductance is proportional to the transmission probability and the density-of-states of the two electron systems. The explicit dependence of the density-of-states factors is originally proposed by the pioneering theoretical work of Bardeen (1961), now referred to the transfer- Hamiltonian method (see Wolf, 1985). Note that usually in this method the probability T(φ, t) is written as M 2, which is the squared transfer matrix element that determines the tunneling transition rate between an initial and final state. Now we can proceed with evaluating the current in a magnetic junction, that is, two magnetic electrodes separated by a nonmagnetic insulator (see Fig. 1.5). The density-of-states of a ferromagnetic material is represented by a simple majority and minority electron band, shifted in energy due to exchange interactions. First, we consider two identical ferromagnetic electrodes with parallel magnetization orientations, separated by an insulating barrier. Assuming that the electron spin is conserved in these processes (Tedrow and Meservey, 1971a), tunneling may only occur between bands of the same spin orientation in either electrode, i.e., from a spin majority band to a spin majority band, and similar for the minorities. Using Eq. (5) and assuming equal transmission for both spin species, we write the 44 (2) (3) (4) (5)

9 Spin-Dependent Tunneling in Magnetic Junctions 9 16 Figure 1.5 Spin-resolved tunneling conductivity G for parallel (top panel) and antiparallel magnetization (bottom), as indicated at right, is proportional to the product of the density-of-states factors at the Fermi level E F. The total current in parallel orientation is governed by Nmaj 2 (E F ) + Nmin 2 (E F ), in the antiparallel case by 2N maj (E F )N min (E F ). The voltage that introduces a net tunneling current across the barrier (indicated by the grey bar) is negligible in this schematics conductance for parallel magnetization as: G P = G + G N 2 maj (E F ) + N 2 min (E F ), where G ( ) is the conductance in the up- (down-) spin channel, and N maj (E F ) (N min (E F )) is the majority (minority) density-of-states at E F.Whenweswitchthe magnetization orientation of one ferromagnetic electrode relative to that of the other ferromagnetic electrode, the axis of spin quantization is also changed in that electrode. Tunneling between like spin orientations now means tunneling from a majority to a minority band, and vice versa. The conductance for antiparallel aligned magnetization is then simply: G AP = G + G 2N maj (E F )N min (E F ). It is immediately clear that conductances are different for parallel and antiparallel magnetizations. In other words, ferromagnetic tunnel junctions display a magnetoresistance when an external field is used to switch between these magnetic orientations. This tunnel magnetoresistance (TMR) is usually defined as the difference in conductance between parallel and antiparallel magnetizations, normalized by the antiparallel conductance, or, alternatively, as the resistance change normalized by the parallel resistance: TMR G P G AP = R AP R P. (8) R P Note that the equality of the two definitions for TMR is only valid for very small bias voltage, since in that case the inverse tunnel resistance R 1 = I/V is identi- 44 G AP 44 (6) (7)

10 10 H.J.M. Swagten cal to the conductance di/dv. In literature on MTJs, another, more pessimistic definition of TMR is used as well, normalizing the resistance change by the resistance in antiparallel instead of parallel orientation. However, throughout the review, Eq. (8) will be strictly applied to quantify the magnetoresistance ratio in magnetic junctions. Using Eqs. (6) and (7), it is easily derived that TMR is equal to [N maj (E F ) N min (E F )] 2 /[2N maj (E F )N min (E F )]. We can generalize this for two different magnetic electrodes, resulting in the well-known Julliere-formula for the magnetoresistance of MTJ s (Julliere, 1975): TMR = 2P LP R, (9) 1 P L P R where P L(R) is the tunneling spin polarization in the left (right) ferromagnetic electrode. The tunneling spin polarization of each electrode is defined as P = N maj(e F ) N min (E F ) N maj (E F ) + N min (E F ), and is simply the normalized difference in majority and minority density-of-states at the Fermi level. From these equations it is immediately seen that in the limit of zero polarization of one of the electrodes, no TMR is expected. On the other hand, for a full polarization of ±1, the TMR becomes infinitely high. These fully polarized materials (one spin channel is absent at the Fermi level) are referred to as being half-metallic, and have been intensively investigated in this field; see also section 4.4. In an experimental study, Julliere (1975) is the first to use Eqs. (9) and (10) for TMR in Fe-Ge-Co junctions, although in principal with a different interpretation of tunneling spin polarization. N(E F ) is defined as an effective number of tunneling electrons to stress the fact that the tunneling process is not only governed by the (static) density-of-states at E F. We will return to this crucial point later on. Nevertheless, it should be emphasized that the Julliere equation in its simplest form demonstrates the fundamental role of the tunneling spin polarization of the ferromagnetic electrode in understanding the observed TMR in magnetic junctions. The tunneling spin polarization of individual magnetic electrodes can be measured with a so-called superconducting tunneling spectroscopy (STS) technique that uses a superconductor (in most cases Al) to probe the spin imbalance in tunneling currents. In more detail, in a ferromagnetic-al 2 O 3 -Al junction a magnetic field splits up the sharply-peaked density-of-states of the superconducting Al electrode, which leads to an asymmetry in the conductance G(V ) that reflects the amount of spin polarization. In section 3 this will be further introduced, here only a numerical example will be given. The tunneling spin polarization for Co is experimentally determined to be around +0.42, whichviaeq.(9) corresponds to a TMR effect of more than 40% for Co-Al 2 O 3 -Co MTJs. This is only slightly above the observed (low-temperature) value. For the moment, it seems that we can use this formula as a phenomenological equation that nicely connects tunneling polarization P to the magnitude of the magnetoresistance. However, as we will see below, the physics of spin-polarized tunneling is much more complex and needs a dramatic reconsideration of these phenomena (10)

11 Spin-Dependent Tunneling in Magnetic Junctions Beyond the elementary approach Although the model we have introduced captures some of the basic physics in magnetic tunnel junctions and is rather illustrative on a tutorial level, it fails to predict a number of experimental observations. These observations for TMR include, for instance: strong dependence of TMR on the applied bias voltage V and temperature T sensitivity of TMR on the electronic structure of the barrier-ferromagnetic interface region, not just the bulk density-of-states (as suggested by Eqs. (9) and (10)) relevance of the electronic structure of the barrier, in some cases even leading to an inversion of TMR. Here we will briefly introduce some of the advanced theories to better appreciate these observations, focusing at this point on the tunneling spin polarization for its fundamental role in the physics of magnetic tunnel junctions. A more detailed treatment will be postponed for sections 3and4. Later on in this review (Table 1.2 in section 3) we will show that the tunneling spin polarization of the 3d ferromagnetic metals are all positive, andinthe range of 40 60%. According to the definition of Eq. (10), the positive sign of the polarization relates to a dominant majority density-of-states at the Fermi level. If one considers the band structure and density-of-states of the 3d metals, however, the situation is completely reversed. As an example, Fig. 1.6 shows the (calculated) density-of-states of Co and Ni, both having a surplus of minority states of the Fermi level. This would suggest a negative tunneling spin polarization, and completely contradicts the experimental observations. This dichotomy was recognized already in the seventies when pioneering experiments in the field of superconducting tunneling spectroscopy were reported on ferromagnetic-superconducting junctions (Tedrow and Meservey, 1971a, 1971b, 1975). Theoretically, Stearns (1977) has shown that the conductance in a tunnel junction is not simply determined by the electron density-of-states at the Fermi level, but should include the probability for them to tunnel across an ultrathin barrier. Especially the most mobile s-like electron states are able to tunnel with a much larger probability as compared to the d electrons due to their different effective mass. Based on this, Stearns could explain the positive spin polarization by considering the spin asymmetry of the s-like energy Figure 1.6 Density-of-states of the elemental metals fcc Cu (a), fcc Ni (b), and hcp Co (c), obtained from self-consistent band-structure calculations using the Augmented Spherical Wave (ASW) method. From Coehoorn (2000). 46

12 12 H.J.M. Swagten bands, thereby neglecting the contribution from the rapidly decaying d-like wave functions in tunneling experiments. More recently, another advanced aspect of spin-polarized tunneling is reported. Slonczewski (1989) emphasizes that spin-dependent tunneling is not a process solely related to the (complex) electronic properties of the ferromagnetic electrodes. He has analytically calculated the tunneling current between free-electron ferromagnetic metals within the WKB approximation (see Eq. (1)), assuming that tunneling electrons have a very small parallel wave vector, close to k = 0. Byexplicitly matching the electron wave functions at the barrier interfaces, the tunneling spin polarization is calculated as: P = P 0 κ 2 k F,maj k F,min, κ 2 + k F,maj k F,min where k F,maj and k F,min are the Fermi wave vectors, and κ the imaginary component of the wave vector of electrons in the barrier with k = 0 at the Fermi level, corresponding to κ = (2m e φ/ h 2 ) 1/2 with φ the height of the barrier. The first term P 0 is equal to the earlier result in Eq. (10). The second term, however, contains the properties of the barrier as well, and is due to the discontinuous change of the potential at the interface with the barrier. As a result of this interface factor, the polarization becomes greatly dependent on the band parameters in relation to the height of the barrier, with the possibility to even change the sign of P.Thisisinfact a first demonstration that tunneling spin polarization is not an intrinsic property solely determined by the ferromagnetic electrode. A similar conclusion is reached in free-electron calculations where the conductance is analytically obtained by matching the freeelectron wave functions (and its derivatives) at the two interfaces (MacLaren et al., 1997). In this free-electron calculation, also electrons with k 0 are considered, although k is assumed to be strictly conserved upon tunneling. In Fig. 1.7 the freeelectron magnetoresistance calculated by Slonczewski (1989) and MacLaren et al. (1997) is plotted as a function of polarization P = (k F,maj k F,min )/(k F,maj + k F,min ), which is equivalent to P 0 in Eq. (11). For thick barriers, the solutions in the calculation of MacLaren et al. (1997) approach the model of Slonczewksi based on the WKB approximation, whereas no correspondence is found with the Julliere expression. However, it should be stressed that the predictability of this elementary, simplified free-electron model is rather poor. As already pointed out by Harrison (1961), this is related to the suspicious absence of density-of-states factors in the transport characteristics. MacLaren et al. (1997) and Zhang and Levy (1999) emphasize that, generally, these free-electron calculations (including the Julliere model) fail to predict the observed magnetoresistance behavior in magnetic junctions, and its dependencies on, e.g., barrier thickness, barrier height, and bias voltage. Nevertheless, there are some attempts to directly use Slonczewski s or other free-electron calculations to investigate how TMR behaves as a function of the model parameters. For an example, see the work of Tezuka and Miyazaki (1998) on the variation of TMR with the Al 2 O 3 barrier height. After the work of Slonczewski (and free-electron calculations by others), a great number of advanced theoretical investigations have been published to further explore the physics of TMR and tunneling spin polarization; see for example the (11)

13 Spin-Dependent Tunneling in Magnetic Junctions Figure 1.7 (a) Calculations of the magnetoresistance (R AP R P )/R AP as a function of the tunneling spin polarization P = (k F,maj k F,min )/(k F,maj + k F,min ). The Julliere curve is based on Eq. (9) although using the pessimistic definition of TMR = 2P 2 /(1 + P 2 ). Calculations within the model of Slonczewski are performed for a barrier height φ of 3 ev. In the free-electron calculations (labelled MacLaren), the thickness of the barrier t is5å, 20Å, and200å(same 18 barrier height). The inset schematically shows k 19 conservation used in the free-electron model. 19 (b) Energy versus density-of-states used in free-electron calculations, showing the parabolic 20 bands for majority and minority electrons on each side of the insulating barrier. E F is the Fermi level. In the calculations the bias voltage is assumed to be small, ev φ. Adapted from MacLaren et al. (1997). 22 review paper of Zhang and Butler (2003). Along with that, experimental evidence has become gradually available that shows, e.g., the decisive role of the barrierelectrode combination for spin-polarized tunneling. Other exciting observations have been reported, such as the role of crystallinity and orientation of the magnetic electrode, oscillations in TMR due to the presence of nonmagnetic layers favoring quantum well states, and unprecedented, giant TMR in junctions when incorporating half-metallic electrodes or, more recently, crystalline MgO barriers. Especially in sections 3and4these developments will be extensively addressed. 1.4 Scope of this review It is the purpose of this review to introduce the reader to the most important aspects of spin-polarized tunneling. We have just seen that spin polarization in MTJs is a complex parameter heavily dependent on the details of the potential the electrons experience when crossing the barrier region. This in turn strongly influences the fabrication process of MTJs, where obviously utmost care should be taken in designing and characterizing the barrier and the interface regions with the ferromagnetic metals. Barriers in MTJs are traditionally made out of oxidized Al for their relative ease to create superior coverage of the metallic electrode, together with the observation of large magnetoresistances. A huge research effort could be witnessed in the late nineties to optimize the oxidation process for enhancing the functionality and reliability of MTJs. Furthermore, a number of oxidation methods have been explored in great detail, in particular the use of an oxygen plasma to gradually 44

14 14 H.J.M. Swagten oxidize a previously deposited Al layer. The physical properties and optimization of the barrier and adjacent ferromagnetic layers are the topic of section 2. Alsointhis section the basic design rules for a magnetic junction will be discussed together with elementary transport properties, such as the dependence of TMR on bias voltage, barrier thickness, and temperature. In section 3 we return to the physics of tunneling spin polarization. Details will be given on the experimental method involving superconducting probe layers, followed by a more in-depth discussion on the basic fundamental ingredients. Topics of interest are the relation between tunneling spin polarization and the ferromagnetic magnetization, the relevance of the barrier-electrode interface region including the local chemical bonding, and the relevance of the symmetry of the wave functions of tunneling electrons. Section 4 reviews a number of crucial experiments in the field of TMR in magnetic junctions. Especially those topics will be highlighted that have contributed to the understanding of the underlying physics of spin tunneling, e.g., addressing the role of the interfaces with the barrier, the (local) density-of-states of the magnetic layers, half-metallic or epitaxial ferromagnetic electrodes, and tunneling across crystalline barriers (such as MgO or SrTiO 3 ). The review will be concluded by briefly considering some of the promising directions within this field of magnetic junctions or, in a wider perspective, the field of hybrid devices where tunnel barriers are often combined with new materials to create new physics or functionality. This includes, for example, the development of allsemiconductor MTJs, the use of magnetic semiconductors as (spin-filter) barriers, and the realization of three-terminal magnetic tunnel transistors Basis Phenomena in MTJs The fabrication of a properly operating insulating tunnel barrier, separating the magnetic electrodes, has developed as a wide and very active research field where many aspects on oxide growth, characterization, magnetism, and transport are being considered. Although there are several ways to fabricate barriers for MTJs, a clear distinction can be noticed between crystalline and amorphous barriers. The amorphous Al 2 O 3 barriers are most extensively studied due to the ability to serve as an excellent barrier with a sufficiently small density of pinholes (i.e., electrical shorts between top and bottom metallic electrode). Usually, alumina barriers are created by depositing a thin Al layer that is subsequently oxidized by thermal (natural) or plasma-enhanced oxidation. Figure 1.8 shows a prototypical example of the magnetic-field dependence of the resistance in a magnetic junction consisting basically of FeMn-Co-Al 2 O 3 -Co, with the alumina barrier formed by plasma-oxidizing an Al layer. The tunneling resistance or current across the Al 2 O 3 barrier is measured in the so-called 4-point geometry by contacting the bottom and top electrodes as indicated in Fig. 1.8c. Recently, there is an increasing amount of studies focusing on junctions with crystalline or even epitaxial barriers, such as the widely investigated MgO and SrTiO 3. In some cases, this yields magnetoresistance ratios superior to those with alumina barriers with the added advantage to be able to accurately 44

15 Spin-Dependent Tunneling in Magnetic Junctions Figure 1.8 In (a) the room-temperature magnetoresistance is shown of an mm 15 MTJ fabricated with UHV magnetron sputtering through metal shadow masks. The arrows indicate the orientation of magnetization. The structure is schematically shown in (b): 17 Si(100)/SiO 2 /50 Å Ta/50 Å Co/100 Å FeMn/35 Å Co/23 Å Al + oxidation/150 Å Co/50 Å Ta The top-view layout of these junctions in (c) indicates the actual 4-point geometry for the resistance measurement. After LeClair (2002). 19 model the transport processes in these better defined systems. The discussion on other, crystalline barriers materials will be postponed for section 4. On the other hand, Al 2 O 3 -based junctions are a perfect playground to address a great number of basic physics in magnetic junctions, let alone the huge interest from industrial labs for the incorporation of these barriers in MTJ-based sensors and magnetic memories (see, e.g., Parkin et al., 2003). In this section, a number of basic phenomena in Al 2 O 3 -based MTJs will be reviewed, including the most relevant fabrication and characterization tools. The topics are: 22 basic properties of MTJs, emphasizing general tunneling transport characteristics, methods for switching the magnetization in junctions, and the basic behavior of TMR oxidation of ultrathin metal layers, such as plasma and natural oxidation, in relation to the performance of TMR devices optimizing barriers for TMR: under- and over-oxidation, pinholes, dielectric breakdown, thermal stability, and alternative amorphous barriers. In somewhat more detail, the first part (section 2.1) introduces the basic voltage dependence of tunneling current in relation to the thickness and electron potential of the insulating barrier, supplemented with a few experimental examples. We will also shortly focus on the magnetization reversal of the magnetic layers, aiming at the realization of two macroscopic, magnetically stable states of the ferromagnetic layers: antiparallel versus parallel. As we have seen in section 1, in these two states the total tunneling current (sum of spin-up and spin-down current) is essentially different in a magnetic junction. In the example of Fig. 1.8 the magnitude of the resistance change is more than 25% (using [R AP R P ]/R P,Eq.(8)) when switching 44

16 16 H.J.M. Swagten from the parallel to the antiparallel state, which is accomplished by using so-called exchange biasing, one of the most widespread magnetic engineering tools. The basic behavior of the magnitude of the magnetoresistance effect will be discussed next in section 2.1, for instance focusing on how TMR depends on oxide thickness, temperature, and bias voltage. The second part of this section is devoted to the oxide layer that is sandwiched between the ferromagnetic layers (section 2.2). In view of the fact that the current is exponentially dependent on thickness and height of the barrier (apart from many other details), the preparation and characterization of the oxide layers is the most critical step in the junction fabrication. The available oxidation procedures will be reviewed mainly in relation to the magnitude of TMR and the resistance R of a magnetic tunnel junction, both rather crucial when assessing device applications for MTJs (see for instance the introduction on MRAM in section 1).Thisisfollowed in section 2.3 by considering a number of key issues in this area, including overand underoxidation of ultrathin Al layers, the role of metallic shorts or pinholes and dielectric breakdown when the barrier becomes extremely thin, thermal stability of MTJs for processing or operation at elevated T, and the use of alternative barriers to further tune the device (magneto)resistance. Finally, it is worth mentioning that the use of a great number of experimental tools will be discussed in this section (in particular in section 2.3.1), such as X-ray photoelectron spectroscopy (XPS), Rutherford backscattering spectroscopy (RBS), transmission electron microscopy (TEM), ballistic electron emission microscopy (BEEM), and optical or ellipsometric characterization. All these tools have added considerably to the understanding of how physical or chemical properties of the barrier are related to the tunneling transport Basic magneto-transport properties This section reviews the basic experimental observations in electrical transport and magnetic behavior of magnetic tunnel junctions, and is aiming at explanations mostly on a phenomenological level. These observations can be summarized as follows: tunneling current I is nonlinear in applied bias voltage V conductance di/dv is approximately parabolic in voltage V, expect for small bias resistance R at low bias scales inversely with junction area A, and grows exponentially with barrier thickness magnetization M of two ferromagnetic layers adjacent to the barrier can be switched independently by several magnetic engineering methods TMR is rather independent of barrier thickness t, except for extremely thin barriers TMR decays with temperature T and with applied bias voltage V. 44 As mentioned before, more advanced approaches to address the underlying mechanisms for spin-polarized tunneling will be reviewed in section 3 and section 4.

17 Spin-Dependent Tunneling in Magnetic Junctions Tunneling transport in junctions It is shown in Eqs. (3) (5) that a net tunneling current is induced across a tunnel junction when applying a finite bias voltage between the ferromagnetic electrodes of an MTJ. A straightforward I(V)measurement is a useful tool to directly assess the existence and properties of the tunneling barrier. The Ohmic behavior as derived in Eqs. (3) (5) is only valid for small applied bias voltage, and should be reconsidered for higher voltages where the I(V)curve becomes essentially non-linear. For symmetric tunnel junctions with identical electrodes, Simmons (1963) has analytically calculated the tunneling current using the WKB approximation (see Eq. (2)) which is valid for thick and high barriers: I(V) = αa ( φ ev ) [ ] exp βt φ ev 2 2 αa ( ) [ ] 13 t 2 13 φ + ev exp βt φ + ev, (12) 16 t 2 16 with, α = e/(2πh), β = 4π 2m e /h (m e the effective electron mass in the barrier conduction band), V the applied voltage, t the barrier thickness, A the barrier area, and φ the average barrier height above the Fermi level t [V(x) E 0 F ] dx/t. Here we neglect the effect of the image charges on the shape of the barrier potential (Simmons, 1963), which, due to the tendency to round off the potential at the outer edges of the insulator, leads to an increase of tunneling current; see Hirai et al. (2002) for data on MTJs. The Simmons equation is later adapted by Brinkman et al. (1970) to include an asymmetry in the barrier potential, with φ the potential difference between right and left electrode. Generally speaking, the potentials the electrons experience when transported across a junction is not automatically symmetric in space. First of all, when employing two different metallic electrodes, their nonequal work functions will create an electrical field across the barrier, leading to an intrinsically tilted barrier potential. Apart from that, the barrier itself is often intrinsically asymmetric related to the preparation. For instance, when oxidizing an Al thin film by a post-growth oxidation process, the stoichiometry of the oxide may vary in the direction perpendicular to the layer planes due to over- or under-oxidation. Moreover, this oxidation procedure may create different interfaces with the electrodes, by which, even when using the same electrode materials, the asymmetry almost naturally arises. We will come back to this in section 2.3. The tunneling current for asymmetric barriers is approximated by a Taylor expansion to the third power (Brinkman et al., 1970): [ ( ) ( ] βe β2 e )V t φ t 2 41 I = R 1 0 V V 2 +. (13) φ 3/2 96 φ 42 R 0 is the Ohmic low-bias resistance of the junction given by: 44 R 0 = 2t exp(βt φ) eaαβ φ, (14)

18 18 H.J.M. Swagten which, as expected, scales inversely with area, and rapidly grows with thickness and height of the barrier. In the case of φ = 0, the current is cubic in applied voltage V, equivalent to a parabolic conductance, one of the basic properties of transport across tunneling barriers: G di (15) dv = 1 ( ) β2 e 2 t 2 + V 2. R 0 32R 0 φ The quadratic increase in conductance is in principle valid only for small V and simply reflects the fact that the effective barrier height becomes smaller when a voltage is applied across the junction. At higher voltages, however, higher order terms in Eq. (13) have to be included, and eventually at energies ev exceeding the barrier height, also the effect of a reduction of the effective barrier width (Rottlander et al., 2002). In experimental studies on MTJs, these formulas given by Simmons (1963) and Brinkman et al. (1970) have been extensively used to characterize the barrier characteristics, viz. the barrier height, including its asymmetry, and the thickness of the barrier. It should be kept in mind, however, that these formulas are based on free-electron-like calculations using single parabolic bands for the metallic electrodes. This means that the spin-dependence of the density-of-states of the magnetic electrodes is not explicitly incorporated, which is clear from the absence of these density-of-states factors in Eqs. (12) (15). One can show that this is related to the fact that the group velocity of electrons at E F (which determines the rate of attempts to penetrate the barrier) decreases inversely proportional to the density-of-states at the Fermi level; see also the discussion by Harrison (1961). The widespread use of these equations can be explained by the possibility to at least compare the barrier parameters of junctions grown in different laboratories, and offers a first-order indication of the quality of the tunneling transport of an MTJ. One example out of the rich existing literature is given in Fig. 1.9a, showing the predicted parabolic conductance, in this case of a CoFe-Al 2 O 3 -CoFe junction (Oliver and Nowak, 2004). At low bias an additional anomalous conductance is observed especially at low temperatures, which we will discuss further in section The extracted barrier thickness and barrier height are shown in Figs. 1.9b and 1.9c, respectively. The different parameters in parallel and anti-parallel case are directly related to the presence of spin-dependent tunneling, since, as we argued before, the Simmons or Brinkman equations do not contain density-of-states factors, and the conductance is entirely determined by t and φ. The deviations in t and φ when temperature is beyond 200 K are indicative for the presence of additional conduction processes, such as an inelastic, spin-independent hopping conductance that is dependent on both voltage and temperature (Oliver and Nowak, 2004). In another case (Dorneles et al., 2003), the fitted barrier thickness and area of a Al-Al 2 O 3 -Al junction is found to deviate considerably from the actual nominal values (e.g. obtained from X-ray diffraction or transmission electron microscopy). This hints to a tunneling process governed predominantly by so-called hot spots, small areas where the barrier thickness or barrier height is effectively much smaller than for the remaining part of the junction. Due to the exponential growth of tunnel resistance with t (see Eq. (14)), the slightest corrugation at the barrier-electrode interfaces 22 44

19 Spin-Dependent Tunneling in Magnetic Junctions Figure 1.9 (a) Parallel and anti-parallel conductance G = di/dv at T = 5 K of a junction consisting of 50 Å Ta/250 Å PtMn/22 Å CoFe/9 Å Ru/22 Å CoFe/5 Å Al + oxidation/10 Å CoFe/25 Å NiFe/150 Å Ta. The solid lines are parabolic fits using the Brinkman expression 16 (Eq. (13)). From these fits the temperature dependence is extracted of (b) the barrier thickness and (c) the average barrier height. After Oliver and Nowak (2004). leads to lateral fluctuations in the barrier thickness, by which the current will be almost completely dominated by these hot spots. It was shown theoretically by Bardou (1997) that even when a barrier is controlled in the Ångstrom regime the tunneling transport can be governed by just a few probable paths due to statistical fluctuations. Moreover, when metallic shorts (pinholes) are present in junctions with extremely thin oxides, the barrier parameters are further obscured by a parallel metallic-like current shunting the true tunneling processes (Akerman et al., 2001). In that case extracting parameters by fitting to Eqs. (12) or (13) is clearly losing its physical significance (see also section 2.3.3). A clear demonstration of the ambiguities involved in extracting barrier parameters is facilitated by internal photoemission studies, from which the barrier height in thin-film tunneling structures can be adequately extracted. For early experiments in this direction see, e.g., Kadlec and Gundlach (1976), Nelson and Anderson (1966), and Crowell et al. (1962). Conceptually, the technique is rather straightforward, see Fig. 1.10b. One shines monochromatic light onto a junction structure, and measures the resulting photocurrent. The incident photons will excite electrons in the electrodes, gaining an amount of energy equal to the photon energy. When electrons are photo-excited to an energy higher than the internal barrier height φ, some of the electrons will be able to enter the conduction band of the insulator. After leaving the barrier at the other side, they will be responsible for a net photocurrent when the opposite contributions from the two electrodes do not cancel. From the onset of this current as a function of the photon energy, the barrier height can be accurately determined (shown in Fig. 1.10a), and is strongly deviating from the barrier potential as derived from fits to the Brinkman equation (Koller et al., 2003). Lateral fluctuations of the tunneling current can be adequately addressed by scanning probe microscopies. Costa et al. (1998) are the first to use the atomic force microscope with a conducting tip in contact with a naturally oxidized epi

20 20 H.J.M. Swagten Figure 1.10 (a) Photoconductance as a function of photon energy for a structure of glass/35 Å Ta/30 Å NiFe/100 Å IrMn/25 Å NiFe/15 Å CoFe/17 Å Al + oxidation/40 Å CoFe/100 Å NiFe/35 Å Ta, plasma oxidizing the Al for 200 sec. The light is incident on 17 the top electrode; no additional bias voltage is applied. The inset shows the (average) barrier height as extracted from fitting to the Brinkman equation as well as from photoconductance, as a function of oxidation time. (b) Schematics of photocurrent generation, showing the energy 19 across a tunnel junction. Electron excitation by light is indicated with hν. E 20 F is the Fermi 20 energy, φ L,R is the barrier height for the left and right electrode. Adapted from Koller (2004). 22 taxial Co layer to map the strong fluctuations in tunneling current. Ando et al. (1999, 2000a) have used a more realistic junction without top electrode (Ta-NiFe- IrMn-Co-Al 2 O 3 ), from which a wide distribution in barrier height can be directly determined, favoring tunneling only from a few hot spots in the barrier. In the atomic-force-microscope studies of Luo et al. (2001) on Co-Al 2 O 3, the observed current fluctuations are attributed to thickness inhomogeneities on a nanometer scale. In a more advanced approach using ballistic electron emission microscopy or BEEM (Kaiser and Bell, 1988), the Al 2 O 3 barrier height can be directly measured on a local scale. Electrons emitted from a conductive tip are injected into the metal insulator metal system at variable energy as determined by the voltage between tip and surface. These injected, hot electrons can only pass the Al 2 O 3 barrier potential when their energy exceeds the barrier height (Rippard et al., 2001; Kurnosikov et al., 2002). Rippard et al. (2001) use structures containing Al 2 O 3 grown on top of Si substrates to create a well-defined Schottky barrier of typically 0.8 ev for selecting the hot electrons. In this work, the alumina barrier height (around 1.22 ev), is found to be rather independent of the deposition method (sputtering versus evaporation), the nominal Al thickness, and the oxidation conditions. In Fig. 1.11, BEEM images directly demonstrate that local barrier height fluctuations emerge upon thinning down of the Al thickness from 6.5 Å to around 4.5 Å (before oxidation). Note that BEEM is only sensitive to the local height of the barrier potential; lateral variations in the barrier thickness are not resolved. In follow-up studies, Rippard et al. (2002) combined BEEM with scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). They have demonstrated that due to variations in the local atomic structure of ultrathin 44

21 Spin-Dependent Tunneling in Magnetic Junctions Figure 1.11 Ballistic electron emission microscopy (BEEM) image (a) of an evaporated junction consisting of Si(111)/75 Å Au/12 Å Co/6 7 Å Al + oxidation/12 Å Co/30 Å Cu. The grey 28 scale is proportional to the collected current of hot electrons. (b) The BEEM current for a thinner barrier (4 5 Å Al + oxidation) shows much stronger variations due to an increase of barrier height fluctuations. Adapted from Rippard et al. (2001). barriers, low-energy extended electron states may support conduction channels at energies below the alumina barrier height. This contradicts the common belief that for ultrathin barriers only metallic pinholes are an important issue for the collapse of spin-polarized transport properties (see section for more details on the effects of pinholes). In another combined BEEM-STM study, Perrella et al. (2002) have found that mobile O 2 adsorbates are present on the surface of an oxidized Al layer, having localized energy states located 1 2 ev above the Fermi level. By thermal annealing (or by electron bombardment) it is possible to drive the adsorbates into the oxide thereby reducing the local transport via these low-energy channels (see Mather et al., 2005, and also the X-ray photoelectron spectroscopy results of Tan et al., 2005). This may be important for the fabrication of high-quality MTJs, since a thermal treatment of a full junction with two electrodes could homogenize the chemisorbed oxygen that is trapped close to the interface with the oxide layer. As a consequence, this could increase the effective barrier height and reduce the (unde- 44

22 22 H.J.M. Swagten 14 Figure 1.12 Junction resistance R versus the area A. (a) Results on Si(100)/200 Å Pt/40 Å 14 NiFe/100 Å FeMn/80 Å NiFe/10 30 Å Al + oxidation/80 Å Co/200 Å Pt structured with e-beam and optical lithography. Adapted from Gallagher et al. (1997). (b) Structured junctions 16 of glass/10 Å Si/100 Å Co/10 14 Å Al+oxidation/170 Å NiFe/40 Å Al by optical lithography andwithshadowevaporation(boeve et al. (1998)). 17 sired) oxidation of the top electrode. For optimization studies on MTJs, including the effects of over-oxidation and annealing; see section 2.3. We now return again to the Simmons and Brinkman equations (12) (15),which also show that the current I in an MTJ is, obviously, linearly scaling with lateral area A of a junction. In other words, the resistance should be inversely proportional to the junction area. This is successfully tested by Gallagher et al. (1997) and Boeve et al. (1998) for Ni 80 Fe 20 -Al 2 O 3 -Co and Co-Al 2 O 3 -Ni 80 Fe 20 junctions, by varying the junction area over up to 5 orders of magnitude using micro-fabrication with e- beam or optical lithography, or during evaporation with the help of shadow masks. The area scaling is illustrated in Fig As a natural consequence of this, the resistance-area product R A can be considered as an area-independent property of a magnetic junction, by which different junctions (from various laboratories) can be compared; see again Eq. (14). In the application of MTJ s this product of resistance and area plays a crucial role, since it determines the resistance noise of the device. When devices are progressively reduced in lateral dimensions (smaller A), the resistance will naturally rise as well as the thermal or Johnson noise that is proportional to RT. Results on noise characterization in MTJ devices, including theroleoflow-frequency1/f noise and the relation to the magnetic switching can be found in a number of publications; see Nowak et al. (1999); Ingvarsson et al. (1999, 2000); Smits (2001); Nazarov et al. (2002); Park et al. (2003); Jiang et al. (2004a). Apart from the noise issue in devices, it is also important that the read-out speed of a memory or sensor (as determined by the RC time) does not increase due to a further reduction of the junction area A (Tehrani et al., 2003; Das, 2003). Furthermore, for an optimal read-out of an MRAM cell, the resistance of the MTJ should match with the underlying transistor (see Fig. 1.3a). For future CMOS technology nodes, the required reduction of R A is roughly scaling with the typical feature size within CMOS (Das, 2003). These considerations explain 22 44

23 Spin-Dependent Tunneling in Magnetic Junctions Figure 1.13 (a) Resistance times junction area R A and (b) TMR as a function of nominal Al thickness. The tunnel junctions consist of 90 Å Ta/70 Å NiFe/40 Å CoFe/t Al + oxidation/30 Å CoFe/250 Å IrMn/30 Å Ta, in which the Al layer is optimally oxidized by a remote oxygen plasma. The junctions are patterned down to 1 2 µm. After de 18 Freitas (2001). the huge effort in reducing R A, e.g. by either reducing the barrier width, or by exploring alternative (energetically lower) barriers; see section and section 4. Another implication of the Simmons and Brinkman equation is the fundamental exponential decay of the current (or, equivalently, exponential growth in resistance) with the thickness of the tunneling barrier (Eq. (14)). This is experimentally demonstrated in Fig. 1.13a, where R A is plotted against the barrier thickness for a large number of CoFe-Al 2 O 3 -CoFe junctions (de Freitas, 2001). Note that these junctions have also been annealed after the deposition process, which, however, does not considerable affect the junction resistance. The main purpose of the post-deposition anneal step is to enhance the TMR as seen in Fig. 1.13b. We will come back to this later on, see section Engineering and switching the magnetic constituents The central part of an MTJ is a sandwich of two ferromagnetic layers, separated by a barrier with a thickness usually below Å. The ferromagnetic layers adjacent to the insulating barrier are typically a few nanometer in thickness, and should be backed with one or more layers to manipulate the magnetic switching. This is necessary to switch the magnetic orientation from a parallel state of the two magnetization vectors to an antiparallel state, which is the basic requirement to observe tunnel magnetoresistance in a ferromagnetic-insulating-ferromagnetic junction (see section 1.2). Creating an antiparallel magnetization state can be realized in several ways, as shown schematically in Fig for three important magnetic engineering schemes. The most straightforward realization is the use of two ferromagnetic materials having different magnetic anisotropy, of which an experimental example has been shown earlier in Fig The magnetization will be antiparallel in a field 44

24 24 H.J.M. Swagten 17 Figure 1.14 Magnetic engineering in magnetic tunnel junctions. In (a) two layers are used with different coercivities H C. Using an antiferromagnetic layer in (b) creates a wide range of 18 antiparallel orientation governed by H 19 ex. In (c) exchange biasing is combined with antiferromagnetic coupling across a metallic spacer to further improve the field range of antiparallel 19 orientation, together with the magnetic and thermal stability. Note that the schematic behavior 21 of M is shown over a much wider field range as compared to (a) and (b), to fully show the decoupling of the artificial antiferromagnet governed by the antiferromagnetic coupling strength J AF. 23 range H C1 < H < H C2 with H C1,2 the coercivities of the soft and hard magnetic layer, respectively (Fig. 1.14a). A serious drawback of this engineering scheme has been reported by Gider et al. (1999). When the magnetization of the softest magnetic layer is repeatedly reversed by magnetic field cycling, the other, magnetically harder layer is progressively demagnetized, equivalent to erasing the MTJ memory when used in an MRAM. Using Lorentz electron microscopy and micromagnetic simulations, the hard-layer magnetization decay is found to result from large fringe fields surrounding magnetic domain walls in the magnetically soft layer (McCartney et al., 1999). To avoid domain-wall formation and motion, the soft layer can be reversed by coherent rotation (Gider et al., 1999), which, in an MRAM architecture, can be simulated by subsequent switching with two current pulses from two orthogonal conduction lines below and above the junction cell (Schmalhorst et al., 2000a). In another study on Co 80 Fe 20 -Co-Al 2 O 3 -Ni 80 Fe 20 junctions, magnetic interactions between domains in the soft and hard magnetic lead to the effect of domain duplication, which in turn affects the magnetoresistance in these MTJs (Rottlander et al., 2004). Generally, however, in most cases the use of an antiferromagnet in direct exchange contact with one of the ferromagnetic layers is preferred above the hard-soft system. Due to unidirectional anisotropy induced by the antiferromagnetic layer, the hysteresis loop of the exchange-biased (pinned) magnetic layer will be shifted in field with respect to the free magnetic layer. This naturally creates an antiparallel field range between 0<H < H ex,withh ex the strength of the exchange bias field 44

25 Spin-Dependent Tunneling in Magnetic Junctions 25 (neglecting the coercivities of the two layers); see Fig. 1.14b. In Fig. 1.8 an example is given of an elementary exchange-biased MTJ, consisting of a stack of the following sequence: SiO 2 -Ta-Co-Fe 50 Mn 50 -Co-Al 2 O 3 -Co-Ta. The Ta-Co layers grown directly on top of the substrate reduce the roughness and provide a proper (111) texture for the FeMn layer, by which an exchange bias field of more than 10 ka/m is established in this case. Usually these systems are additionally heated above the blocking temperature of the antiferromagnet, and subsequently cooled in the presence of an external magnetic field to enhance the interface interactions between ferro- and antiferromagnet. Other antiferromagnetic layers such as metallic PtMn or IrMn compounds are frequently used as exchange-biasing materials, in particular for their better thermal stability (higher blocking temperature). Also insulating, antiferromagnetic NiO films can be applied to exchange-bias one of the ferromagnetic electrodes (Shang et al., 1998a), in this case grown by reactive evaporation of Ni in an oxygen environment. Further details on these procedures as well as on the physics of magnetic engineering by exchange biasing can be found in review papers by Nogues and Schuller (1999) and Coehoorn (2003). In a third engineering method, the single exchange-biased magnetic layer is replaced by an antiferromagnetically coupled sandwich of two ferromagnetic layers separated by an ultrathin metallic spacer. This is the so-called artificial antiferromagnet (AAF) or synthetic antiferromagnet (Sy-AF) as proposed by Parkin (1995) and used, e.g., by Willekens et al. (1995); see also the review paper of Parkin et al. (2003). The magnetization behavior of the three ferromagnetic layers is schematically shown in Fig. 1.14c. Antiparallel orientation of the free and fixed layer on each side of the barrier is induced when the coupled, fixed layer and exchangebiased layer are approximately of equal thickness (Strijkers et al., 2000). In that case, not only the antiparallel field range is superior to the exchange biasing scheme in Fig. 1.14b, but also the two antiferromagnetically coupled layers are magnetically stable with minimal stray field that could affect the magnetization of the free layer. Especially when the lateral dimensions of MTJs become very small in sensor or memory applications, this magnetic rigidity is crucial. Moreover, when the free layer and the pinned layer are ferromagnetically coupled due to their correlated roughness (so-called orange-peel coupling, see Néel, 1962), the antiferromagnetically coupled sandwich of pinned and exchange-biased layer can be tuned (by layer thicknesses and coupling strength) to optimally control the switching of the free layer; see, for example, Vanhelmont and Boeve (2004). Although these advanced modifications in the junction stack are crucial for engineering the field sensitivity of MTJ-based sensors and memories (Engel et al., 2002; Parkin et al., 2003; Tehrani et al., 2003; Pietambaram et al., 2004), we will not further explain more details here. Issues in the magnetic behavior of (sub)micrometer MTJs are generally related to the size dependence of the switching (Gallagher et al., 1997; Lu et al., 1997; Koch et al., 1998; Kubota et al., 2003), the effect of boundary roughness of small magnetic elements due to the patterning process (Meyners et al., 2003), dipolar interactions between MRAM cells (Janesky et al., 2001), thermal stability of the magnetization (Pietambaram et al., 2004), and so on. Regarding the implementation of MTJs in MRAM technology, the dynamics of magnetization reversal is obviously also of utmost importance. Strategies to 22 44

26 26 H.J.M. Swagten 17 Figure 1.15 In (a) the Stoner Wohlfarth astroid shows the switching stability as a function 17 of the normalized hard-axis and easy-axis applied magnetic fields for coherent rotation of ellipsoidal particles. Measurements of the easy-axis switching field versus hard-axis applied field of µm 2 MTJ cells are shown in (b). Open circles are the average switching fields with applied fields swept quasi-statically. Closed circles are data on the same bit cells, but now 21 with magnetic field pulses with 20 ns duration. Adapted from Slaughter et al. (2002) switch the magnetization of the free layer by sending current through the word and bit lines of the MRAM array (see Fig. 1.3) are being widely developed by a number of research groups, see, e.g., Lu et al. (1999); Boeve et al. (1999); Sousa and Freitas (2000); Engel et al. (2002); Slaughter et al. (2002); de Boeck et al. (2002); Gerrits et al. (2002); Tehrani et al. (2003); Parkin et al. (2003). As a typical example within this research area, Fig shows the switching fields of micrometer-size patterned MTJ cells, using both quasi-static and fast current pulses (Slaughter et al., 2002). Apparently, in the regime of 20 ns pulses, coherent Stoner Wohlfarth rotation is still applicable, without the need to consider more complex dynamical behavior (Koch et al., 1998). To improve the magnetic stability of switching one particular MRAM cell, without affecting the other cells along a row, a new scheme has been developed recently. This so-called toggle MRAM uses an artificial antiferromagnet (see earlier) as the free magnetic system, leading to a remarkably improved robustness of cell switching (Engel et al., 2005; Yamamoto et al., 2005) Electrical measurement of TMR Usually the resistance or conductance of a magnetic junction is determined from a 4-terminal measurement. A power supply (or current source) is connected to the bottom and top electrode and one measures the tunneling current (or voltage difference) between the other two terminals; see Fig. 1.8c. An important possible pitfall of such a conductance measurement on a MTJ is related to a laterally inhomogeneous current flowing through the barrier when the resistance of the barrier 44

27 Spin-Dependent Tunneling in Magnetic Junctions 27 is too low as compared to the resistance of the electrode. This is first recognized by Moodera et al. (1996) demonstrating a strong artificial increase of TMR, and is later verified by van de Veerdonk et al. (1997b) using a finite-element approach to model the current crowding in the barrier region of a tunneling device. As discussed by Moodera et al. (1996), also the pioneering data by Miyazaki and Tezuka (1995a) are suffering from an apparent amplification of TMR. Sun et al. (1998a) report on geometrically enhanced TMR in mm 2 -size junctions when the junction resistance is less than 5 times the resistance of the electrode over the junction area. In another regime, when the junction radius is much smaller than the width and length of the leads, Chen et al. (2002) have developed an analytical method to correct for the artificial changes of R A and TMR in such a device-like geometry. It is interesting to mention that the tunnel conductance or resistance can be measured also without the need for two electrodes defining the tunneling area for electrical transport. By applying a voltage across in-line contacts touching the top of a planar (unpatterned) tunneling structure, a current will not only flow through the top conducting layers, but partially also via the tunneling barrier through the bottom part of the stack. It is shown by Worledge and Trouilloud (2003) that four micrometer-spaced probes can be used to reliably determine the (field-dependent) resistance of an MTJ, which is further refined to reduce the experimental errors involved in the positioning of the probes (Worledge, 2004). Especially for testing MTJ devices on a full wafer level, this method is believed to be extremely fast and convenient in assessing, e.g., the uniformity of the resistance or switching fields over a large area TMR: basic behavior, role of bias voltage and temperature In this part three subjects will be treated. First, some basic characteristics of TMR will be shortly reviewed on a phenomenological level, such as the experimental relation with the tunneling spin polarization P, the dependence of TMR on the thickness of the barrier, and the effect of annealing. The use of CoFeB compounds as an alternative magnetic electrode material will be discussed in some detail for its intriguing capability to considerably enhance TMR. Thereafter both the temperature dependence and the bias voltage dependence of TMR will be considered along with an introductory survey of the mechanisms proposed to explain these experimental data. Basic behavior of TMR, including the use of CoFeB In section 1 it is derived that the magnitude of the magnetoresistance in MTJs is directly determined by the tunneling spin polarization via TMR = 2P 1 P 2 /(1 P 1 P 2 ). Although the physics behind the polarization P is far from understood (and will be further explored in section 3), it is clear that tunneling spin polarization of the electrodes at the interface with the barrier offers a direct way to tune TMR. For instance, Co x Fe 1 x and Ni x Fe 1 x are frequently applied in actual devices because of their high values of polarization and TMR (see Kikuchi et al., 2000 for the effect of CoFe composition on TMR). In Fig. 1.13b itisshownthatco 80 Fe 20 -Al 2 O 3 -Co 80 Fe 20 junctions display TMR of 40% at room temperature for nominal Al thicknesses above approximately 8 Å (before oxidation). The rather constant TMR for increasing alumina thickness is a common observation in amorphous Al 2 O 3 -based MTJs, also when using 44

28 28 H.J.M. Swagten other ferromagnetic electrodes. For thinner barriers it is generally observed that TMR is suppressed, see again Fig. 1.13b, most likely due to the increasing density of metallic shorts (see section 2.3.3). It is also observed that annealing of junctions, up to roughly C, greatly improves the TMR (Parkin et al., 1999b; Freitas et al., 2000). This effect has been attributed to a redistribution of the oxygen in the alumina barrier (Sousa et al., 1998) possible combined with a change of the interface structure. We will get back to this later on (section 2.3.4). Recently, the magnetoresistance for Al 2 O 3 -based junctions is significantly improved by the use of CoFeB as a soft ferromagnetic electrode material, for instance by sputtering it from a target with composition Co 73.8 Fe 16.2 B 10 (Cardoso et al., 2004; Ferreira et al., 2005a), or from a Co 60 Fe 20 B 20 target (Wang et al., 2004; Dimopoulos et al., 2004a; Wiese et al., 2004). In the paper of Wang et al. (2004), a room-temperature TMR of 70.4% is achieved which would translate to a tunneling spin polarization of around 51%, probably even higher at low temperatures. Indeed, for Co 72 Fe 20 B 8, Paluskar et al. (2005b) measure a tunneling spin polarization of +53.5% using superconducting junctions which is above the polarization of all other 3d elements or compounds (for more details see section 3). Currently, studies are aiming at understanding these high TMR effects, which may stem from the as-deposited amorphous character of CoFeB, possibly reducing the roughness of the bottom electrode and improving the interface quality (Dimopoulos et al., 2004b; Bae et al., 2005). Upon annealing up to around C, it is observed that these systems may become crystalline depending, e.g., on the composition (B content), film thickness (Wiese et al., 2004; Cardoso et al., 2005), or on the character of the adjacent layers (Bae et al., 2005). However, it is shown by Paluskar et al. (2005b) that the tunneling spin polarization of their thick CoFeB films remains almost unaffected by annealing in ultra-high vacuum conditions. It could be that the electronic structure of CoFeB is not very sensitive to (amorphous crystalline) structural changes as suggested by first-principle calculations on Fe-B alloys (Hafner et al., 1994). Bae et al. (2005) have used MTJs with three different bottom pinned electrodes, Co 32 Fe 48 B 20,CoFe-Co 32 Fe 48 B 20 -CoFe, and CoFe. In the former two B-containing electrodes, the TMR appears to be higher than for the electrode with only CoFe (after annealing). Given the fact that the tunneling spin polarization is determined by the interface with Al 2 O 3 only (section 3), the authors suggest that the surface flatness and interface quality may be rather important for obtaining high TMR with CoFeB electrodes. In section 4, junctions combining CoFeB electrodes with crystalline MgO barriers will be further discussed. These materials turn out to be superior for their enormous magnitude of TMR. The remainder of the data described in this section will be dealing with electrodes not containing these CoFeB electrodes, but rather traditional 3d elements or compounds such as Co, CoFe, and NiFe covering the majority of existing papers in this field. 22 Temperature dependence of TMR The temperature dependence of the (magneto)resistance in MTJs has received enormous attention, both for fundamental interest as well as for applications in sensors and MRAMs operating almost exclusively at room temperature. In a similar way, also the transport behavior at a non-zero, finite applied bias voltage is extremely relevant, which will be the topic 44

29 Spin-Dependent Tunneling in Magnetic Junctions Figure 1.16 Temperature and voltage dependence of TMR in a junction consisting of Si(100)/SiO 2 /50 Å Ta/50 Å Co/100 Å FeMn/35 Å Co/23 Å Al + oxidation/150 Å Co/50 16 Å Ta. In (a) the low-bias normalized TMR (with respect to low T ) is shown as a function of temperature, together with the tunnel resistances in parallel and anti-parallel orientation. 18 Panel (b) shows the voltage-dependence of TMR at T = 5 K. Both the resistance change (R AP R P )/R P and conductance change (G P G AP )/G AP are shown. V 1/2 corresponds to the bias voltage where TMR has dropped to 50% of its zero-bias value. After LeClair (2002) of the following subsection. Generally, three processes are believed to somehow contribute to the T dependence of TMR: a thermal reduction of magnetic moment (polarization) at the barrier interface, directly affecting the magnitude of TMR inelastic tunneling due to electron-magnon (spin-wave) scattering at the barrier interfaces thermally-assisted hopping conductance via impurities or defect states located in the barrier region. In Al 2 O 3 -based magnetic junctions, the temperature dependence of the magnetoresistance is intensively studied, and it is generally seen that TMR gradually decreases with temperature. Figure 1.16a shows that TMR (for low bias voltages) is reduced by more than 25% when heating the junction from T = 5 KtoT = 300 K, which is derived from the change in the parallel and antiparallel resistance (see the figure). To understand why TMR is reduced for higher T, one should first of all realize that an increasing temperature broadens the Fermi distribution of the tunneling electrons, which allows electrons with higher energies to tunnel across the barrier. As long as k B T φ, a criterion well fulfilled at room temperature, this leads to an increase of the low-bias tunnel conductance as G(T )/G 0 = CT / sin(ct ), with C = (2π 2 k B t/h)(2m e /φ) 1/2,andwithG 0 equal to 1/R 0 in Eq. (15). This, however, corresponds to a resistance drop of only a few percent between 0 and 300 K for realistic values of barrier thickness and height, in contrast to experimentally observed changes in R P and R AP (see, for example, Fig. 1.16a). A first approach in further understanding the decaying TMR is proposed by Shang et al. (1998b). The 44

30 30 H.J.M. Swagten zero-bias conductance of a magnetic junction is written as: G P,AP (T ) = G 0CT sin(ct ) [1 ± P LP R ]+G inelastic (T ). The prefactor of the first term G 0 CT / sin(ct ) is the aforementioned enhanced tunneling conductance by smearing of the Fermi functions, P L,R is the tunneling spin polarization of the left and right electrode, where the + refers to parallel oriented magnetizations, to antiparallel. The second term in Eq. (16) is representing a spin-independent inelastic contribution to the current, and is believed to originate from hopping conductance via imperfections in the Al 2 O 3 barrier. The role of scattering by impurities in the barrier is separately studied by Jansen and Moodera (2000) in artificially doped barriers, e.g. by plasma oxidizing an Al-Si-Al trilayer, with a Si thickness of Å. When using magnetic ions (Ni instead of Si), the inelastic nature of spin scattering is reflected in a more pronounced temperature dependence of TMR. Returning to the analysis of Shang et al. (1998b), it is instructive to calculate the magnetoresistance from Eq. (16) using the definition given in Eq. (8), yielding TMR = 2P L P R /[1 P L P R + G inelastic (T )/G(T )]. This explains the reduction of TMR with temperature whenever a nonzero inelastic tunneling term is present. Apart from that, also the polarization P L,R itself is depending on temperature which is shown theoretically by MacDonald et al. (1998). Due to the presence of thermally excited spin waves the polarization in the Julliere formula can be effectively written as P(T) = P 0 [m(t )/m 0 ] with m(t ) the saturation moment at the interface of the ferromagnetic layer with the barrier, and P 0 and m 0 the zero-temperature polarization and magnetic moment, respectively. Using the approach captured by Eq. (16) including the polarization suppression by thermally excited spin waves, a good agreement with temperature-dependent experiments has been reported by Shang et al. (1998b). Another approach to model the temperature dependence of tunnel magnetoresistance is given by Davis et al. (2001). In this case, tunneling is treated purely elastically within a free-electron model (Slonczewski, 1989, see also section 1.3) without incorporating additional inelastic conduction channels. In a free-electron model the tunneling spin polarization of the (Fermi) electrons can be expressed as (k F,maj k F,min )/(k F,maj +k F,min ),withk F,maj,min =[2m maj,min (E F U maj,min )/ h 2 ] 1/2, m the effective electron mass, and U maj,min the bottom of the exchange-split parabolic bands. The temperature dependence of TMR now arises from the T dependence of the exchange splitting U maj U min and is reported to be nearly proportional to M(T) (Shimizu et al., 1966). From fitting the model calculations to experimental data (Davis et al., 2001), it is shown that a small drop in magnetization between 0 and 300 K may lead to a substantial variation of TMR in accordance with the experiments. This implies a rather prominent role of intrinsic band structure effects in understanding the T -dependence of transport in MTJs. In an alternative theoretical approach (Zhang et al., 1997a), the reduction of TMR with temperature is described in terms of inelastic magnon (spin wave) scattering. By the emission or absorption of magnons during the tunneling process (16)

31 Spin-Dependent Tunneling in Magnetic Junctions 31 across the insulating barrier (involving a reversal of spin), TMR is more efficiently reduced with temperature than for elastic tunneling only. Using a detailed analysis of the conductance of exchange-biased junctions, Han et al. (2001) have found an excellent agreement with the magnon-assisted inelastic excitation model, which includes a proper description of the bias voltage dependence of TMR. We will return to the model of Zhang et al. (1997a) below. Bias-voltage dependence of TMR Since the discovery of magnetoresistance in alumina-based junctions, the significant suppression of TMR with increasing bias voltage V has been subject of a great number of experimental and theoretical studies. In Fig. 1.16b the typical reduction of TMR with applied bias voltage in a Co-Al 2 O 3 -Co junction is shown using two different representations, viz. as R/R P and as G/G AP. Obviously, the resistance and conductance change only coincide at sufficiently small bias voltage when R = V /I is identical to 1/G = dv /di. The suppression of TMR with voltage is critically important when operating MTJs devices at finite voltage, and a huge research effort is seen in optimizing and understanding the decay of TMR. Usually the voltage where TMR = R/R P is reduced by 50%, indicated in the figure by V 1/2, is taken as a representative fingerprint of the bias-voltage dependence. From the huge amount of reports on the bias-voltage dependence of TMR, it is seen that V 1/2 is typically in the order V in Al 2 O 3 -based magnetic junctions. As to the explanations of the V dependence, several mechanisms have been proposed so far: 22 spin-mixing due to electron-magnon scattering in the magnetic electrodes, at the interfaces with the barrier additional tunnel conductance channels provided by defect and impurity states in the barrier region intrinsic modification of the barrier shape, combined with the spin-dependent band structure of the magnetic electrodes. In Eq. (15), it is shown that in the WKB approximation the conductance of a tunnel junction is quadratic in voltage, as experimentally observed for high enough voltages (see Fig. 1.9). At low bias voltage, however, both the conductances in parallel and anti-parallel configuration strongly deviate from the parabolic law, and a quasi-linear, so-called zero-bias anomaly is universally observed in Al 2 O 3 - containing MTJs. Zhang et al. (1997b) and Bratkovsky (1998) have shown that the excess energy of the tunneling electrons as provided by the applied voltage is capable of collectively excite magnons at the ferromagnet-barrier interface, thereby inducing an additional inelastic conductance contribution that is linear in bias voltage, viz. G(V ) V for voltages V k B T C /e with T C the Curie temperature of the magnetic electrode. Due to the reversal of the electron spin associated with the creation of a magnon, TMR is naturally decaying with voltage in this regime. For higher voltages, the lifetime of the magnons becomes too short and the additional inelastic conductance levels off upon further increase of V. Han et al. (2001) have carefully measured the bias dependence of exchange-biased Co 75 Fe 25 -Al 2 O 3 -Co 75 Fe 25 junctions, not only measuring I(V) and di/dv curves, but also measuring d 2 I/dV 2, 44

32 32 H.J.M. Swagten so-called inelastic tunneling (IET) spectra. Using the magnon-assisted inelastic excitation model of Zhang et al. (1997b), their data are reasonable well captured by the calculations, provided that the wavelength-cutoff energy of the spin-wave spectrum is different for parallel and anti-parallel magnetization (Han et al., 2001). From a theoretical point of view, also mechanisms other than (interface) magnetic excitations have been proposed to explain the suppression of TMR with voltage. This includes the effect of the intrinsic band structure, and impurities in the barrier. The latter contribution is specifically addressed by intentionally adding a δ- doped ultrathin layer within the Al 2 O 3 barrier (Jansen and Moodera, 1998, 2000). Only in the case of magnetic impurities, a stronger bias dependence has been observed and is attributed to spin-exchange scattering. Intrinsic band structure effects can be understood by realizing that already in elementary free-electron calculations the TMR is decaying with voltage (Zhang et al., 1997b); see section 1.3 and Fig. 1.7 for details on the free-electron model. This is due to the fact that the overall conductance is enhanced by applying a significant bias, simply due to an effectively reduced barrier height by tilting the barrier potential with voltage. On the other hand, the difference between conductance in the parallel and anti-parallel orientation is only slightly affected, since the voltage adds additional energy dependencies to the density-of-states (or spin polarization) thereby diminishing the imbalance between the number of majority and minority tunneling states. Assuming that tunneling in Fe-Al 2 O 3 -Fe is dominated by a single free-electron-like spin-resolved d band, Davis and MacLaren (2000) have found a fair agreement with the data of Zhang and White (1998), suggesting that the behavior of TMR with applied voltage has an intrinsic component resulting purely from the underlying electronic structure. This is corroborated by free-electron calculations and experiments on the bias dependence of Co-Al 2 O 3 -Co junctions (Xiang et al., 2002, 2003), in which a reasonable variation of the Co density-of-states over energy is required to describe the resistance and TMR over a broad range of bias voltages. Again this hints to the relevance of intrinsic electronic properties for the bias dependence of spin tunneling (see section 4.3 for a further discussion of other experimental results). Finally, it is expected that by applying a bias voltage across a magnetic junction, it should be possible to extract specific density-of-states features of the ferromagnetic electrodes from the conductance or TMR. However, this turns out to be far from trivial, and only a limited number of experimental studies are available. Excellent examples are reported for junctions containing, e.g., epitaxial La 2/3 Sr 1/3 MnO 3 electrodes, or Fe combined with MgO barriers. This will be discussed in section 4. Another point of interest for the bias dependence is raised by the experiment of Valenzuela et al. (2005). They have produced a lateral double-barrier tunneling device basically consisting of CoFe-Al 2 O 3 -Al-Al 2 O 3 -NiFe, where the Al is laterally extended, separating the ferromagnetic electrodes and tunnel barriers over a distance between 1500 and Å. Due to this, the spin-dependence of the electrons tunneling out of one electrode and tunneling into the other electrode can be disentangled. From their experiments it is suggested that tunneling into the empty states of the ferromagnetic electrode is dominating the reduction of TMR with increasing bias voltage, probably due to the intrinsically reduced polarization of the 22 44

33 Spin-Dependent Tunneling in Magnetic Junctions 33 hot electrons and the matching of the wave functions at the interfaces with the tunneling barrier (Valenzuela et al., 2005). 2.2 Oxidation methods for Al 2 O 3 barriers The breakthrough of high room-temperature magnetoresistance in MTJs as reported by Miyazaki and Tezuka (1995a) and Moodera et al. (1995) is strongly related to the successful fabrication of well-controlled, uniform tunneling barriers. Many investigations related to the search for MTJs with improved properties like high TMR, low RA product, large V 1/2 and breakdown strength, and strong thermal stability, are intimately connected to improved control over the barrier region. As we have seen in the introduction, this is due to the physics of TMR and tunneling spin polarization, determined primarily by the barrier and the interfaces with the ferromagnetic electrodes. Consequently, a careful control over the barrier and interface regions is indispensable. Related to this, a wide variety of barrier oxidation and preparation techniques has been explored since the pioneering experiments, which includes: plasma oxidation, using a DC or RF-generated O-plasma ion-beam oxidation thermal or natural oxidation in an O 2 atmosphere UV-light assisted oxidation oxidation by ozone or by O radicals direct Al 2 O 3 deposition. 22 In this subsection, we will focus on these oxidation processes mainly in relation to the magnitude of TMR and R A, since both are, among other properties such as electrical noise (section 2.1.1), decisive parameters for future device implementation of progressively down-scaled junctions. In Fig. 1.17, a compilation of some of the existing data is shown for Al 2 O 3 -based junctions prepared by these different oxidation techniques; see also Table 1.1. In this section we will restrict ourselves to alumina-type junctions since these are predominantly studied in this field. In later sections other barrier materials (such as SrTiO 3 and MgO) will be discussed separately. With very few exceptions, it is clear that plasma oxidation, indicated by the solid symbols in Fig. 1.17, distinguishes itself from all other techniques by the highest values of TMR, but also with a characteristically high value of R A. Thermally oxidized junctions, grouped mostly in the smaller circle, have in general a low RA product but also a low(er) magnetoresistance. Other techniques, which are explored for their potential of making junctions with both a high TMR and lower R A, are situated between those extremes. Even with such a variety of techniques it currently appears to be practically impossible to enter the upper-left part (low R A, high TMR) of Fig with alumina-based junctions. However, the results obtained with ion-beam oxidation (Ferreira et al., 2005a) are promising for their very low RA combined with reasonably high TMR ratios; see also section Interestingly, junctions with crystalline MgO barriers are reported to exhibit much 44

34 34 H.J.M. Swagten Figure 1.17 TMR versus the RA product (at room temperature) of junctions made by various barrier production techniques. The larger and small circle roughly indicate the plasma and 19 thermally oxidized junctions, respectively. Note that ion-beam oxidation (resembling the use of a regular DC plasma) seems superior for their low RA and high TMR. For the underlying data including references, see Table higher magnetoresistances combined with a relatively low resistance-area product (section 4.6). Before discussing the results reported in literature in more detail, the reader must bear in mind that the observed spread in TMR or R A as seen in Fig may naturally stem from lab-to-lab variations of the structure of the junctions, and, in particular, the barrier region. The structure and morphology of the unoxidized aluminum layer influences the oxidation process and therefore the quality of the resulting barrier. The oxide growth can easily be imagined to be affected when oxidizing an aluminum layer with a grain-like structure. Such a growth mode is intrinsically induced by the layer on which the aluminum is deposited and can vary with the bottom layer material and with deposition technique. For example, Ando et al. (2000b) have shown by atomic-force-microscopy measurements that the roughness of aluminum can be reduced with 80% by replacing the aluminum buffer layer under the bottom electrode by Pt. The deposition parameters and characteristics of the deposition facility can play a huge role. For instance, a small amount of surface contamination can induce a different growth mode of the aluminum layer. Fujikata et al. (2001) report a considerable improvement of TMR in junctions with an intentionally contaminated Ta buffer layer in their junctions. Furthermore, in the case of plasma oxidation and UV-oxidation, the exact lay-out and operation of the oxidation setup can be crucial for the quality of the barrier layer. Therefore, the comparison between oxidation techniques found in literature should be considered with great care. 44

35 Spin-Dependent Tunneling in Magnetic Junctions 35 2 Table 1.1 A selection out of the vast literature on room-temperature low-bias TMR and R A for 2 3 alumina-based magnetic tunnel junctions. Oxidation methods are categorized in thermal oxidation, 3 4 plasma oxidation, ion beam oxidation, UV-assisted oxidation, ozone-enhanced oxidation, 4 5 oxidation by radicals, and reactive deposition. Only the electrodes next to the Al 2 O 3 barrier are 5 6 indicated. In several cases the results are obtained after a post-deposition anneal. Data by Li 6 and Wang (2002), indicated with, aretakenatt = 18 K. TMR with is measured at a 0.3 V bias voltage Method Electrodes TMR (%) R A( µm 2 ) Reference Thermal Fe CoFe Tsuge and Mitsuzuka (1997) Thermal NiFe NiFe Matsuda et al. (1999) Thermal Co NiFe Parkin et al. (1999b) Thermal NiFe NiFe Chen et al. (2000) Thermal NiFe NiFe Ohashi et al. (2000) Thermal CoFe CoFe Sun et al. (2000a) Thermal CoFe CoFe Song et al. (2000) Thermal CoFe CoFe Zhang et al. (2001b) Thermal CoFe CoFe Zhang et al. (2001b) Thermal CoFe CoFe Moon et al. (2002) Thermal Co Co Diouf et al. (2003) Thermal CoFe CoFe 22 8 Wang et al. (2003) Thermal CoFe CoFe Das (2003) Thermal CoFe CoFe Zhang et al. (2003b) Thermal CoFe CoFe Zhang et al. (2003b) Thermal CoFe CoFe Shang et al. (2003) Plasma Co CoFe Moodera et al. (1996) Plasma Co NiFe Nassar et al. (1998) Plasma NiFe NiFe Wee et al. (1999b) Plasma Co Co Gillies et al. (1999) Plasma CoFe CoFe Parkin et al. (1999b) Plasma CoFe CoFe (10 20) 10 3 Sun et al. (1999) Plasma CoFe CoFe Sun et al. (1999) Plasma Co Co LeClair et al. (2000a) Plasma CoFe CoFe Ando et al. (2000b) Plasma CoFe CoFe 50 (1 10) 10 3 Ando et al. (2000b) Plasma NiFe NiFe 30 (10 100) 10 3 Chen et al. (2000) Plasma CoFe CoFe Park and Lee (2001) Plasma Co Co (1 100) 10 3 Kuiper et al. (2001a) Plasma CoFe CoFe Dimopoulos et al. (2001a) Plasma CoFe CoFe Tsunoda et al. (2002) Plasma CoFe CoFe 48 ( ) 10 3 Tsunoda et al. (2002) Plasma CoFe CoFe Lohndorf et al. (2002) (continued on next page)

36 36 H.J.M. Swagten 1 Table 1.1 (Continued) 1 Method Electrodes TMR (%) R A( µm 2 ) Reference Plasma NiFeCo NiFeCo Engel et al. (2002) Plasma NiFe NiFe Song et al. (2003) Plasma Co NiFe Song et al. (2003) Plasma CoFe CoFe 10 3 Das (2003) Plasma Co Co Koller et al. (2003) Plasma CoFe CoFe 37 (5 10) 10 3 Kim et al. (2003) Plasma CoFeB CoFe Wang et al. (2004) Plasma CoFeB CoFeB Wang et al. (2004) Ion beam CoFe CoFe 40 ( ) 10 3 Cardoso et al. (1999) Ion beam NiFe Co Roos et al. (2001) Ion beam CoFeB CoFeB Ferreira et al. (2005a) Ion beam CoFeB CoFeB Ferreira et al. (2005a) UV-assisted NiFe NiFe Song et al. (2000) UV-assisted NiFe NiFe Song et al. (2000) UV-assisted Co NiFe Girgis et al. (2000) UV-assisted NiFe NiFe Covington et al. (2000) UV-assisted Co NiFe Boeve et al. (2000) UV-assisted NiFe Co Rottlander et al. (2000) UV-assisted Co Co Rudiger et al. (2001) UV-assisted NiFe NiFe Li and Wang (2002) UV-assisted CoFe CoFe Das (2003) Ozone CoFe CoFe 30 (8 24) 10 6 Park and Lee (2001) Ozone CoFe CoFe Park and Lee (2001) Radicals Co Co Shimazawa et al. (2000) Radicals CoFe NiFe 40 (1 3) 10 3 Kula et al. (2003) Reactive depo. NiFe NiFe > Chen et al. (2000) Reactive depo. Fe(211) CoFe Yuasa et al. (2000) Plasma oxidation Plasma oxidation is currently the most widely applied method for producing aluminum oxide for MTJs with the highest values of TMR for amorphous barriers. As mentioned above, Moodera et al. (1995) are the first to reproducibly produce MTJs using plasma oxidation, and many groups have followed using numerous variations on plasma oxidation. A DC glow plasma is easy to set up (see Fig. 1.18b) and is therefore most commonly applied. Nassar et al. (1998) have applied an AC O 2 /Ar rf-plasma for the production of MTJs. A TMR of 6% is found with an R A of 200 M µm 2. The low TMR and high RA product suggest that the bottom electrode is oxidized in the process. An inductively coupled plasma (ICP) is generated without electrodes by Ando et al. (2002) and Song et al. (2003), which means that there is no contamination by sputtering of electrode material. This method is therefore thought to produce less impurities in the tunnel barrier. However, there is no 44

37 Spin-Dependent Tunneling in Magnetic Junctions Figure 1.18 (a) Differential ellipsometry to in-situ monitor the amount of oxidizing metal as a function of time. The bottom two curves are taken on a Si/SiO 2 /10 Å Al sample using natural oxidation followed by plasma oxidation (open symbols), and for plasma 17 oxidation only (closed). The upper curve represents plasma oxidation of a full stack of 19 Si/SiO 2 /50 Å Ta/70 Å Co/100 Å FeMn/35 Å Co/23 Å Al. In (b) a picture of an oxidation chamber as taken from Knechten (2005) shows the DC glow discharge due to the high (negative) potential of the ring-shaped electrode. See Knechten et al. (2001) conclusive evidence that impurities in the barrier due to sputtering of the electrode are causing a degradation of MTJ properties. A plasma generated by radio-frequency or microwave radiation has been successful in plasma oxidation of silicon and is also applied for aluminum oxidation, for example by Sun et al. (1999) and Yoon et al. (2001). Plasma oxidation is very fast as compared to many other oxidation methods. For example, Park and Lee (2001) optimally oxidize 18 Å of aluminum in approximately 40 seconds, and Kuiper et al. (2001a) need only 20 seconds of plasma oxidation to optimally oxidize 15 Å of aluminum. To monitor these dynamical processes, insitu characterization techniques have been developed. Wee et al. (1999a, 1999b) use the Van der Pauw method to in-situ measure the electrical resistance of the Al layer during plasma oxidation from which the tunneling barrier thickness can be estimated. Optical, ellipsometric techniques have been reported by LeClair et al. (2000c), Lindmark et al. (2000), andknechten et al. (2001), using the extreme contrast between the dielectric constant of a metal and that of its oxide. In Fig. 1.18a it is illustrated that the growth of the oxide from a 10 Å and 23 Å Al layer can be monitored with high temporal resolution and with sub-monolayer sensitivity, offering the possibility to investigate the oxidation dynamics in great detail (for more details, see the thesis work of Knechten, 2005). Variations in plasma pressure and plasma composition can improve the performance of MTJs. Following the success of a krypton oxygen mixture (97%:3%) in silicon oxidation (Sekine et al., 2001), junctions with a TMR of 59% have been obtained by Tsunoda et al. (2002). Lee et al. (2003) have found that the rough- 44

38 38 H.J.M. Swagten ness of the barrier interfaces can be tuned by adding a small amount of Zr to the aluminum. At a doping level of 9.9% Zr, the barrier interfaces, as observed with transmission electron microscopy, are the smoothest and the TMR is highest. Tunnel junctions made by plasma oxidation with a low resistance-area product are, e.g., fabricated by Ando et al. (2000b). The barrier layer in their tunnel junctions is made by inductively-coupled-plasma oxidation of 8 Å Al. After annealing, the junctions with a 10 seconds oxidation time display an RA product of 230 µm 2 combined with a TMR of 31%. When the oxidation time of the Al layer is longer (30 60 seconds) the maximal TMR is raised to roughly 50%, although now with a higher RA of typically a few k µm 2. Using CoFeB compounds as ferromagnetic electrodes (see also section 2.1.4) andaar/o 2 plasma for Al oxidation, very large magnetoresistances of more than 70% are reported by Wang et al. (2004). However,the resistance of these junctions is very high, around 24 M µm 2. Other investigations are concentrating on further optimizing these CoFeB-based alumina junctions; see for example Pietambaram et al. (2004), Wiese et al. (2004), andcardoso et al. (2005). Ionized atom-beam oxidation Roos et al. (2001) use an ionized oxygen atom beam of low energy (30 80 ev) to oxidize the aluminum. MTJs with a 8 14% TMR and a high R A of M µm 2 are produced. Cardoso et al. (1999) and Freitas et al. (2000) use an ion beam for the deposition of the layers and for the oxidation process as well. The ion beam is created by inserting a grid between a high-power (80 W) Ar/O plasma and the sample, and by applying a voltage (typically 30 V) over the grid, accelerating oxygen ions towards the sample. With this technique, junctions with a TMR of 40% and an RA product of around 500 µm 2 were created. The high quality is explained by the better layer-by-layer growth as compared to the more frequently applied sputter deposition. The oxidation times for optimal TMR are comparable to plasma oxidation, in the order of 60 seconds. Recently, an even smaller RA product of around 2 15 µm 2 has been established together with a TMR of around 20% (Ferreira et al., 2005a, 2005b). When an artificial antiferromagnet is present to engineer the top CoFeB electrode (see sections and 2.1.2), R A values of µm 2 are combined with a TMR of 40 45%. The ion-beam oxidation is performed in three steps with increasing plasma reactivity and leads to under-oxidized barriers from an initial 9 Å Al layer. As can be seen from the data points on ion-beam oxidation in Fig. 1.17, these junctions are very promising for device applications in sensors and memories due to the unique combination of low RA and high TMR (Ferreira et al., 2005a) Thermal and natural oxidation In order to create junctions with lower specific resistances for device applications, the barriers in MTJs have become progressively thinner. For the oxidation of Al layers of 10 Å or less, plasma oxidation is thought to be too aggressive (due to the high-energy particles involved) and not well controllable, possibly resulting in damage to the interface with the bottom electrode. Therefore, for very thin layers often natural or thermal oxidation is chosen. We note that for oxidation in an oxygen atmosphere at room temperature normally the term natural oxidation is used, 44

39 Spin-Dependent Tunneling in Magnetic Junctions 39 whereas thermal oxidation refers to oxidation in an oxygen atmosphere where the sample is usually but not necessarily at an elevated temperature. For most junctions reported in literature, oxidation at room temperature is used. Tsuge and Mitsuzuka (1997) and Matsuda et al. (1999) use pure natural oxidation, and a TMR of 13% is found with an RA product of only 1.5 k µm 2. The barrier is made by exposure of 20 Å Al to 0.27 bar of pure oxygen for one hour. The fact that a barrier made from an initial 20 Å Al results in an RA product that is three orders of magnitude lower than plasma-oxidized junctions starting with the same initial Al thickness, suggests that the barrier has an inhomogeneous thickness and the tunneling current runs through the thinnest parts of the barrier. The low TMR is probably due to unoxidized aluminum suggesting that the oxidation in this case is rather inhomogeneous. Generally speaking, it is observed that the resistance of plasma-oxidized junctions is much higher than their naturally oxidized counterparts. The explanation of this striking difference is not yet clear, and maybe directly related to the much higher oxidation rates for plasma oxidation, combined with the fact that the energetic O atoms in the plasma more easily oxidize the pinholes (Knechten, 2005). Natural oxidation is a slow process if the aluminum layer is typically thicker than 5 Å. For instance, the optimal oxidation of 10 Å Al takes 15 hours at room temperature, as reported by Das (2003). In order to reduce processing time, two cycles of deposition of Al and subsequent oxidation are used. The oxidation time resulting in optimum TMR is thereby reduced from 15 hours to 2 2 hours (Das, 2003). An identical technique is used by Moon et al. (2002) who report MTJs with 30% TMR and an RA product of only 140 µm 2. Extremely small values of R A are reported by Wang et al. (2003) (see also Zhang et al., 2001b, 2002, 2003b), and is set to 8 µm 2 while these junctions have a TMR of 22%. Ohashi et al. (2000) have produced a functional low-resistance tunnel magnetoresistive sensor for use as hard disk read-head. The MTJ is made by natural oxidation of 8.5 Å Al in a pure oxygen atmosphere for 20 minutes, resulting in a TMR of 14% and an R A value of 14 µm UV-light assisted oxidation Oxidation assisted by UV-light irradiation has been tried as a faster method of aluminum oxidation as compared to natural oxidation which is due to an increased reactivity by ozone generation. Since the damage due to high-energetic species is avoided as compared to plasma oxidation, UV-oxidized junctions possibly combine alowr A with a high TMR. Boeve et al. (2000) and Girgis et al. (2000) are the first to report results on MTJs made with UV-assisted oxidation. They oxidize a sputter-deposited 13 Å Al layer for one hour in an oxygen atmosphere of 100 mbar, assisted by an in-situ ultraviolet lamp. They find an RA product of 60 k µm 2 and a TMR of about 20%. Compared with their naturally oxidized junctions, UVoxidation results in higher TMR but also in higher R A. Their plasma-oxidized junctions, which are identical except for the oxidation method, give a higher TMR but similar R A values. Later experiments have resulted in junctions with a TMR of 10%, with a typical RA product of 1 k µm 2 (Rottlander et al., 2000). Li and Wang (2002) have prepared junctions by UV-assisted oxidation of only 5 Å Al, 44

40 40 H.J.M. Swagten resulting in a few percent TMR but an R A as low as 3.2 µm 2. By oxidation of 4 Å Al, an extremely low R A of approximately 0.6 µm 2 is found. The TMR in these junctions has, however, dropped to 2%. Probably this is due to metallic shorts between the ferromagnetic layers Other oxidation and deposition processes Oxidation using ozone The high reactivity of ozone suggests that by using ozone for aluminum oxidation, shorter oxidation times with respect to natural oxidation are possible, and that possibly larger oxide thicknesses as compared to natural oxidation are attainable within reasonable oxidation times. An oxygen ozone mixture to oxidize aluminum for MTJs is used by, for instance, Park and Lee (2001) and Park et al. (2002). In a comparison between ozone-oxidized and plasma-oxidized junctions, they report slightly higher TMR values (33%) for junctions oxidized by ozone, whereas the RA product of ozone-oxidized junctions is one order lower with a lowest value of 10.5 k µm 2. The process is still very slow; 50 minutes of oxidation are necessary to produce the junction described here. Junctions made by thermal oxidation have TMR values close to that of the ozone-oxidized junctions (30%), but with considerable lower R A of 140 µm 2 (Moon et al., 2002). Radical oxidation Shimazawa et al. (2000) report on experiments with oxidation using a beam of oxygen radicals, arguing that this can be an energetically low and slower process as compared to plasma oxidation, thus suitable for the oxidation of ultrathin Al layers. The radicals are produced by a microwave (electron cyclotron resonance) in approximately 10 3 mbar of oxygen. Junctions are created with R A values of 350 µm 2, and with a TMR of about 11%. Kula et al. (2003) applied radical oxidation as well, resulting in junctions that have a TMR of 40% and a minimal R A of only 2 k µm 2. In a comparison with natural and plasma oxidation, for radical oxidation a higher TMR is reported as well as a medium RA product in between thermal and plasma oxidized junctions. The possible advantage of radical oxidation over plasma oxidation might be the absence of more energetic particles such as ions. 22 Direct deposition of Al 2 O 3, atomic layer deposition As an alternative to oxidation processes, direct deposition of Al 2 O 3 layers has been tried in various forms. First of all, reactive sputtering has been applied to create Al 2 O 3 layers. By sputtering aluminum in an argon atmosphere which contains a few percent oxygen, in principle a homogeneous and stoichiometric Al 2 O 3 layer can be obtained as shown by Koski et al. (1999). The roughness of thick films ( 1 µm) grown on pure Si was reported to be about 7 Å. Chen et al. (2000) have tried to improve the method by preventing the oxidation of the bottom electrode by first depositing 7 Å of pure Al before adding 5 Å of reactively sputtered Al 2 O 3. Since their method resembles plasma oxidation, the results are comparable to their plasma oxidation results, with a TMR of 18% and an RA product of 1 M µm 2. Yuasa et al. (2000) have produced reactively sputtered, amorphous alumina barriers on top of crystalline Fe electrodes (see section 4.3). The Al 2 O 3 is created by evaporation of Al at an O 2 pressure of around mbar. Due to the direct deposition method, it is possible to use wedges of variable alumina thickness, facilitating the study of magneto-transport as a function 44

41 Spin-Dependent Tunneling in Magnetic Junctions 41 of the thickness of the barrier layer in a single sample (Yuasa et al., 2000). Another method of direct Al 2 O 3 deposition is atomic layer deposition (ALD). In principle, this technique allows for deposition of very thin dielectric films with excellent conformality, uniformity, and atomic-level thickness control; see, for example, Paranjpe et al. (2001). Although tunneling transport has been demonstrated across sputtered exchange-biased magnetic junctions incorporating ALD-based Al 2 O 3 barriers, no appreciable TMR has been measured (Bubber et al., 2002). 2.3 Towards optimized barriers In the following subsections, experiments on the optimization of the oxidation process will be briefly outlined. In many studies, the optimization is performed mainly in relation to the magnitude of TMR, the resistance R of the junction, the temperature dependence of TMR and R, and, finally the bias voltage dependence. These properties have been reviewed in the previous sections. Here we will focus on optimization schemes in relation to the following prominent issues: over- and under-oxidized barriers barrier pinholes and dielectric breakdown thermal stability upon junction annealing the use of alternative (amorphous) oxides. We will start the discussion on these items with a short overview of the experimental tools that have been successfully applied in this area, focusing again on the properties of Al 2 O 3 barriers Tools for oxidation monitoring and optimization Apart from the direct measurement of (tunneling) transport characteristics, a number of diagnostic tools have been used to examine and further optimize the oxidation processes. Electrical and optical tools to in-situ monitor the oxidation dynamics have been mentioned in section 2.2.1, with which the transformation of Al into its oxide can be followed with submonolayer precision. Surface sensitive techniques are frequently applied to study the chemical composition of the junction and in particular the alumina barrier, e.g. using X-ray photoelectron spectroscopy (Mitsuzuka et al., 1999; LeClair et al., 2000c; de Gronckel et al., 2000; Kottler et al., 2001), Rutherford backscattering spectroscopy (Sousa et al., 1999; Gillies et al., 1999), and electron recoil detection (Gillies et al., 2000). In Fig. 1.19a the latter two techniques have been applied to measure the oxygen content in junctions of Ta-NiFe-IrMn-NiFe-Co-Al 2 O 3 -Co-NiFe-Ta for variable oxidation time. The observed ln(t) behavior hints to a simple logarithmic growth law which is proposed by Mott (1947) to describe the oxidation of thin metal films, typically below 40 Å. In this type of oxidation model, it is assumed that the aluminium ions are diffusing through the growing oxide and react with the oxygen at the outer interface. This has been confirmed by Kuiper et al. (2001a) using an isotope technique in which an Al layer is shortly oxidized with 16 Obefore continuing with 18 O; see Fig. 1.19a. In this study, secondary ion mass spectrometry depth profiles indicate that 16 O moves to larger depth with increasing 18 O oxidation 44

42 42 H.J.M. Swagten 11 Figure 1.19 (a) O content of junctions consisting of 35 Å Ta/30 Å NiFe/100 Å IrMn/25 Å NiFe/15 Å Co/15 Å Al + oxidation/40 Å Co/100 Å NiFe/35 Å Ta for variable oxidation time, 12 as measured by Rutherford backscattering spectrometry (RBS) and elastic recoil detection analysis (ERD). Two-step oxidation using 18 O 14 2 and 16 O 2 oxygen isotopes is performed to 14 establish that Al is the moving species during oxidation. (b) Oxide thickness for comparable 15 junctions extracted from cross-section transmission electron microscopy (XTEM) as shown in panel (c) for a multilayer of 50 Å Co/15 Å oxidized Al. This suggests an intermediate oxidation step with increasing O content at constant oxide thickness. After Gillies et al. (1999) and Kuiper et al. (2001a). 18 time, while 18 O is incorporated close to the surface. This points to Al as the moving species during plasma oxidation. Cross-section transmission electron microscopy (XTEM), used for instance by Boeve et al. (2001) and Bruckl et al. (2001), is used to further characterize the growth and morphology of the films. For instance, XTEM on Co-Al 2 O 3 multilayers by Gillies et al. (2000) shows an excellent contrast between the metal and oxide layers, see Fig. 1.19c, from which the oxide thickness can be followed as a function of oxidation time as shown in Fig. 1.19b. When comparing this to the left panel of the figure, it is suggested that oxidation of the barrier is governed roughly by distinct steps. In the first stage the oxygen rapidly penetrates through the total Al layer, then a homogenization stage follows where the O content steadily increases at a fixed oxide thickness, until, at the third step, the Co electrode starts to oxidize. To continue the discussion on characterization tools, Shen et al. (2003) have combined XTEM with electron holography to directly measure the shape of the barrier and its interfaces. An ac-impedance technique is applied by Gillies et al. (2000) in order to characterize the dielectric properties of the barrier layer. Analyzing the data by modelling the tunneling across the oxide with an RC network, complementary information on the structure of the barrier and its evolution with plasma oxidation time has been extracted. Similarly, Landry et al. (2001) have used ac-impedance data on NiFe-Al 2 O 3 -NiFe junctions to determine an interfacial contribution to the capacitance and to extract the electron screening length in the NiFe electrodes. The complex capacitance of magnetic junctions has been measured also by Huang and Hsu (2004), in their case over a frequency range from 10 2 to 10 8 Hz in CoFe-Al 2 O 3 -CoFe junctions. From the analysis of the so-called Cole Cole diagrams, a significant sensitivity to the oxidation process of the metallic Al layers is reported, being able to clearly discriminate between different stages in the oxidation process

43 Spin-Dependent Tunneling in Magnetic Junctions Over- and under-oxidation In order to study the effect of the oxidation process or just to find the optimum oxidation parameters, often a series of samples at various stages of oxidation is produced. This is usually done in one of the following two ways: either by depositing a number of identical samples, oxidized with various oxidation times (see for example Sun et al., 1999; van de Veerdonk, 1999; Gillies et al., 1999, 2000; Song et al., 2000; Tehrani et al., 2000; Park et al., 2002), or by depositing a series of samples with a range of Al thicknesses, all oxidized at once (see Moodera et al., 1997; Song et al., 2000; Tehrani et al., 2000; Boeve et al., 2001; Freeland et al., 2003). The first method requires a rather time-consuming experiment due to the number of oxidation steps, since each oxidation includes a long pump-down stage and possibly sample transport. An example of such an optimization is shown in Fig. 1.20a. The second method, making one batch of samples with a range of Al thicknesses, usually allows for more rapid experiments because only one oxidation step is necessary, see Fig. 1.20b. The use of a wedge (a film with a lateral variation in thickness by linear displacement of a shutter during deposition) is even faster and yields detailed and accurate information (Fig. 1.20c), see for example the work of LeClair et al. (2000c), Song et al. (2000), andcovington et al. (2000). In another method, applied by Nowak et al. (2000) and Song et al. (2000), a series of samples at various stages of oxidation is made by a single deposition and a single oxidation step in a plasma that is not uniform over the wafer. The interpretation of the results in terms of oxidation conditions of this method is of course much more complicated. From the data shown in Fig it is evident that oxidation of the barrier is a critical process, where both over- and underoxidation are detrimental to the TMR effect. Underoxidation leaves the Al layer partially in its metallic state which effectively reduces the tunneling spin polarization of the carriers. The detrimental effect Figure 1.20 (a) TMR as a function of oxidation time for as-deposited junctions comprised of Co 90 Fe 10 /17 Å Al + oxidation/co 90 Fe 10 (Koller, 2004). (b) Results for Ni 80 Fe 20 /t Al + oxidation/ni 80 Fe 20 (Moodera et al., 1997), and (c) for as-deposited and 43 annealed (250 C) junctions of Ni Fe 20 /t Al (wedge) + oxidation/ni 80 Fe 20 (Covington et al., ). Note that the original data from Moodera et al. (1997) have been corrected to match 45 the definition of TMR in Eq. (8). See the references for the composition of the full junction stack as well as for details of the Al oxidation. 46

44 44 H.J.M. Swagten of foreign impurities has been addressed by Jansen and Moodera (1998, 2000) by δ-doping the Al 2 O 3 barrier with nonmagnetic and magnetic elements. Generally, a strong reduction of TMR is observed and interpreted via the effect of additional impurity-assisted tunneling channels combined with spin-flip processes when magnetic impurities are involved. Theoretically, in a number of papers the effects of impurity-assisted tunneling in magnetic junctions have been addressed; see for example Bratkovsky (1997), Zhang and White (1998), andjansen and Lodder (2000). Parkin (1998) and LeClair et al. (2000d) alternatively demonstrate this effect of reduced TMR by adding a very thin nonmagnetic film at the barrier interface. As an example, the addition of 1 monolayer of Cu at the interface between the bottom electrode and the barrier reduces TMR by more than a factor of 2; see also section 4.1. The reduction of TMR is also observed for overoxidation, although now the quenching of polarization will be induced by the presence of antiferromagnetic oxides at the barrier interfaces (Moodera and Mathon, 1999). This leads to additional spin flip conductance channels, by which the spin polarization (and TMR) will be suppressed. Using the diagnostic techniques mentioned above, it is established by several groups that an asymmetry in the barrier potential, easily detected by fitting IV -data with the Simmons or Brinkman formula (Eqs. (12) and (13), respectively), is accompanied by a low TMR. Correspondingly, the maximum TMR is found when the junction has a minimal asymmetry, see the work of Sun et al. (1999), Covington et al. (2000), andoepts et al. (2001). Both over- and under-oxidation cause this asymmetry by creating different electrode-barrier interfaces. An asymmetry in the barrier height is directly observed by Koller et al. (2003) in photoconductance measurements (see section 2.1.1). Due to the large sensitivity of the technique to the presence of Al close to the tunnel barrier, the disappearance of a negative contribution to the photocurrent is correlated to the complete oxidation of the barrier layer and the corresponding maximum in TMR. In order to increase the TMR by preventing such an asymmetry, some modifications to the simple oxidation process are often applied. In several cases, this is accomplished by creating a reservoir of oxygen at the interface with the bottomelectrode. In a later step, this reservoir is supposed to fill the Al-Al 2 O 3 from the bottom up in order to create a more homogeneous barrier. A first method in which such a reservoir is used is to first slightly oxidize the surface of the bottom electrode before deposition of Al. The subsequent deposition of Al will cause an unstable situation since generally the Gibbs free energy of ferromagnetic oxides is larger than that of Al 2 O 3 (Dean, 1992). Since Al 2 O 3 has a lower energy, the oxygen will naturally move into the aluminum. This is applied by Sun et al. (1999) and Kuiper et al. (2001b). In the latter case, the authors find that, when the Co bottom electrode is partially oxidized, up to 10 Å of Al can be completely oxidized by oxygen from this reservoir. A second method involves a small amount of intentional over-oxidation. A gradient of Al or O now exists in the barrier layer. After the top electrode and capping layers are deposited, the junction is subjected to an annealing step, i.e. the sample is brought to higher temperatures in a non-reactive argon (or high vacuum) atmosphere. This causes the oxygen to leave the bottom electrode and move into the 22 44

45 Spin-Dependent Tunneling in Magnetic Junctions 45 barrier, often resulting in a more homogeneous barrier layer. This procedure was performed by, for instance, Song et al. (2000) and Dimopoulos et al. (2001b). In all cases, the barrier is homogenized and the TMR is increased. From photoconductance experiments on tantalum-oxide MTJs with variable oxidation time of Ta (Koller et al., 2005b), the shift of the maximum TMR with anneal temperature is accompanied by a similar shift of the oxidation time where the asymmetry of the barrier potential is absent. The experiments of Koller et al. (2005b) directly support the idea that for obtaining the highest magnetoresistance ratio one should anneal MTJs that would be characterized as slightly over-oxidized in the as-deposited state. It is suggested that this result can be understood by a homogenization of the oxygen distribution in the barrier, possibly combined with a change of the bottom barrier-electrode interface. Several groups have attempted to create more homogeneous barriers by applying a two-step process, each step being comprised of Al deposition and oxidation. Yoon et al. (2001) applied this technique with plasma oxidation, Das (2003) with natural oxidation, and Zhang et al. (2003b) with a slow ion-beam process. For all methods, it is reported that the TMR increases slightly in junctions with a two-step process with respect to a one-step process. The RA product increases, often by as much as one order of magnitude (see for example Zhang et al., 2003b). In a comparison between single-step and two-step oxidation experiments, Yoon et al. (2001) have found from X-ray photoelectron spectroscopy that indeed the concentration of O in the barrier is more homogeneous than from single-step plasma oxidation. The TMR of the two-step oxidized junctions is higher, although not much Pinholes and dielectric breakdown A pinhole is a path of relatively high conduction between the two electrodes, through the barrier layer. Often this is a metallic short due to inhomogeneous oxidation, or a very thin part of the barrier due to inhomogeneous deposition of the aluminum prior to oxidation. Generally, pinholes can decrease the TMR in two ways. First, for very thin barriers (or for barriers created from a too rough Al) a strong magnetic coupling between the two electrodes may be present in regions where the electrodes are in direct contact. In that case, the free layer no longer fully switches independently from the bottom electrode, and results in a decrease of TMR (Wang et al., 2003; Zhang et al., 2003b). The second, more generally observed, detrimental aspect of pinholes is that the largest part of the current through the junction will run via a normal metallic contact instead of the desired spindependent tunneling across the insulator. This shunting of the tunneling current via metallic shorts has been widely observed and is analyzed by simply modelling a tunnel resistor with an ohmic resistor in parallel (Oliver et al., 2002). Direct visualization of pinholes is usually difficult due to the extremely small dimensions, probably down to a few atomic distances. With the use of a nematic liquid crystal deposited on the sample, pinholes can be directly visualized (Oepts et al., 1998). Upon heating their liquid crystal above the clearing point (56.5 C), the material changes from the nematic state showing optical anisotropy, to the isotropic state. By operating the junction just below the clearing point, the power dissipation due to the pinholes are then visualized as black spots; see Fig. 1.21a. Schad 44

46 46 H.J.M. Swagten 10 Figure 1.21 (a) Polarized light picture of liquid crystal on top of a shadow-evaporated Co/20 22 Å Al 2 O 3 /Co 50 Fe 50 junction. The black spot in the middle of the junction surface is the location of a pinhole at a breakdown site (after Oepts et al. (1998)). In (b) a scanning-electron-microscopy 12 image is made after the electrochemical growth of cauliflower-like Cu islands on an oxidized 12 Å Al layer grown on top of 125 Å NiFeCo. After Schad et al. (2000). et al. (2000) have developed a method for pinhole imaging using electrodeposition of Cu. Selective nucleation at the metallic pinhole locations produces characteristic cauliflower-like structures that can be easily visualized. An example is shown in Fig. 1.21b. An indirect way to detect pinholes is based on the magnetic field generated by the large current density flowing through the pinholes. Due to the vertical direction of the current ( to the plane of the layers), these magnetic fields are in the plane of the free magnetic layer and are thus able to shift or deform the switching behavior from which the location of the pinhole may be extracted (Oliver et al., 2002, 2004). Indirect indications for pinholes are widely reported. E.g., Zhang et al. (2003b) have found evidence for pinholes from resistance and TMR data, combined with the observation of magnetic coupling between the free and fixed magnetic layers. Whereas junctions with a barrier of nominal thickness t Al 6.5 Å are reported to be pinhole-free, thinner nominal Al layers are clearly suffering from the presence of pinholes. Han and Yu (2004) report on junctions in which the aluminum (9 Å) is underoxidized. The as-deposited junctions show a very low resistance (820 µm 2 ) and practically no TMR (0.5%), consistent with transmission-electron-microscopy measurements showing pinholes with a diameter of a few nm. However, after an annealing step, both the junction resistance and the TMR increase enormously to 30 k µm 2 and 43%, respectively, which is related to the reduction of pinholes and an improvement of the interfaces with the barrier. Moon et al. (2002) show that in otherwise identical conditions, the usage of a two-step oxidation process instead of a single-step process increases the TMR as well as the RA product. Together with the observation that the magnetic coupling between the bottom and top electrode is reduced in junctions fabricated by the two-step process, they conclude that twostep oxidation creates tunnel barriers with a much lower pinhole density. Pinholes in magnetic tunnel junctions have also triggered a reconsideration of the criteria formulated by Rowell and others (see, e.g., Brinkman et al., 1970) to discriminate true tunneling conductivity from other metallic-like current paths (Garcia, 2000; Jonsson-Akerman et al., 2000; Rabson et al., 2001; 22 44

47 Spin-Dependent Tunneling in Magnetic Junctions Figure 1.22 Ramped current stress measurements (a) and a constant voltage stress measurement 15 (b) on junctions consisting of 30 Å Ta/30 Å NiFe/100 Å IrMn/100 Å CoFe/10 Å Al + oxidation/40 Å CoFe/60 Å NiFe/50 Å Cu. The arrows indicate the average breakdown voltage V BD and the moment of breakdown t BD. Adapted from Das (2003). Akerman et al., 2001). According to these Rowell-criteria the conductance G (1) should vary exponentially with barrier thickness, (2) is parabolic in bias voltage, (3) scales with the junction area, and (4) displays a weak insulator-like temperature dependence; see also section 2.1. Although these are necessary criteria for tunneling, it is shown by Oliver et al. (2002) that they do not rule out the existence of pinholes, especially for junctions with ultrathin (<10 Å Al) barriers. Since the detection of pinholes is generally not a straightforward experiment and may not be inferred from the presence of all tunneling criteria, it is suggested that the examination of the insulator breakdown mechanisms will reveal the true nature of the barrier quality including the presence of pinholes (Oliver et al., 2002). Breakdown of tunneling barriers is crucial in the assessment of the lifetime of MTJ-based devices and has been studied by electrically stressing the system until the oxide breaks. Experimentally, this is achieved by, for instance, ramping the bias voltage between the electrodes. At the breakdown voltage, highly conductive paths (pinholes) are created that shunt the remaining tunnel resistance, thereby quenching the TMR (Oepts et al., 1998, 1999; Shimazawa et al., 2000; Rao et al., 2001; Schmalhorst et al., 2001; Oliver and Nowak, 2004). Several mechanisms have been proposed to explain breakdown of the oxide layer, and these are related to the presence or generation of traps or defects in the barrier that percolate at the point of breakdown. From these voltage-ramp experiments the so-called E-model for dielectrical breakdown in MTJs is successfully confirmed (see, e.g., Oepts et al., 1999), in which it is assumed that traps are generated when the electric field breaks the dipoles in the oxide. More recently, constant voltage (or constant current) tests for MTJs are performed, where the current (voltage) changes are monitored during time (Das et al., 2003; Nakajima et al., 2003). A typical experiment of breakdown via current ramping or at constant voltage is shown in Fig It is found by Das et al. (2003) that 22 44

48 48 H.J.M. Swagten breakdown (or pre-breakdown) in UV-light assisted and plasma-oxidized barriers intrinsically occurs due to electric field-induced generation of single traps in the oxide, similar to the breakdown mechanism in SiO 2. At the position of the trap, the tunnel barrier potential is locally distorted, leading to dramatic changes in the local conductivity. Extrinsically, breakdown is also strongly related to the quality of the barrier in terms of post-deposition processing defects, e.g., at the perimeter of the junction (Nakajima et al., 2003), or via imperfections induced already during deposition. Especially when the barrier becomes very thin, the inevitable growth-related pinholes may grow due to strong Joule heating at the pinhole area upon electrically stressing the barrier, eventually leading to a junction breakdown as reported by Oliver et al. (2002). In these naturally oxidized junctions a clear distinction can be observed between extrinsic breakdown due to the presence of pinholes and more robust junctions that exhibit intrinsic breakdown. The latter show the highest TMR rather independent of RA product, in contrast to pinhole-richer samples with lower TMR and a strong dependence on R A. This strongly suggests that the tendency for lower TMR and RA in naturally oxidized junctions (see Fig. 1.17) maybere- lated to the presence of pinholes with, generally, a much higher density than for plasma-oxidized junctions Thermal stability In Fig. 1.13, shown earlier in this section, it is demonstrated that TMR in magnetic junctions can be enhanced by a thermal treatment of the system. This generally applies to junctions annealed at temperatures up to around C, see, e.g., Sato et al. (1998); Sousa et al. (1998); Parkin et al. (1999b); Cardoso et al. (2000b). The enhancement may be attributed to an improvement of the active ferromagneticbarrier-ferromagnetic region of the junction due to structural changes or diffusion processes, for instance due to homogenization of the oxygen in the as-deposited alumina (see section 2.3.2). Also oxygen at the interface with the bottom electrode, due to a slight over-oxidation, may be released by a thermal treatment which increases the tunneling spin polarization and TMR. The impact of annealing on the barrier properties has been directly determined from photoconductance measurements (Koller et al., 2004), suggesting that highest TMR can be obtained by annealing MTJs that are slightly over-oxidized in the as-deposited state. Apart from the barrier-related thermal effects, also structural and magnetic changes in the frequently used antiferromagnetic layers are believed to play an important role (Cardoso et al., 2000b; Koller et al., 2004), which is reflected in the thermal stability of the exchange bias field, similar as reported for exchange-biased GMR systems (Coehoorn, 2003). Indeed, when not using exchange biasing in junctions with artificial antiferromagnets or with only two layers with different coercivities, the rise of TMR with annealing temperature is only modest (Parkin et al., 1999a; Schmalhorst et al., 2000b). Contrary to the rise of TMR at relatively low anneal temperatures, annealing above C leads to a severe degradation of the tunnel magnetoresistance (Parkin et al., 1999b; Cardoso et al., 2000b, 2000c). In Fig. 1.23a an example of the TMR collapse is shown for a CoFe-Al 2 O 3 -CoFe-IrMn junction. Since incorpora

49 Spin-Dependent Tunneling in Magnetic Junctions Figure 1.23 (a) TMR measured at T = 300 K as a function of post-deposition annealing temperature for Ni 80 Fe 20 /Co 82 Fe 18 /15 Å Al+oxidation/Co 82 Fe 18 /Ir 26 Mn 74 (Cardoso et al., c) annealed in a vacuum of 10 6 mbar. In (b) the open symbols show tunneling spin polarization for Al/Al 2 O 3 /Co (CoFe)/FeMn superconducting junctions annealed in a vacuum at a base pressure of <10 9 mbar. Closed symbols are identical junctions but now without the top FeMn layer. Adapted from Kant et al. (2004b) and Paluskar et al. (2005a). 19 tion of MTJs into existing semiconductor technology requires thermal stability up to at least 400 C (especially in view of the development of MRAM), the suppression of TMR in that regime has attracted enormous attention. The current belief is that one of the main reasons for the collapse of TMR is related to the diffusion of Mn out of the antiferromagnetic layer (such as FeMn, IrMn, PtMn). The impact of this is twofold. First, the strength of the exchange biasing may be reduced and the adjacent ferromagnetic layer may suffer from a reduction of the magnetization or at least an effect on the switching behavior (Cardoso et al., 2000b). Secondly, it has been demonstrated that the Mn diffuses over considerable distances as determined by Rutherford back-scattering (Cardoso et al., 2001). When reaching the ferromagnetic-barrier interface, it can obviously deteriorate the tunneling spin polarization. To prevent the Mn from diffusing, anti-diffusion barriers have been implemented in particular between the barrier and the antiferromagnetic layer. However, no conclusive picture emerges from these experiments. As an example, Ta barriers do indeed stop the Mn from diffusing towards the barrier (Cardoso et al., 2000a) although it does not avoid the TMR to collapse. Annealing studies with barriers of variable thickness (t Al between 7 Å and 15 Å) demonstrate that only changes at the CoFe-Al 2 O 3 interfaces (e.g. the roughness) or in the barrier itself can explain the observed TMR degradation (Cardoso et al., 2001). When exposing the bottom electrode to a nitrogen plasma prior to the deposition of the Al layer, Shim et al. (2003) observe a reduction of the Mn diffusion along with a better TMR at an anneal temperature of 270 C. Oxidic CoFeTaO x diffusion barriers introduced by Fukumoto et al. (2004) shift the TMR collapse towards higher anneal temperatures as well, although they simultaneously use thin alumina diffusion layers to separate thecofefromthenifeintheco 90 Fe 10 -Ni 81 Fe 19 top electrode. This suggests 22 44

50 50 H.J.M. Swagten that also Ni migration (in this case in the top electrode) may be relevant for the TMR collapse. By a rapid thermal anneal process of only 10 seconds as compared to conventional anneals of hours, Lee et al. (2002) have shown that TMR has become thermally more robust probably due to an abrupt change of the oxide barrier parameters. The important role of structural changes in the barrier is particularly relevant for ultrathin barriers (Cardoso et al., 2001). In that regime, TMR is most sensitive to changes in the interfacial region at the bottom of the barrier as determined from Rutherford backscattering and atomic-force microscopy, and leads to a reduced thermal robustness of TMR. To directly study the possible degradation of the Al 2 O 3 barrier or its interfaces in relation to TMR, Kant et al. (2004b) have measured the tunneling spin polarization as a function of anneal temperature by superconducting tunneling spectroscopy (which will be explained in more detail in section 3). As mentioned in the introduction, the tunneling spin polarization P is directly responsible for the magnitude of the magnetoresistance effect via the simple equation TMR = 2P /(1 P 2 ).Itis shown that annealing of Al-Al 2 O 3 -Co junctions does not affect the tunneling polarization up to anneals at T = 500 C, demonstrating the intrinsic thermal stability of the barriers and its interfaces. Also when FeMn is additionally grown on top of the Co layer, the polarization is still not degrading with anneal temperature, despite the fact that Mn strongly diffuses above T = 300 C as independently shown by X-ray photoelectron spectroscopy (Paluskar et al., 2005a). It is suggested by the authors that this is in qualitative agreement with the work of Kim and Moodera (2002), who report that Mn concentrations as high as x = 30% in Al-Al 2 O 3 -Co 1 x Mn x junctions have only a weak negative effect on the tunneling spin polarization (section 3) Alternative barriers for MTJs As we will show in the following sections 3and4, there is an increasing number of papers devoted to alternative barriers for magnetic tunnel junctions, leading to intriguing phenomena such as extremely large magnetoresistance ratios and unusual bias voltage dependencies. In the early years of tunnel magnetoresistance, the search for alternative barriers has been inspired, among other aspects, by the possibility to tune the performance of these devices by the insulator band gap and therefore the potential energy of the barrier. Not only is this directly affecting the RA product of a magnetic junction (Eq. (14)), also within free-electron models (see, e.g., Slonczewski, 1989) the barrier height is a critical parameter that determines the magnitude and even the sign of TMR; see also the work of Tezuka and Miyazaki (1998). Also within more recent models the insulator, and in particular the interfaces and bonding with the ferromagnetic electrodes, are critically important for the spin-dependent tunneling processes, as extensively discussed in the following sections. Another aspect of considering alternative barriers is related to the issue of overoxidation (section 2.3.2). To prevent degradation of the underlying ferromagnetic electrode by overoxidation, the metal atoms of the barrier materials should behave sufficiently electronegative and react preferentially with the supplied oxygen. It appears from experimental studies that, apart from Al 2 O 3, a number of 44

51 Spin-Dependent Tunneling in Magnetic Junctions 51 candidates are available in this respect. However, as we will see below, the results for alternative amorphous (or partially polycrystalline) barriers are not dramatically different from the observations when regular Al 2 O 3 is used, although for tuning tunnel device properties in specific applications these studies may be extremely valuable. This is in striking contrast to the use of crystalline or epitaxial barriers, leading to crucial modifications of the tunneling properties (section 4). One of the first attempts to use alternatives for alumina as a barrier has been reported by Platt et al. (1996, 1997), andsmith et al. (1998) using a number of reactively sputtered oxides. In case of oxidized Hf, Mg, and Ta, a sizable magnetoresistance has been observed, although only when the junctions are cooled down to liquid-nitrogen temperature. The lack of TMR at ambient conditions is attributed to the vacuum break necessary to change their shadow masks. Hafnium is used by Wang et al. (2002) in ultrathin Hf/Al bilayers that are naturally oxidized in pure O 2. Magnetoresistances of more than 10% are reported at room temperature together with low RA products. The presence of unoxidized Hf (2.5%) close to the bottom electrode as determined by X-ray photoelectron spectroscopy, has improved the continuity and conformality of these amorphous barriers. As discussed later on in section 3.3.3, in the work of Sharma et al. (1999) the use of composite Ta 2 O 5 -Al 2 O 3 barriers may change the sign of TMR, an effect that strongly depends on the bias voltage. More recently, a positive 2.5% TMR at room temperature is reported by Rottlander et al. (2001) for junctions with plasma-oxidized Ta 2 O 5 barriers. Up to 10% room-temperature magnetoresistance has been found in exchange-biased Ta 2 O 5 junctions, not further improving after post-deposition annealing up to almost 300 C, see Gillies et al. (2001). Oxidation by oxygen release during the anneal of a partially oxidized Co electrode also does not improve this figure, although the RA product is somewhat higher than for the regular plasma-oxidized barriers. Direct information on the properties of the Taoxidized barrier can be obtained via photoconductance measurements as shown by Koller et al. (2004), which is facilitated by the low band gap of Ta 2 O 5 ( 4.2 ev) as compared to alumina. Due to optical electron-hole pair generation in the barrier itself and subsequent transport in the electric field, the sign and magnitude of the barrier asymmetry can be determined quite accurately. Moreover, the oxidation time where the asymmetry becomes zero is found to coincide with a maximum in the magnetoresistance ratio. This is argued to be due to the complete oxidation of the barrier material, resulting in a symmetric tunnel barrier. In a follow-up study, Koller et al. (2005a) have shown that TMR strongly depends on the thickness of the Ta 2 O 5 layer, possibly due to structural modifications in the barrier or at the interfaces. Barriers of AlN y and AlO x N y have been produced by Sharma et al. (2000) yielding up to 18% magnetoresistance at room temperature when using a mixture of O 2 and N 2 for plasma oxidation (nitridation). This magnitude is similar to those produced with the regular Al 2 O 3 barriers although with a lower RA product. When using pure N 2, however, TMR is significantly lower, never exceeding 16% (see also Shang et al., 2001). Wang et al. (2001a) have combined nitrogen-containing barriers (AlN) and ferromagnetic electrodes (Fe 93.8 Ta 2.4 N 3.8 ) to minimize possible tunneling spin polarization losses during post-deposition anneal of their structures

52 52 H.J.M. Swagten After annealing at 225 C, magnetoresistances of 17% are reported, degrading after anneals above 250 C. From Rutherford backscattering it is found that a significant amount of O 2 (< 10%) is incorporated in the barrier, which is generally a major concern in preparing barriers by nitridation due to the extremely high reactivity of oxygen. For comparable junctions using CoFe alloys a TMR of more than 30% is observed after annealing (Wang et al., 2001a). Schwickert et al. (2001) have used AlN and AlO x N y barriers, as well as AlN-Al that is naturally oxidized. Although typically more than 10% of magnetoresistance is obtained, the use of ultrathin Al 2 O 3 is still superior in terms of TMR and RA product. Boron nitride (BN) barriers are used by Lukaszew et al. (1999) on top of an epitaxial structure consisting of Si(100)- Cu(100)-Co(100). When combined with a polycrystalline Co or Ni layer on top of the BN barrier, room-temperature TMR of up to 25% has been observed. Wang et al. (2001b) have fabricated junctions with crystalline ZrO x barriers by plasma oxidation of thin Zr layers (typically 5 Å). The as-deposited barriers appear to consist of both ZrO and ZrO 2 phases. After annealing, interfacial oxygen incorporated partially at the bottom CoFe electrode is released into the barrier, resulting in a considerable increase of TMR, up to a magnitude of around 20%. Upon natural oxidation of Zr-Al bilayers, the barrier becomes amorphous and smoother than for crystalline ZrO x or pure amorphous Al 2 O 3, with TMR ratios that are still exceeding 15% after annealing. In another approach, the addition of impurities in an Al 2 O 3 barrier is thought to create another microstructure within the insulator and at its interfaces with the electrodes. Lee et al. (2003) find that the addition of Zr to the Al, prior to oxidation, severely affects the structural properties of the barrier. At a 9.9 at.% Zr-alloyed Al-oxide barrier, a very smooth amorphous alloy phase is established with excellent TMR of almost 40%. Yttrium oxide (YO x )barriers obtained from plasma oxidation of an Y film are also reported to be well-defined, smooth, and amorphous, giving rise to around 25% TMR at room temperature (Dimopoulos et al., 2003). As mentioned earlier, also in section 4 alternative barriers for magnetic tunnel junctions will be treated. In that case the tunnel barriers are no longer structurally amorphous (or at most polycrystalline), but are designed to become crystalline and epitaxially matched to the electrode(s), e.g. by employing molecular beam epitaxy or pulsed laser deposition Tunneling Spin Polarization As we have seen in the introduction (section 1), the degree of spin polarization is the key ingredient for the magnetoresistance effect in magnetic junctions. Generally, however, in literature the physical property of spin polarization is defined in several different ways. To start with, spin polarization is sometimes related directly to magnetization of ferromagnetic metals, i.e. the difference between the number of spin-up and spin-down electrons. In transport experiments, it is clear that another definition needs to be used, and electrons at the Fermi level are ruling the spin polarization. However, our first-order definition 44

53 Spin-Dependent Tunneling in Magnetic Junctions 53 P =[N maj (E F ) N min (E F )]/[N maj (E F ) + N min (E F )], seeeq.(10), isstillfarfrom realistic. We are dealing with 3d ferromagnetic materials having both heavy d electrons as well as light s electrons at the Fermi level. This seriously complicates our view on spin polarization, since generally d states in these metals are dominating the magnitude of the density-of-states, whereas the mobile s states are responsible for electrical transport. Moreover, one should carefully define spin polarization in relation to the actual geometry and electrical properties of the device. As an example, in studying giant magnetoresistance (GMR) in all-metallic multilayers it is a useful concept to consider the spin polarization of current in the bulk of the magnetic layers. It is believed that Andreev reflection spectroscopy on a (nano)contact between a ferromagnetic metal and a superconductor is a valuable technique to directly measure this current spin polarization of bulk ferromagnets (Soulen et al., 1998; Upadhyay et al., 1998; Mazin, 1999; Nadgorny et al., 2000; Strijkers et al., 2001; Kant et al., 2003). In a tunneling experiment, obviously a completely different regime of current polarization is considered: tunneling spin polarization is the polarization in electrical current when electrons are tunneling from a ferromagnetic metal through a nonmagnetic barrier layer. As shown by Mazin (1999), thisdrasti- cally changes the physics behind the polarization of electrical current and should not be confused with the polarization obtained from Andreev reflection spectroscopy or from other techniques, such as photo-emission studies (see, e.g., Sicot et al., 2003). In this section a further introduction will be presented on the wide range of complexities in understanding the underlying physics of tunneling spin polarization and its intimate relation to TMR. This will be preceded by the experimental procedure to determine the polarization via transport in magnetic-superconducting junctions at low temperatures, so-called superconducting tunneling spectroscopy or STS. Also an overview will be presented of existing data on tunneling spin polarization and the relation with the physics involved How to measure spin polarization? The tunneling spin polarization as introduced in Eq. (10) can be measured straightforwardly by superconducting tunneling spectroscopy. For an extended review of this technique, see Meservey and Tedrow (1994), or the thesis work of Worledge (2000), Kaiser (2004), andkant (2005). In STS one uses a superconducting electrode as a detector for the tunneling spin polarization in the following way. The tunneling current in junctions at a finite, low bias voltage is in first order governed by the density-of-states factors and the tunneling probability, see Eqs. (4) and (5). In the case of one superconducting electrode this reads: G N(E F )T (φ, t)ρ(ev), with N(E F ) the metal density-of-states at the Fermi level, ρ(ev) the superconducting density-of-states at an energy ev,andwithv the bias voltage between superconductor and magnetic metal. A measurement of the conductance is therefore directly reflecting the density-of-states of a superconductor, having sharp peaks at an energy ±, the energy difference between the energy level of the Cooper pairs and the single-electron states (see Fig. 1.24a). Only at voltages exceeding 44 (17)

54 54 H.J.M. Swagten 21 Figure 1.24 Calculated conductance of a superconductor-metal tunnel junction as sketched in the bottom right panel. In part (a) the zero-field conductance is shown both at 0 K (thin line) and at T = 0.1T c (rounded curve). In panel (b) (d) the spin-up and spin-down density-of-states in the superconductor are Zeeman split by a magnetic field B(μ B B = 0.6 ) at T = 0.1T c. 24 The polarization of the tunneling electrons is zero in (b), +40% in (c), and 80% in (d). The thin dashed and solid line in graph (b) represent the conductance due to the individual spin-up 26 and spin-down density-of-states, respectively, both at 0 K. 26 ± /e the electrons can tunnel into the empty single-electron states of the superconductor or vice versa, from the superconductor towards the metal. It is crucial to note that the magnitude of is typically around 1 mev, whereas the densityof-states of a metal shows variations on an energy scale of ev s. As a result, the conductance in Eq. (17) exclusively reflects the peak-shaped density-of-states of the superconductor. At finite temperatures, thermal broadening of the Fermi level reduces the sharpness of the peaks as can be seen in Fig. 1.24a. The density-of-states of a metal N(E) as observed in the conductance measurement is the sum of the density of spin-up states and the density of spin-down states. In absence of a magnetic field, no energy is required to flip the spin of an electron, and, accordingly, the contributions to the conductance of the spin-up and spin-down density-of-states coincide. This situation changes when a magnetic field is applied parallel to the plane of the tunnel junction. The magnetic field penetrates the superconductor uniformly since the thickness of the superconducting electrode is much smaller than the penetration depth of the magnetic field. In presence of the field, the spin-up electrons (assumed to have their moment parallel to the field) are lowered in energy with respect to spin-down electrons, corresponding to an energy difference of 2μ B B,whereμ B is the electron magnetic moment, and B the mag- 44

55 Spin-Dependent Tunneling in Magnetic Junctions 55 netic induction. Consequently, the magnetic field shifts the density of spin-up states of the superconductor to lower energy and the density of spin-down states to higher energy. Figure 1.24b shows how these energy shifts lead to four maxima in the conductance. The maxima are clearly resolved when the Zeeman splitting 2μ B B is large as compared to k B T, defining the sharpness of the maxima. The maximum applicable field is limited by the critical field B c of the superconducting electrode. Critical fields larger than 4 T can be obtained with aluminum superconducting electrodes, and, typically, fields of 2 to 3 T are applied. With μ B 58 µev/t and k B 86 µev/k one finds that a temperature below 1 K is required to clearly resolve the Zeeman splitting. The two conductance maxima at low bias, those numbered 1 and 2 in Fig. 1.24b, give a direct indication of the spin polarization of the tunneling electrons. Since at the position of maximum 1 the density of spin-up states is zero, maximum 1 is a direct measure of the spin-down conductance. Likewise, maximum 2 is a direct measure of the spin-up conductance. In the example of Fig. 1.24b the spin-up and spin-down conductance are equal, i.e., the tunneling spin polarization is zero. When dealing with ferromagnetic metals with a positive tunneling spin polarization, there are more majority (spin-up) electrons available for tunneling than minority (spindown) electrons, by which maximum 2 becomes larger than 1 as shown by the calculated curve in Fig. 1.24c. In this particular example, the polarization is 40% and positive, since we assumed that tunneling is dominated by majority electrons. To a good approximation, the tunneling spin polarization P is given by the relative difference between the height of maxima 2 and 1, 22 P G 2 G 1, (18) G 2 + G 1 as indicated in the figure. A most accurate extraction of the polarization is obtained by fitting a model to the measured conductance curve, which will be discussed later on. To finish the introduction of the spin-polarized tunneling technique, we consider the case of negative polarization as shown by the calculation in Fig. 1.24d. Here maximum 1 is larger than 2, which means that the tunnel current is dominated by minority electrons. The superconductor used in the tunnel junctions for spin-polarized tunneling measurements is usually aluminum. There are two main reasons responsible for this important role of aluminium. First, aluminum is a superconductor with a low atomic number. Clear observation of the Zeeman split spin-up and spin-down superconducting density-of-states is possible only when the spin orbit scattering rate in the superconductor is low and thus requires a low atomic number. Consequently, other common superconductors such as niobium, lead and tantalum are not suitable and only a few superconductors other than aluminum can be used (Meservey and Tedrow, 1994). The second reason for the almost exclusive role of aluminum in spin-polarized tunneling is the defect and pinhole-free amorphous Al 2 O 3 tunnel barrier obtained by exposing metallic aluminum to oxygen; see section 2.2. Consequently, most published work is based on Al-Al 2 O 3 -metallic tunnel junctions obtained by oxidation of the top part of an aluminum electrode followed by deposition of the normal metal on top of the Al 2 O 3 barrier. However, also inverted 44

56 56 H.J.M. Swagten structures of metallic-al 2 O 3 -Al have been successfully used in STS experiments (see, e.g., Kaiser and Parkin, 2004), showing that the tunneling spin polarization can be different as compared to the system with Al underneath the oxide. In the inverted structures, especially reactive Ni-containing bottom electrodes (Ni 1 x Fe x ) are susceptible to oxidation when Al is plasma oxidized prior to deposition of the top (superconducting) Al layer. These oxidized Ni-Al interfaces then lead to a significant reduction of spin polarization. Zeeman splitting in the conductance of Al(superconducting)-Al 2 O 3 -metallic tunnel junctions is observed for the first time by Tedrow and Meservey (1971b) in the early 70 s (see also Meservey and Tedrow, 1994). Soon thereafter values of the tunneling polarization in junctions with various magnetic top electrodes are obtained. These early polarizations are determined simply from the differences in the conductance maxima using a procedure similar as given in Eq. (18). This procedure, however, leads to a small but significant underestimation of the polarization since it does not take into account the effect of orbital depairing and a finite spin orbit scattering rate on the spin-up and spin-down superconducting densityof-states (Monsma and Parkin, 2000a; Worledge and Geballe, 2000a). Depairing in a superconductor is caused by the presence of a magnetic field. The field can originate from different sources, but, most relevant for our purpose, it is the external applied field that induces an orbital motion of the electrons by the Lorentz force breaking up the Cooper pairs (Tinkham, 1996). In experiments, a small out-ofplane external magnetic field can seriously broaden the conductance curves shown in Fig It is estimated that typically the field should be aligned with the plane of the superconducting layer better than 0.05 to avoid too much broadening to accurately extract spin polarization (Kant, 2005). As mentioned before, to reduce the effect of spin orbit coupling Al is almost exclusively used in STS due to its low atomic mass, which minimizes an effective mixing between the spin-up and spin-down channels. However, also in the case of Al, spin orbit interaction should be taken into account in analyzing conductance curves to extract tunneling spin polarization unambiguously (Worledge and Geballe, 2000a). In Fig an example is given of a measurement of the tunneling spin polarization using STS in a system of Al-Al 2 O 3 -Co and Al-Al 2 O 3 -Co 90 Fe 10, yielding in both cases a positive spin polarization (note the similarity with the conductances shown in Fig. 1.24c). The theoretical, solid curves in Fig are based on the socalled Maki-theory that takes into account the required corrections for spin orbit interaction and orbital depairing in the superconductor; see Maki (1964), Merservey et al. (1980), andworledge and Geballe (2000a) Data on tunneling spin polarization An overview of available data on tunneling spin polarization is listed in Table 1.2 and Table 1.3. Generally, the 3d metallic ferromagnets have a positive polarization between +30% and +55% when tunneling across amorphous Al 2 O 3 is considered. The polarization does not scale with the magnetic moment μ μ of Ni, Co,andFe,being0.62μ B, 1.75μ B,and2.22μ B per atom, respectively. Although this is not surprising, even using the first-order definition of tunneling spin po- 44

57 Spin-Dependent Tunneling in Magnetic Junctions Figure 1.25 Conductance G as a function of bias voltage V of a UHV-sputtered junction consisting of (a) Al/Al 2 O 3 /Co and (b) Al/Al 2 O 3 /Co 90 Fe 10, taken at T = 0.3 K in zero field 20 and in an external field of several Tesla (as indicated). The total thickness of superconducting Al electrode and Al 2 O 3 barrier is typically smaller than 50 Å. The top electrodes are 200 Å in 22 thickness and capped with 60 Å Ta. The solid lines are theoretical fits using the Maki-theory 23 (Worledge and Geballe, 2000a) yielding the indicated tunneling spin polarization. Reproduced from Kant (2005). 24 larization involving the density-of-states at the Fermi level (Eq. (10)), there has been some debate on this issue in literature, in particular in the beginnings of spinpolarized tunneling experiments; see Meservey and Tedrow (1994), and also Tezuka and Miyazaki (1996). The absence of correlation between magnetic moment and tunneling spin polarization also applies to alloys between Ni, Fe and Co, again in contrast to the early experiments (Paraskevopoulos et al., 1977)). The data on Ni 1 x Fe x alloys as shown in Table 1.3 have been used by van de Veerdonk et al. (1997a) to verify that magnetic moment μ μ is not linearly related to tunneling spin polarization P ; seefig Also in the case of spin polarization obtained by Andreev reflection spectroscopy using metal-superconducting contacts, data on Ni 1 x Fe x are almost independent of x, again demonstrating the absence of correlation with magnetic moment or magnetization. A similar conclusion is reached by Kim and Moodera (2002) and Kaiser et al. (2005b) in their STS studies of Co 1 x Mn x and Co 1 x Pt x binary alloys, respectively. In both cases, the spin polarization is almost insensitive to the Mn (Pt) concentration up to x 0.40 (see Table 1.3), whereas the magnetization in that regime is linearly suppressed with increasing Mn (Pt) content. Surprisingly, however, the Co 1 x V x systems studied by Kaiser et al. (2005b) exhibit an approximately linear relationship between tunneling spin polarization and magnetization for x roughly below

58 58 H.J.M. Swagten 1 Table 1.2 Tunneling spin polarization (P ) values obtained with the STS technique for a number 1 2 of elementary ferromagnetic materials, as well as for some crystalline ferromagnetic electrodes 2 or barriers indicated by an asterisk ( ). Data with a dagger ( ) are not corrected for depairing and spin orbit coupling. The Al 4 2 O 3 indicated with the double dagger ( ) hasavariablethickness, between 6.4 Å and 16 Å (Munzenberg and Moodera, 2004). Polarization data obtained by 4 Monsma and Parkin (2000a) and Panchula et al. (2003) are for junctions where the Al sputter target is slightly doped with Si System P (%) Reference Al/Al 2 O 3 /Ni Moodera and Mathon (1999), Kim and Moodera (2004), Monsma and Parkin (2000a) Ni /Al 2 O 3 /Al 25 ± 2 Kim and Moodera (2004) Al/Al 2 O 3 /Co 40 ± 2 Moodera and Mathon (1999), Monsma and Parkin (2000a), Kant et al. (2004c) Al/Al 2 O 3 /Fe 42 ± 2 Moodera and Mathon (1999), Monsma and Parkin (2000a), Kant et al. (2004c) Al/Al 2 O 3 /Fe Munzenberg and Moodera (2004) Al/Al 2 O 3 /Gd 13 ± 4 Merservey et al. (1980) Al/Al 2 O 3 /Gd 13 Kaiser et al. (2005a) Al/Al 2 O 3 /Tb 5 ± 2 Merservey et al. (1980) Al/Al 2 O 3 /Dy 6 ± 2 Merservey et al. (1980) Al/Al 2 O 3 /Ho 6 ± 2 Merservey et al. (1980) Al/Al 2 O 3 /Er 5 ± 2 Merservey et al. (1980) Al/Al 2 O 3 /Tm 3 ± 2 Merservey et al. (1980) 22 NiMnSb /Al 2 O 3 /Al 28 ± 2 Tanaka et al. (1999) Al/Al 2 O 3 /Mn 45 Sb Panchula et al. (2003) La 0.67 Sr 0.33 MnO 3 /SrTiO 3 /Al 72 ± 1 Worledge and Geballe (2000b) SrRuO 3 /SrTiO 3 /Al 10 ± 1 Worledge and Geballe (2000c) Co/SrTiO 3 /Al 31 Thomas et al. (2005) CrO 2 /Cr 2O 3 /Al 100 Parker et al. (2002) Al/MgO/Co 30 ± 2 Kant et al. (2004c) Al/MgO /Co 39 Kaiser et al. (2005a) Al/MgO/Fe 30 ± 2 Kant et al. (2004c) Fe /MgO /Al Parkin et al. (2004) Co 70 Fe 30 /MgO /Al Parkin et al. (2004) An intriguing STS experiment addressing the relation between magnetization and tunneling polarization is performed with ferrimagnetic alloys between Co and Gd (Kaiser et al., 2005a); see Table 1.3. At the compensation point of the alloy, the magnetization is absent due to an equal sublattice contribution. Nevertheless, the tunneling spin polarization at this point can still be very large and can even 44

59 Spin-Dependent Tunneling in Magnetic Junctions 59 3 Table 1.3 Tunneling spin polarization (P ) values obtained with the STS technique for a number of alloys. Data with a dagger ( ) are not corrected for depairing and spin orbit coupling. 3 Crystallinity of MgO barriers is indicated by an asterisk ( ). Barriers in junctions by Monsma and Parkin (2000a), Kaiser et al. (2005a),andKaiser et al. (2005b) are made with a Si-doped Al sputter target. For data on CoV and CoPt alloys employing AlN as a barrier, we refer to Kaiser et 7 al. (2005b) 7 System P (%) Reference Al/Al 2 O 3 /Co 90 Fe 10 48±1 Paluskar et al. (2005a) Al/Al 2 O 3 /Co 84 Fe 16 50±2 Moodera and Mathon (1999), Monsma and Parkin (2000a) Al/Al 2 O 3 /Co 60 Fe Monsma and Parkin (2000a) Al/Al 2 O 3 /Co 50 Fe 50 50±1 Moodera and Mathon (1999), Monsma and Parkin (2000a) Al/Al 2 O 3 /Co 40 Fe Monsma and Parkin (2000a) Al/Al 2 O 3 /Co 72 Fe 20 B Paluskar et al. (2005b) Al/Al 2 O 3 /Ni 4 Fe van de Veerdonk et al. (1997a) Al/Al 2 O 3 /Ni 12 Fe van de Veerdonk et al. (1997a) Al/Al 2 O 3 /Ni 17 Fe van de Veerdonk et al. (1997a) Al/Al 2 O 3 /Ni 25 Fe van de Veerdonk et al. (1997a) Al/Al 2 O 3 /Ni 30 Fe van de Veerdonk et al. (1997a) Al/Al 2 O 3 /Ni 40 Fe Monsma and Parkin (2000a) Al/Al 2 O 3 /Ni 47 Fe van de Veerdonk et al. (1997a) Al/Al 2 O 3 /Ni 60 Fe Monsma and Parkin (2000a) Al/Al 2 O 3 /Ni 74 Fe van de Veerdonk et al. (1997a) Al/Al 2 O 3 /Ni 78 Fe van de Veerdonk et al. (1997a) Al/Al 2 O 3 /Ni 81 Fe Monsma and Parkin (2000a) Al/Al 2 O 3 /Ni 86 Fe van de Veerdonk et al. (1997a) Al/Al 2 O 3 /Ni 90 Fe Monsma and Parkin (2000a) Al/Al 2 O 3 /Ni 95 Fe 5 34 Monsma and Parkin (2000a) 22 Al/Al 2 O 3 /Co 90 Mn Kim and Moodera (2002) Al/Al 2 O 3 /Co 73 Mn Kim and Moodera (2002) Al/Al 2 O 3 /Co 68 Mn Kim and Moodera (2002) Al/Al 2 O 3 /Co 30 Mn 70 9 Kim and Moodera (2002) Al/Al 2 O 3 /Co 95 V 5 30 Kaiser et al. (2005b) Al/Al 2 O 3 /Co 89 V Kaiser et al. (2005b) Al/Al 2 O 3 /Co 87 V Kaiser et al. (2005b) Al/Al 2 O 3 /Co 83 V Kaiser et al. (2005b) Al/Al 2 O 3 /Co 77 V 23 8 Kaiser et al. (2005b) Al/Al 2 O 3 /Co 74 V 2 Kaiser et al. (2005b) (continued on next page) 44

60 60 H.J.M. Swagten 1 Table 1.3 (Continued) 1 System P (%) Reference Al/Al 2 O 3 /Co 90 Pt Kaiser et al. (2005b) Al/Al 2 O 3 /Co 81 Pt Kaiser et al. (2005b) Al/Al 2 O 3 /Co 65 Pt Kaiser et al. (2005b) Al/Al 2 O 3 /Co 60 Pt Kaiser et al. (2005b) Al/Al 2 O 3 /Co 44 Pt Kaiser et al. (2005b) Al/Al 2 O 3 /Co 34 Pt Kaiser et al. (2005b) Al/Al 2 O 3 /Co 26 Pt Kaiser et al. (2005b) Al/Al 2 O 3 /Co 95 Gd 5 26 Kaiser et al. (2005a) Al/Al 2 O 3 /Co 93 Gd 7 31 Kaiser et al. (2005a) Al/Al 2 O 3 /Co 79 Gd 21 12, 8, 0, +9 Kaiser et al. (2005a) Al/Al 2 O 3 /Co 70 Gd Kaiser et al. (2005a) Al/Al 2 O 3 /Co 62 Gd Kaiser et al. (2005a) Al/Al 2 O 3 /Co 50 Gd Kaiser et al. (2005a) Al/Al 2 O 3 /Co 39 Gd Kaiser et al. (2005a) Al/Al 2 O 3 /Co 31 Gd Kaiser et al. (2005a) Al/Al 2 O 3 /Co 24 Gd 76 2 Kaiser et al. (2005a) Co 77 Gd 23 /MgO /Al 28, +23 Kaiser et al. (2005a) Co 68 Gd 32 /MgO /Al 28 Kaiser et al. (2005a) Co 40 Gd 60 /MgO /Al 22 Kaiser et al. (2005a) Al/MgO /Co 77 Gd Kaiser et al. (2005a) Al/MgO /Co 68 Gd Kaiser et al. (2005a) Figure 1.26 (a) Tunneling spin polarization P measured by STS at T = 0.4 K and (b) low-temperature saturation magnetization μ SAT of Ni 1 x Fe x compounds. NiFe is either evaporated from a tungsten boat (open symbols) or from an e-gun (solid). The results for Ni samples 42 prepared under ultra-high vacuum conditions (squares) are taken from Moodera and Mathon 44 (1999), andkim and Moodera (2004). Note in (a) the lower polarization of Ni 25 Fe 75,probably due to a structural transformation in this range of composition. The solid curve in (b) is taken from Bozorth (1993). Adapted from van de Veerdonk et al. (1997a). 46

61 Spin-Dependent Tunneling in Magnetic Junctions 61 change sign (see section for a more detailed discussion). As a final remark to the ongoing debate on the linearity between tunneling spin polarization and magnetization, Hindmarch et al. (2005a) have used Co-Al 2 O 3 -Cu 38 Ni 62 junctions using CuNi as an electrode with a low Curie temperature of around 240 K. Due to this, the temperature-dependent magnetization of CuNi can be directly related to the tunneling spin polarization of CuNi as a function of temperature. The latter is extracted from TMR(T ) = 2P Co (T )P CuNi (T )/[1 P Co (T )P CuNi (T )] with P Co (T ) separately determined from Co-Al 2 O 3 -Co junctions. Although the relation between P and M is found to be strictly nonlinear, it can be reproduced by a theoretical model that incorporates tunneling via multiple s and d (hybridized) bands. This will be further addressed in section 3.3. At present, there is no complete theoretical picture enabling realistic predictions of tunneling spin polarization for the 3d metals as listed in Table 1.2. Nevertheless, there are a number of arguments that lead to a qualitative understanding of sign, and to some extent, the magnitude of P. This will be further discussed in the next section (3.3), where the ingredients for tunneling spin polarization will be discussed in some detail. Also shown in Table 1.2 are the 3f and 4f ferromagnetic materials, showing generally a rather low polarization. This, combined with the low Curie temperature of these elements, explains that only a few papers have addressed these materials for implementation in magnetic junctions. Data have also been gathered to test the half-metallicity of some materials (such as La 0.67 Sr 0.33 MnO 3, NiMnSb or CrO 2 ) in tunneling experiments, which will be the topic of section 4.4. Onlyin the case of CrO 2, a true 100% tunneling spin polarization is found. Related to this, the table shows the tunneling spin polarization when using crystalline ferromagnetic materials, in some cases combined with a crystalline insulating barrier as well. Surprisingly, in SrRuO 3 -SrTiO 3 -Al epitaxial junctions a negative spin polarization is measured, only rarely reported in STS experiments. Epitaxial junctions will be treated extensively in section 4, which includes a detailed discussion on the use of MgO as a barrier material. As can be seen from Table 1.2, the crystallinity of MgO is crucial for obtaining a very high tunneling spin polarization, in some cases up to +85%. It is instructive at this point to recall the intimate relation between tunneling spin polarization and magnetoresistance as suggested by Eq. (9), which for equal magnetic electrodes reads: TMR = 2P 2 /(1 P 2 ).InFig a compilation is shown of TMR in a ferromagnet-insulator-ferromagnet junction with equal electrodes as a function of the tunneling spin polarization measured with STS; see Table 1.2 and Table 1.3. Generally, it is observed that the Julliere formula (solid curve) is reasonably well describing the data (see, however, also the earlier discussions in Miyazaki and Tezuka, 1995b and Lu et al., 1998). This seems to justify to use of Julliere equation, despite a number of theoretical contributions addressing its validity (for example, see MacLaren et al., 1997; Qi et al., 1998; Tsymbal and Pettifor, 1998; Mathon and Umerski, 1999; Belashchenko et al., 2004). However, it should be emphasized that the number of data in Fig is rather limited and is in some cases obtained by a correction of TMR for the electrodes being unequal. Furthermore, it obviously passes over the rich physics behind spin polarization and TMR, 22 44

62 62 H.J.M. Swagten 16 Figure 1.27 TMR measured in magnetic junctions with equal electrodes as a function of the tunneling spin polarization determined by STS. Some of the TMR data are based on MTJs with different electrodes, and have been corrected accordingly. See Table 1.2 and Table 1.3 for references on tunneling spin polarization. 19 and suggests that the Julliere formula should be used only as a phenomenological equation that links these physical quantities. It is important to mention that some of the tunneling spin polarization data collected in Table 1.2 and Table 1.3 have been subject to significant changes over the past decades. This can be partially explained by the progress in deposition and surface characterization tools, in particular after the realization of high room-temperature TMR in the mid-nineties. As an example, the polarization of Ni has increased from 5% to 33% over the years, as reported in a number of review papers (Meservey and Tedrow, 1994; Moodera et al., 1999a; Moodera and Mathon, 1999). As we will show below, the tunneling spin polarization is quite sensitive to the ferromagnet-insulator interface conditions, which can obviously vary from lab to lab, and may significantly improve over the years. Recently, an even higher value for P Ni of 46% has been reported, now obtained by employing cleaner interfaces between polycrystalline Ni and amorphous alumina using ultra-high vacuum conditions (Kim and Moodera, 2004). The extreme sensitivity of the polarization for tiny structural changes close to the barrier is clearly demonstrated by Monsma and Parkin (2000b). They observe a suppression of tunneling spin polarization of Ni when measured over a large number of weeks, from 28% directly after deposition to 16% after almost a year, which is the result of a slowly evolving chemical reaction between Ni and the alumina barrier Ingredients of tunneling spin polarization As discussed before, it is crucial to have a solid interpretation of tunneling spin polarization measured in an STS experiment. In this section we will introduce the ingredients for tunneling spin polarization not covered by the elementary formula given by Eq. (10). This includes the following aspects: 44

63 Spin-Dependent Tunneling in Magnetic Junctions 63 the dominant role of the ferromagnet-insulator interfaces, instead of bulk properties of the ferromagnetic electrodes variations in the mobility or Fermi velocity of tunneling electrons (sp- or d-like), and dissimilarity in tunneling transmission coefficients, being dependent on the character of the electron wave functions relevance of the tunneling barrier due to the chemical bonding in the interface region between ferromagnet and insulator Density-of-states at the barrier interfaces The spin polarization of electrical current measured in metal-superconductor junctions is generally defined as the relative difference in the spin-up and spin-down current or conductance: P = G G G + G. When using Eq. (17) with an equal transmission across the barrier for both spinup and spin-down electrons, the tunneling spin polarization reads [N maj (E F ) N min (E F )]/[N maj (E F ) + N min (E F )]. This is identical to what is derived in section 1, Eq.(10). It suggests that the tunneling spin polarization is determined by the Fermi electrons of the bulk ferromagnetic material which is in striking contrast with existing experimental observations. In fact, a crucial point in understanding tunneling polarization and TMR is that it is not governed by the bulk density-ofstates but rather by the local density-of-states at the interfaces with the barrier. Quite generally, the tunneling process samples the density-of-states only over a few Fermi wavelengths, in particular in the strongly perturbed region at the metal-insulator interface. Due to the strong screening of the Fermi sea in a metal, bulk metal electrons are simply not aware of the interface with the barrier until they are approaching it at a few monolayers. This has been theoretically recognized already in the early days of electron tunneling in superconducting as well as in normal metal junctions (see, e.g., Appelbaum and Brinkman, 1969, 1970; Mezei and Zawadowski, 1971). Along with experimental data on the relevance of interfaces (see below and also in section 4), a number of calculations have appeared to provide a more solid theoretical basis for the interfacial sensitivity of spin-polarized tunneling (Tsymbal and Pettifor, 1997; de Boer et al., 1998; Itoh et al. 1999a, 1999b; Zhang and Levy, 1999; Vedyayev et al., 1999; Uiberacker et al., 2001; Uiberacker and Levy, 2001). The crucial role of the interfaces with the barrier for tunneling spin polarization is experimentally first seen by Tedrow and Meservey (1975) in Al-Al 2 O 3 -Al junctions when a thin layer of Co is inserted at the interface between the top nonsuperconducting Al and the Al 2 O 3 ;seefig. 1.28a. In this STS measurement it is observed that only one or two monolayers of ferromagnetic metal at the interface with the alumina barrier induces a finite spin polarization, saturating to the bulk value at 3 to 5 monolayers only. Also in a more recent experiment a similar interfacial effect is observed, although now in a reversed way (Moodera et al., 1989). The strong tunneling spin polarization in the Al-Al 2 O 3 -Fe system is efficiently suppressed by inserting an ultrathin nonmagnetic layer at the barrier-ferromagnetic (19)

64 64 H.J.M. Swagten 15 Figure 1.28 (a) Tunneling spin polarization P versus thickness t of the ferromagnetic Co layer in a junction consisting of 40 Å Al (superconducting)/15 Å Al 2 O 3 /t Co/50 Å Al (normal). (b) P versus thickness of a nonmagnetic Au interlayer in 40 Å Al + oxidation/t Au/ Å Fe. After Tedrow and Meservey (1975) and Moodera et al. (1989). 18 interface, as shown in the right panel of Fig This proofs the interface sensitivity of (spin-dependent) electron tunneling, and suggests that we should consider the density-of-states at the Fermi level in the tunneling spin polarization as a local interfacial density-of-states rather than a bulk density-of-states. Later on (section 4), we will extensively come back to the role of interfaces and interfacial density-of-states for TMR Weighted the density-of-states factors Apart from the role of the interfaces, there is another ingredient for better understanding the tunneling spin polarization, which is obvious when considering the positive sign of the polarization for traditional ferromagnetic metals such as Fe, Co, and Ni, as we have compiled in Table 1.2. For example, in the case of Co and Ni the dominance of minority electrons at the Fermi level, see Fig. 1.6 in section 1, would result in a negative spin polarization, whereas in the table it is shown that experimentally they have a positive sign, i.e. tunneling is most efficient for majority electrons. The observation that not just the (interfacial) density-of-states is decisive for polarization can be explained by reconsidering the definition of Eq. (10): P = [N maj (E F ) N min (E F )]/[N maj (E F )+N min (E F )]. It is theoretically analyzed by Mazin (1999) that the density-of-states factors N min,maj (E F ) should be weighted with spindependent factors reflecting the possibility that electrons of different symmetry and Fermi velocity are coupled differently to the states in the barrier. This yields an alternative, more physically justified expression of tunneling spin polarization: 44 P = w majn maj (E F ) w min N min (E F ) w maj N maj (E F ) + w min N min (E F ), (20)

65 Spin-Dependent Tunneling in Magnetic Junctions 65 with w min(maj) the spin-resolved weighting coefficients, and, following the earlier discussion in this subsection, with N(E F ) now referring to density-of-states at the interfaces. The weighting factors are suggested to depend on (i) the Fermi velocity of the electrons travelling perpendicular to the tunneling barrier, and (ii) on transmission coefficients for electron tunneling. As an example, the latter dependence on transmission matrix elements can be understood from our earlier expression of conductance in STS experiments, see Eq. (17). When inserting this in the definition of tunneling spin polarization in Eq. (18), it is found that P =[T maj N maj (E F ) T min N min (E F )]/[T maj N maj (E F ) + T min N min (E F )], i.e. densityof-states factors clearly weighted with transmission coefficients. Mazin (1999) has pointed out that for a simple model of a delta-type tunnel barrier with a large barrier height (V(x) = Wδ(x) with W the barrier height approaching infinity) the weighting factors are proportional to vf 2,x,withv F the Fermi velocity. This again stresses the relevance of the orbital character of the tunneling electrons, not only the static density-of-states of Fermi electrons. It is subsequently pointed out by Butler et al. (2001a) from calculations using the layer Korringa Kohn Rostoker technique (MacLaren et al., 1990) that also the electron wave functions parallel to the plane of the layers are critically important for spin-dependent tunneling processes. Matching of the wave functions at the interfaces is influenced by the lateral variations of wave functions of different symmetry, and can dramatically change their decay rate in the barrier region, by which s electrons seem to tunnel more readily than d-like electrons. Note that this is contrary to the free-electron models, where the decay rate for a given k and energy E is uniquely given by exp( 2κt) with κ =[2m(U(x) E)/ h 2 + k 2 ]1/2 ; see section 1. In view of the relevance of barrier transmission probabilities and Fermi velocities of tunneling electrons, it is instructive to have a closer look at the band structure of the 3d ferromagnetic metals commonly used in MTJs. Due to the multi-orbital character of the band structure, both s- and d-type electrons are active around at the Fermi level and may therefore contribute to a (tunneling) current. Especially the dispersive s-like electrons are believed to have the largest tunneling probability due to their small effective mass, see Eq. (1), whereas d electron wave functions are more localized and more rapidly decay in the barrier region. In the pioneering theoretical work of Stearns (1977), it is pointed out that this is consistent with a positive spin polarization of Co and Ni in contrast to the (negative) polarization of the full density-of-states. The sign change between the tunneling polarization from s- and d-states is related to the hybridization between these bands as shown more recently by more sophisticated calculations; see, e.g., Butler et al. (2001a), Mathon (1997), and Mazin (1999). As an example, in the tight-binding calculations of Mathon (1997), the current polarization between two Co electrodes changes from negative to positive when the tight-binding hopping integral is gradually turned off, simulating the transition from metallic GMR-type conduction to tunneling across an insulating interlayer. An interesting consequence of these ideas is that it may be possible to select s or d tunneling states by varying the thickness of the insulator as pointed out in these calculations. When the barrier is sufficiently thick, the itinerant s-like electrons dominate the current. However, the d electrons take over for sufficiently thin 22 44

66 66 H.J.M. Swagten barriers, which could turn the spin polarization from positive to negative. The predicted sign change has not been observed experimentally. Nevertheless, a significant decrease of the positive tunneling spin polarization of Fe is seen when barriers are thinned down to below 10 ÅinAl-Al 2 O 3 -Fe superconducting junctions (Munzenberg and Moodera, 2004). Although these findings are attributed to the increasing role of d-like electron states, it should be mentioned that the data for the thinnest barriers are not very well described by the Maki-theory, possibly related to the extremely small junction resistance in this regime, in some cases only 1 or less. Also it is important to realize that a similar trend of decreasing polarization is observed in the magnetoresistance of MTJs (Freitas et al., 2000; Oliver and Nowak, 2004); see also Fig In these data, the suppression of TMR at lower barrier thickness is related to, e.g., an incomplete coverage of the ultrathin Al leading to more pinholes in extremely thin barriers. In the STS data of Munzenberg and Moodera (2004) this is, however, ruled out by the absence of leakage current around zero bias voltage. In section 3.2, it is pointed out that a number of STS studies show that there is no simple proportionality between tunneling spin polarization and magnetization of the ferromagnetic electrode (see also Fig. 1.26). Moreover, in Co 1 x Mn x (Kim and Moodera, 2002) andco 1 x Pt x (Kaiser et al., 2005b) the spin polarization remains almost constant up to x = 0.40, in contrast to the magnetization that is linearly suppressed. Kaiser et al. (2005b) conjecture that this can be explained by assuming that in Co 1 x Pt x the tunneling rate from Pt atomic sites is much lower than from the highly polarized Co sites, an argument that the authors derive from scanning tunneling microscopy studies (Hofer et al., 2003). In fact, it is expected that the strong bonding of Co to the oxygen at the barrier interface as compared to Pt leads to the enhanced tunneling transmission from Co sites. In a follow-up study, additional evidence for this site-dependent tunneling transmission is found from data on Co 1 x Gd x ferrimagnetic alloys of heavy rare earth and 3d transition metals (Kaiser et al., 2005a). As shown in Fig. 1.29a, the tunneling spin polarization is positive for low x due to the dominant tunneling probability from Co sites, combined with the fact that the Co sublattice is aligned with the field direction in this regime (see Fig. 1.29b). However, at the compensation composition of the ferrimagnetic alloy, i.e. at x 0.20, the polarization abruptly changes sign, since now the Co sublattice magnetization is antiparallel to the field due to the much larger magnetic moment of Gd as compared to Co. Upon further increase of x, the positive contribution from the Gd polarization leads to a sign reversal of P from negative to positive at x 0.75, finally saturating at P =+13% for pure Gd. Using a phenomenological model that simply weights the tunneling spin polarization associated with Co and Gd sites (see the curves in Fig. 1.29a), a tunneling probability is found that is around 20% higher from Co sites than from Gd Bonding across the metal-insulator interface So far, the role of the insulator for tunneling spin polarization is not considered. However, due to the prominent role of the interfacial density-of-states as well as the electron waves of different symmetry decaying in the barrier, it is conceivable that the insulator and in particular the metal-interface region is also crucial for tun- 44

67 Spin-Dependent Tunneling in Magnetic Junctions Figure 1.29 (a) Tunneling spin polarization P of Co 1 x Gd x versus the Gd concentration x measured by STS at T 0.3 KacrossAl 2 O 3 barriers (open symbols) and MgO (solid symbols). The curves represent a model calculation that weights the contribution to P from the available Co and Gd sites. (b) Saturation magnetization M SAT measured with a superconducting-quantum-interference-device (SQUID) magnetometer at T = 10 K of Å Ta/1000 Å Co 18 1 x Gd x /100 Å Ta (open symbols), together with data taken from Hansen 18 et al. (1989) (solid curve). The arrows indicate the alignment of the Co and Gd subsystems in 19 the magnetic field H at the compensation point (vertical line in grey at x 0.20), as well as for concentrations below and above this point. Adapted from Kaiser et al. (2005a) neling spin polarization and TMR. Experimentally, first evidence has been found by a sign reversal of the tunneling spin polarization when replacing Al 2 O 3 for a composite Ta 2 O 5 -Al 2 O 3 barrier combined with Ni 80 Fe 20 electrodes (Sharma et al., 1999). The use of composite Al 2 O 3 -Ta 2 O 5 barriers is inspired by the Julliere formula, TMR = 2P L P R /(1 P L P R ).AfullTa 2 O 5 or Al 2 O 3 barrier will lead to positive TMR due to the product of P L and P R, irrespective of the sign of the individual tunneling spin polarization. Only when the polarization at the left and right side of the barrier are of opposite sign, a negative TMR will be measured. The observed negative TMR for Al 2 O 3 -Ta 2 O 5 is tentatively attributed to the difference between s and d dominated electron tunneling across either a Al 2 O 3 or Ta 2 O 5 barrier when keeping the ferromagnetic electrodes equal. Moreover, the polarization strongly depends on the applied bias voltage in the composite Al 2 O 3 -Ta 2 O 5 junctions,aswellasinsingle-barrierta 2 O 5 junctions, hinting to the relevance of the more pronounced d-like density-of-states as compared to the s-electrons (Sharma et al., 1999). A point of concern regarding these data is raised by Montaigne et al. (2001). By calculations within a free-electron model incorporating a composite barrier they also observe a strong and asymmetric variation of TMR with applied bias voltage; see also similar results by Li et al. (2004). Moreover, it should be noted that the observed sign reversal in these experiments of Sharma et al. (1999) is only indirectly inferred from TMR in full magnetic junctions. Unfortunately, a direct determination of P (including its sign) when tunneling across Ta 2 O 5 is still lacking due to quenching of the required Zeeman splitting by a large spin orbit scattering in Ta-oxide based superconducting junctions (Kant et al., 2004a). 44

68 68 H.J.M. Swagten 13 Figure 1.30 Calculated density-of-states (DOS) averaged over the first two layers of a Co(001) surface as a function of energy. Both the total and s-electron partial density-of-states are 14 plotted for majority electrons (a) and minority electrons (b), showing the opposite sign of the polarization of s- and d-states at the Fermi level. For clarity, the occupied s-based states are shaded and multiplied with a factor of 10. After Tsymbal and Pettifor (1997). After these initial experiments from Sharma et al. (1999), a number of new experiments have been launched using epitaxial oxides grown mostly with pulsed laser deposition (e.g. SrTiO 3 ), which will be reviewed in section 4. All together, strong evidence is found for a critical role of the barrier and its interfaces with the ferromagnetic layers. Putting it differently, by choosing appropriate barriers in spintunneling experiments, one is able to probe wave functions of different symmetry related to the ferromagnetic electrodes. This makes spin tunneling a unique technique for studying specific features of the complex band structure of ferromagnetic thin films and interface regions. From a theoretical point of view, the role of the density-of-states and chemical bonding at the ferromagnet-insulator interface region has been widely addressed. The chemical bonding at the ferromagnet-insulator interface determines the effectiveness of transmission at the interface, which, for electrons of different character, may be markedly different. By combining a tight-binding approximation with ballistic quantum-mechanical transport calculations, Tsymbal and Pettifor (1997) have found that the conductance in a magnetic tunnel junction strongly depends on the type of covalent bonding between the ferromagnet and the insulator, as represented by ss, sp and dd hopping integrals. Figure 1.30 shows the calculated density-ofstates of Co surface layers showing the dominance of minority d-like electrons at the Fermi level, just like the situation for bulk Co (Fig. 1.6). However, for electrons with s (and also p) character the situation is reversed due to s(p)-d hybridization; now the majority electrons are outnumbering the minority electrons at the Fermi level. This has important consequences for the conductance between a ferromagnetic electrode and a non-magnetic metal represented by an s band, when tunneling across an insulator having s-bands separating the energy gap; see Fig Incase of sd bonding at the interface between ferromagnet and insulator, there is a large transmission of the d electrons across the MTJ. Due to the negative d-electron density-of-states, this consequently leads to a negative polarization of the current

69 Spin-Dependent Tunneling in Magnetic Junctions Figure 1.31 Calculated spin-polarized tunneling conductance between a ferromagnetic metal (using the density-of-states as shown in Fig. 1.30) and a nonmagnetic s-like metal across s-like 14 tight-binding bands separated by an energy gap. The conductances are plotted for majority electrons (a) and minority electrons (b) when only ss bonding between ferromagnet and insulator is taken into account (labelled as ss only ), as well as for the case of ss, sp, and sd bonding 17 ( full ). The polarization of the tunneling current due to the s states is positive (+34%) in contrast to that of the full conductance ( 11%). AfterTsymbal and Pettifor (1997). 18 In contrast, when interfacial ss bonding is dominant, only s states of the ferromagnetic film are coupled with those of the insulator. These s states have a strongly reduced minority density-of-states at the Fermi level, which then leads to a positive tunneling spin polarization, in the case of Co with a magnitude of +34%. This is perfectly in line with the experimental observations for alumina barriers (see Table 1.2). To address the interface bonding in relation to the oxidic character of the insulating barriers usually employed, a number of calculations have been reported to address this in detail. However, an accurate prediction of spin polarization (and TMR) for amorphous Al 2 O 3 barriers in MTJs is still lacking due to the enormous theoretical complexities involved, and therefore, in all cases, the barrier is assumed to be atomically ordered. Ab-initio calculations by de Boer et al. (1998) have shown that the spin polarization at crystalline Co-HfO 2 interfaces changes sign with respect to the bulk of Co. Tsymbal et al. (2000) show that by covering a Fe(001) surface with one oxygen overlayer, the spin polarization can be inverted with respect to that of the clean Fe surface. Due to hybridization of the iron 3d levels with the O 2p orbitals and the strong exchange splitting of the antibonding oxygen states, a positive spin polarization in the density-of-states of the oxygen atoms is found at the Fermi level, from there on propagating into the vacuum barrier. In the same spirit, the tunneling spin polarization of ferromagnetic-al 2 O 3 interfaces turns out to be positive when imperfectly oxidized Al (or off-stoichiometric O ions) are assumed to be present at the interface with the amorphous barrier; see Itoh and Inoue (2001). Furthermore, this is only weakly dependent on the choice of ferromagnetic electrodes (viz. bcc Fe, fcc Co, or fcc Ni). Oleinik et al. (2000) have studied the bonding at O- and Al-terminated interfaces between fcc Co(111) and crystalline α-al 2 O 3 with [0001] orientation to better understand the atomic and electronic structure of an alumina-based system 22 44

70 70 H.J.M. Swagten 13 Figure 1.32 (a) Local density-of-states (DOS) of [0001] oriented α-al 2 O 3 at the Fermi energy for majority (closed symbols) and minority electrons (open symbols), as a function of the distance from the interface with ferromagnetic Co. In (b) spin polarization of the density-of-states is plotted using Eq. (10) for the data in (a), showing a sign reversal to positive spin polarization 16 at a distance of 10 Å. After Oleinik et al. (2000). from first principles. As shown in Fig. 1.32a, the alumina density-of-states at the Fermi level decays exponentially with distance from the Co interface, the average decay length being larger for the majority electrons than for the minorities. Close to the interface with Co the spin polarization on the Al and O atoms (using the definition given by Eq. (10) for the local density-of-states) is negative reflecting the negative spin polarization of the density-of-states of Co. Most interestingly, at the interior atoms within the alumina barrier, more specifically beyond 10 Å, the spin polarization becomes positive in line with the STS experiments as discussed earlier (see Table 1.2). The crucial role of interface bonding is further corroborated by calculating the effect of oxidizing a clean Co(111) surface on the tunneling spin polarization (Belashchenko et al., 2004). For sufficiently thick barriers, the transmission function can be factorized into a product of surface transmission functions and a decay factor for the barrier, from which the tunneling current can be determined in a system of Co-vacuum-Al. It is demonstrated that one monolayer of oxygen bonded to the Co(111) surface changes the spin polarization from negative for a barrier of 20 Å to almost +100% due to the creation of an additional tunneling barrier in the minority spin channel. The relation between oxygen adsorption and tunneling spin polarization is further explored by a first-principles Green s function technique applied to crystalline (111)-oriented Co-Al 2 O 3 -Co junctions where O atoms are located at the interface region (Belashchenko et al., 2005a; Tsymbal and Belashchenko, 2005). When the three interface O atoms are bonded to two adjacent Al atoms the spin polarization is found to be negative. In line with the earlier predictions (Belashchenko et al., 2004), a very strong Co O bonding of O positioned inside the large pores at the fcc cobalt interfaces leads to a remarkable enhancement of the tunneling current in the majority channel, thus reversing the tunneling spin polarization. In actual experiments, the positive P found for tunneling across Al 2 O 3 may be fully ruled by the details of interfacial adsorption of oxygen. This is supported by X-ray ab

71 Spin-Dependent Tunneling in Magnetic Junctions 71 sorption spectroscopy and X-ray magnetic circular dichroism experiments (Telling et al., 2004), showing that the polarization at the Co-Al 2 O 3 interfaces increases when the bonding changes from Co Al to Co O. Also the STS experiments of Munzenberg and Moodera (2004) on Al/Al 2 O 3 /Fe junctions as mentioned already in section could be qualitatively interpreted in the light of the interface bonding model of Belashchenko et al. (2005a), since the positive spin polarization for the case of strong Co O bonding is shown to increase with the thickness of the barrier. The above results all show that tunneling spin polarization is a very complex and delicate parameter in MTJs given the variety of ingredients discussed in this section. Nevertheless, the role of the electronic structure and chemistry of the interfacial regions is probably most critical for the sign and magnitude of P, and simple rules of thumb given by s- and d-dominated tunneling as raised earlier may not be entirely justified for amorphous Al 2 O 3 -based junctions. In this respect, experiments using crystalline barriers are much more promising for revealing the true mechanisms behind spin polarization due to the advantage of a more realistic theoretical treatment. In section 4, in particular the use of crystalline barriers such as SrTiO 3 and MgO will be introduced in relation to theoretical analyses. In the case of MgO barriers, the tunnel magnetoresistance appears to be highly sensitive to the symmetry of the propagating states in the electrodes in relation to the way they couple to the evanescent states in the barrier layer. This is in fact another important ingredient for tunneling spin polarization for which strong experimental evidence is currently available, e.g. in epitaxial Fe-MgO-Fe MTJs (section 4) Crucial Experiments on Spin-Dependent Tunneling This section deals with a number of key experiments in the area of magnetic tunnel junctions. Although the field of magnetic tunneling is not settled and is still further developing, contributions are selected that are believed to be critically important for better understanding the physics behind magnetoresistance effects in MTJs. Through the direct relation between tunneling spin polarization and TMR, experiments are to a great extent focused on similar aspects as introduced in the previous section 3. In this section the following themes will be discriminated: the application of thin nonmagnetic layers in MTJs to address the relevance of the ferromagnet-barrier interface region quantum-well observations in TMR by incorporating ultrathin layers for spindependent confinement of tunneling electrons role of the ferromagnetic electrodes, including the use of half-metallic ferromagnets to achieve extremely large TMR role of the tunneling barrier using alternative insulators, including coherent tunneling across crystalline MgO barriers. 44

72 72 H.J.M. Swagten 4.1 The relevance of interfaces: using nonmagnetic dusting layers In the previous sections, we have emphasized that the magnetoresistance effect in magnetic tunnel junctions can be phenomenologically explained by a simple Julliere formula, viz. TMR = 2P L P R /(1 P L P R ), illustrating the leading role of the tunneling spin polarization of the (left and right) electrodes. However, it was argued before that this tunneling spin polarization should be considered with great care (section 3). It is not just related to the electronic properties of the electrodes alone, but sensitively depends on the combined system of (magnetic) electrode and barrier material. In an elegant type of experiment addressing the delicate properties of spin polarization, thin nonmagnetic so-called dusting layers are inserted at the interface between the magnetic electrode and the insulating barrier. When the Julliere formula would be used in a naive manner, the zero polarization of the nonmagnetic interlayer (see Eq. (20) with density-of-states factors at the interfaces) would immediately lead to a vanishing TMR. Considering the subtle role of the interfaces for TMR, however, it is conceivable that this may result in a rich spectrum of interesting physics by engineering structures with nonmagnetic elements incorporated in an MTJ. In one of the earliest experiments in this field, Moodera et al. (1989) have directly measured the spin polarization in Al-Al 2 O 3 -Au-Fe using superconducting tunneling spectroscopy (see section 3, Fig. 1.28), finding that the polarization rapidly decreases with the thickness of the Au layer. Nevertheless, at larger Au thickness the polarization still persists and decreases roughly as 1/t Au.In contrast to this observation, later experiments showed an oscillation of the TMR in Co-Au-Al 2 O 3 -NiFe with increasing Au thickness, suggesting that the spin polarization could change sign by inserting the nonmagnetic interlayer (Moodera et al., 1999b). To unravel these and other inconsistencies, a number of experiments with dusting layers were performed using standard exchange-biased Co-Al 2 O 3 -Co junctions. Generally, a decaying TMR has been found for all nonmagnetic materials employed so far (Moodera et al., 2000). However, the location of the interlayer, either grown on top of the bottom electrode or grown on top of the Al 2 O 3 barrier, is crucial in the suppression of TMR, which is shown in Fig for data reported by LeClair et al. (2000d) using Cu layers up to a thickness of about 10 Å. In the case of Cu on top of the barrier, the decay length ξ (when assuming a phenomenological exponential decay function TMR exp[ t Cu /ξ]) is roughly 7.0 Å, and is much larger than ξ for the Cu layers on top of the bottom electrode. This is attributed to the non-ideal, cluster-like growth of metal layers on top of the amorphous Al 2 O 3 barrier, as verified by X-ray photoelectron spectroscopy and Auger electron spectroscopy (LeClair et al., 2000d), and by analyzing the voltage dependence of the conductance (LeClair et al., 2000b). This non-ideal growth also explains the rather long decay lengths reported by Sun and Freitas (1999), Parkin (1998), andyamanaka et al. (1999) when nonmagnetic layers are grown on top of the insulating barrier. In the case of the Cu on top the bottom electrode, a nearly layer-by-layer growth has been established (LeClair et al., 2000d), by which the observed length scale ξ = 2.6 Å is now more intrinsically related to the decaying tunneling spin polarization

73 Spin-Dependent Tunneling in Magnetic Junctions Figure 1.33 The effect of nonmagnetic dusting layers at the interface between the ferromagnetic electrode and the barrier. TMR, normalized to the magnetoresistance in Co/Al 2 O 3 /Co, is shown as a function of the thickness of the nonmagnetic layer t NM. The data labelled with Al O 3 /Cu refer to Cu layers grown on top of alumina as indicated at 18 right. The other interlayers, viz. Cu, Cr, and Ru, are grown on top of Co (underneath 19 the Al 2 O 3 ), see again the panel at right. The full junction stack is composed of Si(100)/SiO 2 /50 Å Ta/70 80 Å Co/100 Å FeMn/35 50 Å Co/Al 2 O 3 /150 Å Co, capped with Ta or Al, with dusting layers at one of the interfaces with alumina. From LeClair et al (2000d). 22 In the case of dusting with Cr, an even faster intrinsic decay length has been reported by LeClair et al. (2001b). At room temperature it is measured that ξ = 1.25 Å (see Fig. 1.33) which means that the addition of 1.5 monolayer of Cr reduces TMR to only 10% of a control junction without the spacer layer. As an additional proof of the interface-sensitivity of spin-dependent tunneling, a thin Co layer was subsequently deposited on top of the Cr dusting layer, by which the TMR is recovered almost completely. In the case of Cr dusting, the authors argue that the Co-Cr interfaces induce a strong spin-dependent modification of the interfacial density-of-states which enhances scattering of the majority electrons and thereby strongly reduces TMR (LeClair et al., 2001b). This is further substantiated by an anomalous suppression of the low-temperature conductance at small bias voltage, a so-called zero-bias anomaly, again related to the density-of-states modifications at the Cr-Co interface. Additionally, these zero-bias anomalies are used in multiple dusting experiments (employing dusting with Cr as well as Cu) to identify that Cr in contact with Co is the driving source for the additional scattering. A similarly fast decay of TMR has been reported for dusting with Ru, see again Fig However, in this case the polarization changes sign when t Ru 2 Å, and, after reaching a minimum around t Ru = 3 Å, it gradually decays to zero (LeClair et al., 2001a). It is hypothesized that this is due to the presence of an interfacial Co-Ru alloy as evidenced by nuclear-magnetic-resonance experiments on Co-Ru multilayers by Wieldraaijer (2006). Above a critical alloy composition the s electrons are then believed to have a negative tunneling spin polarization (Itoh et al., 1993; 44

74 74 H.J.M. Swagten Stepanyuk et al., 1994; Rahmouni et al., 1999), thus reversing the sign of TMR. In the following (section 4.2), Ru interfacial layers will be employed in epitaxial junctions, leading to an oscillatory TMR upon variation of the dusting layer thickness. As to the explanation of the extremely fast decay of spin polarization by dusting with non-ferromagnetic elements, Zhang and Levy (1998) find theoretically that for uniform nonmagnetic layers a rather long ( Å) length scale is expected from coherent transmission, but that for nonmagnetic layers with thickness fluctuations, only a few monolayers are required to completely quench the TMR. This is in striking contrast to other calculations (Vedyaev et al., 1997; Zhang et al., 1998; Mathon and Umerski, 1999; Itoh et al., 2003) showing oscillatory behavior of TMR when tunneling is fully coherent with a strict conservation of k upon tunneling through the barrier. From the absence of experimental evidence for oscillatory features in the aforementioned dusting experiments (except for Ru dusting with a negative TMR attributed to a Co-Ru alloy) it is assumed that the assumption of k conservation is not very realistic for these structures, even when the bottom electrode is well-grown in a nearly layer-by-layer fashion. The presence of a small amount of roughness, combined with the use of (poly)crystalline electrodes and amorphous Al 2 O 3,preventsk conservation and quenches TMR. Nevertheless, in the case of dusting with Au, Moodera et al. (1999b) have found first indications for the presence of an oscillation in TMR that are explained in terms of a simple free-electron tunneling model. The presence of only one sign reversal of TMR and the strong resemblance with the Ru data of LeClair et al. (2001a) could, however, point to alternative explanations as well. Shim et al. (2003) have reported an unexpected field-dependence of the magnetoresistance when using a bottom Au dusting layer in an exchange-biased Co-Al 2 O 3 -NiFe system, although no indications for quantum-well formation are detected. In section 4.2 the phenomenon of quantum effects in MTJs will be further discussed. We now return to dusting experiments using Cr as a dusting layer. Although Cr has no macroscopic magnetization, it is known to exhibit a layered antiferromagnetic structure when grown on Fe(001), i.e. the spins in each monolayer are opposite to those in the neighboring layers. When spin-polarized tunneling would be intrinsically related to the interface between electrode and barrier (as suggested by the experiments described earlier in this section), this should result in an oscillating magnetoresistance with the thickness of a well-defined Cr dusting layer. Nagahama et al. (2005) have prepared Fe(001)-(t)Cr(001)-Al 2 O 3 -CoFe by molecular beam epitaxy, with a variable thickness t of the (001)-oriented Cr layer. Indeed, after an initial suppression from +15% to less than +1% in agreement with LeClair et al. (2001b), TMR is rapidly changing sign from 3 to up to more than 30 monolayers of Cr, with an oscillation period of 2 monolayers and an amplitude that slowly decays with t Cr.InFig. 1.34a thisisshownfort = 50 K. At room temperature the effect is still present, only the oscillation amplitude is somewhat smaller; see the inset of the figure. The extreme interface sensitivity can be explained by scattering of s-type tunneling electrons of 1 symmetry at the interfacial Cr layer (see the schematics in Fig. 1.34b) due to the absence of a 1 band at the Fermi level of Cr. In passing, we note that 1 bands are extremely important for coherent tunneling across epitaxial MgO barriers as will be discussed in section 4.6. Although 22 44

75 Spin-Dependent Tunneling in Magnetic Junctions 75 Figure 1.34 (a) TMR ratio at T = 50 KandatT = 300 K (inset) as a function of the thickness t of the Cr(001) interlayer in units of Cr monolayers (ML). The junctions consist of 17 MgO(001)/400 Å Cr(001)/1000 Å Au/300 Å Fe(001)/t Cr(001)/17 Å Al 2 O 3 /200 Å CoFe (b) Schematic illustration of the junction structure showing the antiparallel arrangement of neighboring Cr layers, and the scattering of electrons at the Cr/Al 2 O 3 interface. Adapted from Nagahama et al. (2005). 20 in principle also quantum-well states could be formed in the Cr(001) layer, the 2 monolayer oscillation is found not to depend on bias voltage which excludes this possibility. The slight shift of the oscillation phase with bias is again attributed to band-structure effects in the Cr(001) interlayer (Nagahama et al., 2005) Quantum-well oscillations in MTJs The absence of convincing evidence for electron-confinement effects in an MTJ could be related to the loss of quantum coherence of tunneling electrons when polycrystalline thin films are embedded instead of well-defined single-crystalline entities. Using epitaxial junctions, a crucial experiment addressing the quantum confinement of electrons in dusting layers has been reported by Yuasa et al. (2002). They have produced Co(100)-Cu(100)-Al 2 O 3 -NiFe junctions where the Co electrode and Cu dusting layer are essentially epitaxial, the alumina barrier is amorphous, and the top electrode polycrystalline. In this case also the TMR is quenched considerably already for 1 or 2 monolayers of Cu, as shown in Fig. 1.35a. However, for thicker Cu spacers, the TMR is clearly changing sign several times, up to thicknesses of more than 20 Å. Although single sign reversals of TMR have been observed for Ru and Au dusting layers (LeClair et al., 2001a; Moodera et al., 1999b), this oscillatory behavior with t Cu is a first convincing evidence for resonant tunneling of spin-polarized electrons in the Cu quantum well as schematically shown in Fig. 1.35b. In Fig. 1.36b it is further illustrated that only for minority electrons a Cu quantum well exists due to the difference in potential energy between Cu and minority Co. On the other hand, majority electrons in Co and Cu have a very similar electronic structure and are therefore not subject 44

76 76 H.J.M. Swagten 17 Figure 1.35 (a) TMR at T = 2 KandT = 300 K at low bias voltage (10 mv) as a function of the Cu thickness t Cu in a junction consisting of MgO(001)/buffer/200 Å Co(001)/0 32 Å Cu(001)/12 Å Al 2 O 3 /100 Å Ni 80 Fe 20 /Au-cap. The inset shows the saturation magnetic field 19 H 20 SAT for a 50 Å Co(001)/0 45 Å Cu(001)/50 Å Co(001) structure as a function of t Cu as 20 obtained from room-temperature magneto-optical Kerr-effect measurements. (b) Schematics of quantum-well reflections for minority electrons in the Cu layer, only when propagating along 22 k = 0 as indicated in the Fermi surface of fcc Cu in (c). Due to the confinement in z direction, quantum-well states with scattering vectors q 1 and q 2 can be formed in the [001] direction After Yuasa et al. (2002) Figure 1.36 (a) Period of oscillation from TMR data as a function of bias voltage V (open 42 symbols) together with the theoretical curve obtained from the energy dispersion of the band of Cu along the Ɣ-X axis (Segall, 1962). The solid circle is the period estimated from data 44 on interlayer coupling (see Fig. 1.35a). (b) The sign convention for the bias voltage, showing the possibility to trace the quantum-well energies only at positive bias. See Fig for the actual junction structure. After Yuasa et al. (2002). 46

77 Spin-Dependent Tunneling in Magnetic Junctions Figure 1.37 (a) Room-temperature TMR as a function of the Ru thickness t Ru of UHV-sputtered 12 Å Co 90 Fe 10 /t Ru/11 Å Al + oxidation/30 Å Co 90 Fe 10 grown 15 on MgO(110). The applied bias voltage is 15 mv. In (b) the saturation field H 16 SAT of a 16 trilayer 150 Å Co 90 Fe 10 /t Ru/50 Å Co 90 Fe 10 is shown as measured with a vibrating sample 17 magnetometer at room temperature. After Nozaki et al. (2004). 17 to considerable confinement. The TMR oscillation period of about 11 Å almost perfectly agrees with one of the extremal k-vectors in the [001] direction corresponding to electrons tunneling with k = 0 (see Fig. 1.35c), showing that these junctions are close to an ideal magnetic junction where electrons can be injected into the Cu quantum well only when k = 0. Moreover,thesameperiodofoscillation is found by the authors when measuring the interlayer coupling fields in Co(001)-Cu(001)-Co(001) structures, as shown in the inset of Fig. 1.35a. Indeed, in explaining magnetic interlayer coupling an identical interpretation of resonant spin-dependent reflection and transmission of electron waves is used to describe oscillatory exchange fields (see Bruno, 1995 and Bürgler et al., 1999). In contrast to the interlayer studies, tunneling offers the unique opportunity to scan the energy dependence of the resonantly tunneling electrons by applying a variable bias voltage across the barrier as we discussed already in section InFig. 1.36a itshownby Yuasa et al. (2002) that the oscillation period is considerably enlarged for positive bias, corresponding to electrons tunneling into the Cu quantum well formed by the minority electrons. Only in that case a variable bias will probe energy dispersion along the [001] direction thereby changing the period of oscillation as indicated in the calculation of Fig. 1.36a. In the same spirit, Ru interlayers have been grown on epitaxial CoFe bottom electrodes by Nozaki et al. (2004). The partially occupied d band at the Fermi level of Ru is almost equal to the hcp Co minority band, by which only majority electrons are confined to the Ru quantum well (contrary to the aforementioned Cu quantum well). In structures of Co 90 Fe 10 -(t)ru-al 2 O 3 -Co 90 Fe 10 the bottom CoFe layer has a hcp(1010) orientation due to growth on MgO(110). The TMR of the dusted MTJs is displayed in Fig. 1.37a. Although a similar negative TMR has been observed also by LeClair et al. (2001a) as discussed in section 4.1, athigher Ru thickness the magnetoresistance again changes sign, and an oscillation seems to 22 44

78 78 H.J.M. Swagten persist up to t Ru 20 Å. Moreover, these oscillations are nicely correlated with the oscillatory interlayer coupling across Ru, as measured separately by the saturation field of antiferromagnetically coupled trilayers of Co 90 Fe 10 -(t)ru-co 90 Fe 10, see Fig. 1.37b. The TMR ratio of the Ru system changes considerably upon the application of bias voltages. For thick enough Ru interlayers, roughly beyond 10 Å, the asymmetry in the bias dependence becomes very strong, leading to sign changes of TMR for both positive and negative bias voltage. This is in contrast to the observations by Yuasa et al. (2002) with Cu dusting, showing TMR modulations only for the bias direction corresponding to an electron flow from the ferromagnetic electrode into the quantum well. Nozaki et al. (2004) have qualitatively explained this by the contribution of a series of discrete energy levels for the majority electrons confined in the Ru spacer, and a continuous energy spectrum for the minorities. 4.3 Role of the ferromagnetic electrode for TMR Although a number of experiments have clearly pointed out the relevance of interfaces for spin-polarized tunneling, they do not necessarily rule out a (spindependent tunneling) contribution from ferromagnetic material located just behind the interfacial region. In Fig it is observed that a certain thickness of the ferromagnetic layer is required to saturate the tunneling spin polarization in a Al-Al 2 O 3 -(t)co-al superconducting junction. Partially this can be explained by a development of the full ferromagnetic moment which is obviously linked to the spin polarization. To avoid these ambiguities, Zhu et al. (2002) have growna wedgeshaped Co 50 Fe 50 layer inserted between the alumina barrier and a bottom Ni 81 Fe 19 reference layer to eliminate finite-size effects in the magnetization. As shown in Fig. 1.38a, TMR is developing rather slowly with a characteristic length scale of 8 Å (from a fit to the data), demonstrating that apart from interface contributions also a few deeper layers may be relevant. Magnetization measurements (see Fig. 1.38b) show that with increasing CoFe thickness the slope of the moment per area is similar when a CoFe wedge is grown on top of an underlying CoFe layer or on top of a NiFe layer. This confirms that the magnetic moment of the thin CoFe layer is not seriously quenched by strong interface intermixing. In sections 3 and 4.1 it is illustrated that tunneling spin polarization in a ferromagnetic-insulator-ferromagnetic junction is not just a static density-of-states parameter of the ferromagnetic electrode, but depends on the interaction of the mobile electrons with the barrier wave functions and in particular the electronic modifications at the ferromagnetic-barrier interface region. However, in terms of the definition of tunneling spin polarization in Eq. (20), the weighting factors w min(maj) explicitly depend on the Fermi velocities v F, in limiting cases even in a quadratic way (section 3.3.2). It is therefore expected that modifications in the (bulk) ferromagnetic electrodes would affect the spin-dependent tunneling properties, assuming that the interface region should, although certainly altered, resemble the bulk electronic properties. Moreover, from the experiments of Zhu et al. (2002) it is suggested that the interface region is extending into the bulk, at least for a few monolayers away from the interfaces

79 Spin-Dependent Tunneling in Magnetic Junctions Figure 1.38 (a) CoFe thickness dependence of room-temperature TMR in Si/200 Å Ni 81 Fe 19 /60 Å Cu/120 Å FeMn/80 Å Ni 81 Fe 19 /t Co 50 Fe 50 (wedge)/al 2 O 3 /130 Å 15 Ni Fe 19 /500 Å Cu (open circles). The solid circles are reference data on 80 Å 16 Ni 81 Fe 19 /Al 2 O 3 /130 Å Ni 81 Fe 19 without the CoFe, scanned along the same direction to 17 check the uniformity of deposition. (b) Magnetic moment per area (μ/a) measured with a superconducting-quantum-interference-device (SQUID) magnetometer of a Co 50 Fe 50 wedge grown on top of a layer of 150 Å Co 50 Fe 50 (open) or 200 Å Ni 81 Fe 19 (closed). After Zhu et al. (2002). 20 A first observation of the dependence of the interface region of ferromagnetic electrodes relates again to quantum-well formation and is reported by Nagahama et al. (2001). When the coherence of the electron wave function is conserved, quantum-well states can be formed in thin epitaxial ferromagnetic layers located at the interface with the alumina barrier, thereby affecting TMR. By employing epitaxial junctions similar to those shown in Fig. 1.35, no oscillations in TMR have been observed when changing the thickness of the interface layer, probably due to a too small amplitude. However, by inspection of the conductance versus bias voltage, a clear oscillatory behavior is reported for epitaxial Fe(001) layers of 2 to 9 monolayers in junctions of Cr(001)-Fe(001)-Al 2 O 3 -Fe 50 Co 50.InFig a selection of these data is presented, where one should focus on voltages beyond 0.2 V, outside the regime of magnon and phonon-assisted tunneling. Note that these effects are only observed for one bias direction, since the CoFe layers are polycrystalline and thick enough to exclude quantum-well states in the CoFe. In this regime of V 0.2 V, the maxima in conductance are shifting towards lower bias for thicker Fe layers, as expected for a quantum-size effect. The phase of the oscillations is found to be identical for the anti-parallel and parallel configuration, suggesting that only one of the spin bands is active in the quantum-well formation. Indeed, as shown in the insets of Figs. 1.39a and 1.39b, it is known that the Fe minority band is very similar to the band structure of Cr, leading to quantumwell states only in the Fe majority band. Unlike the resistance change R/R P, the conductance change defined as G/G AP does show quantum-well oscillations upon a variation of the applied voltage; see Fig. 1.39b. The arrows in this graph are the bias voltages with maximum conductance. Despite these convincing qualitative observations, no calculations are yet available to explain the observed oscillations, 22 44

80 80 H.J.M. Swagten Figure 1.39 (a) Conductance di/dv versus bias voltage V at T = 2 K after subtraction of a background conductance in junctions consisting of MgO(001)/buffer/200 Å Cr(001)/5, 7, 15 9 monolayers Fe(001)/17Å Al 2 O 3 /200 Å Fe 50 Co 50 /Au-cap. The inset shows the possibility of quantum-well formation for positive bias. In (b) the conductance change (G P G AP )/G AP is shown versus voltage. The arrows indicate the position of the maxima in di/dv at low bias as 17 indicated in (a). The inset shows quantum-well reflections for majority electrons along k 18 = After Nagahama et al. (2001). and probably require the application of advanced transport theory (Nagahama et al., 2001). Now we will review the (few) existing examples of the influence of the crystallographic orientation of the ferromagnetic electrodes on TMR. In a tunnel junction, electrons with a momentum vector perpendicular to the barrier plane are strongly selected by the tunneling process. Due to the anisotropy of the Fermi surface of ferromagnetic electrodes, this momentum filtering should cause the TMR to depend on the orientation of the ferromagnetic electrodes. This is also reflected by the tunneling spin polarization (Eq. (20)), where the weighting of the Fermi velocity is expected to be strongly anisotropic in epitaxial junctions. Yuasa et al. (2000) have prepared Fe-Al 2 O 3 -CoFe MTJs with molecular beam epitaxy in which (only) the bottom electrode is epitaxial with three different orientations: Fe(100), Fe(110), and Fe(211). A wide variation in thickness of the amorphous Al 2 O 3 is created by evaporation of Al in an O 2 atmosphere (section 2.2), combined with the use of a moving shutter during deposition. As shown in Fig there is a distinct difference between the orientations as well as a significant dependence on the thickness of the Al 2 O 3 spacer. As an attempt to explain this anisotropy, the authors have calculated the polarization of the bulk density-of-states in the direction normal to the interfaces, using the so-called layer Korringa Kohn Rostoker approach; see also MacLaren et al. (1997, 1999). This yields 4% for Fe(100), 31% for Fe(110), and 34% for Fe(211). This qualitative agreement with the trend observed in Fig may be somewhat fortuitous since the interfacial modification of the density-ofstates may dominate the polarization anisotropy. Also the tunneling current across amorphous Al 2 O 3 may be dominated by s-like states (see section 3) which is not taken into account in the calculation. Furthermore, the variation of TMR with barrier thickness is unexplained, and might be intrinsically related to the complex 22 44

81 Spin-Dependent Tunneling in Magnetic Junctions Figure 1.40 (a) TMR as a function of the thickness of the Al 2 O 3 barrier in Fe/t Al 2 O 3 /200 Å Fe 50 Co 50 junctions. The Fe layer (either 100 Å or 200 Å) is single crystalline 13 with (100), (110), and (211) orientation and has been obtained by epitaxial growth on proper crystals and seed layers. Data were taken at T = 2 K using a bias voltage of 20 mv. Note that the alumina layers are optimized for all thicknesses (with minimal oxidation at the interfaces) 16 due to reactive deposition of alumina in ultra-high vacuum. The curves are guides to the eye. (b) Cross-sectional transmission-electron-microscopy image of a junction with a nominal barrier thickness of 20 Å. After Yuasa et al. (2000). 18 interplay between electron wave functions of different character decaying differently in different orientations. As a final remark, MacLaren et al. (1999) have theoretically shown that due to the symmetry of the Bloch states at the Fermi level the TMR is expected to be highest for Fe(100) and should increase with the barrier thickness. Both these predictions are in contrast with the experiments of Yuasa et al. (2000). As to the barrier-thickness dependence of TMR, it is suggested by Mizuguchi et al. (2005) that also thinner barriers (below 10 Å) are experimentally accessible in these epitaxial junctions. In-situ scanning tunneling microscopy of alumina on top of epitaxial Fe(100) has revealed that the naturally oxidized Al layer is surprisingly flat showing mono-atomic steps with a 3 Å step height corresponding to one monolayer of Al 2 O 3. Another clear evidence for tunneling spin polarization reflecting the density-ofstates of the ferromagnetic electrode has been reported in Co-Al 2 O 3 -Co junctions in which the buffer layer grown underneath induces a particular growth mode of the Co electrode (LeClair et al., 2002b). When grown on top of a single Ta buffer, their Co grows in a random, polycrystalline fashion with a mixture of fcc, hcp and stacking faults. However, when a Ta-Co-FeMn buffer is used, the Co located at the interface region with Al 2 O 3 is (111) textured and predominantly fcc-structured. The difference in tunneling transport between these two cases is best visible in the (normalized) voltage-dependent conductance change G/G AP (V ). Note that as in the case of quantum confinement in Fe, see Fig. 1.39, the TMR itself is not conclusive for finding the spin-dependent transport features. To eliminate effects of magnons and spin-independent excitations from e.g. phonons which are symmetric in bias voltage, the odd part of the conductance change G/G AP (V >0) G/G AP (V < 0) is analyzed and shown in Fig. 1.41b. The original data are presented in the left panel of the figure. The strong minimum seen in the fcc data can be qualitatively explained by a modified elastic tunneling model using free-electron like bands de

82 82 H.J.M. Swagten Figure 1.41 (a) Conductance di/dv versus voltage V in parallel (P) orientation for Si(100)/SiO 2 /buffer + Co/23 Å Al + oxidation/150 Å Co/50 Å Ta. The open symbols refer to 15 a buffer and magnetic bottom electrode composed of 50 Å Ta/50 Å Co/100 Å FeMn/50 Å fcc(111) Co. Closed symbols refer to 50 Å Ta/poly 50 Å Co where poly relates to polycrystalline and polyphase Co as determined by nuclear magnetic resonance (Wieldraaijer, 2006) (b) Odd part of G/G AP versus voltage for fcc Co and poly Co. The solid line is based on 18 a calculation with the modified elastic tunneling model (Davis and MacLaren, 2002). Data are all taken at T = 5 K. In all cases V >0refers to electrons tunneling from the top to the bottom 20 electrode. After LeClair et al. (2002b) rived from ab-initio electronic-structure calculations. In this model, the conductance is dominated by the contribution of the highly dispersive, s-hybridized density-ofstates of fcc Co to reflect the fact that in these Al 2 O 3 junctions electrons with s character are decisive for spin-dependent tunneling (corresponding to positive tunneling spin polarization; see section 3). In particular, the presence of two sharp peaks in the s-derived density-of-states above and below E F, as well as a dispersive minority band just above E F, are key to the observed behavior (LeClair et al., 2002b; Davis and MacLaren, 2002). Inspired by these results, Hindmarch et al. (2005b) have measured the odd part of the conductance and TMR in junctions with a Cu 38 Ni 62 magnetic electrode having a Curie temperature of around 240 K. Due to the low T C of this alloy, the energy of the bottom of the minority spin bands close to the Fermi energy can be followed for temperatures up to the magnetic phase transition. From the odd part of G/G AP versus bias voltage, it is observed that slightly above the Fermi level the band minimum remains fixed in energy until the temperature is raised to around T = 190 K. Beyond this point it abruptly drops below the Fermi level, which is consistent with a Stoner-like collapse of the effective exchange splitting of energy bands responsible for tunneling. 4.4 Towards infinite TMR with half-metallic electrodes The implementation of electrodes with a (nearly) 100% tunneling spin polarization, the so-called half-metallic materials, is expected to yield infinite TMR as indicated by the Julliere formula 2P L P R /(1 P L P R ) with P L and P R equal to ±1. Experimentally 44

83 Spin-Dependent Tunneling in Magnetic Junctions 83 as well as theoretically, an ongoing intensive research effort is devoted to these materials and their implementation; see, e.g., Pickett and Moodera (2001). Although many predictions of half-metallic behavior have been reported this is verified experimentally only in a few cases, including La 0.7 Sr 0.3 MnO 3 (Park et al., 1998a; Soulen et al., 1998), NiMnSb (Ristoiu et al., 2000), and CrO 2 (Ji et al., 2001). In the latter case of CrO 2, Parker et al. (2002) verified the near +100% tunneling spin polarization directly in a superconducting-tunneling-spectroscopy experiment on CrO 2 -Cr 2 O 3 -Al and CrO 2 -Cr 2 O 3 -Pb junctions (see Table 1.2). For La 2/3 Sr 1/3 MnO 3 a polarization of +72% is measured using the same technique (Worledge and Geballe, 2000b). The use of these materials in ferromagnetic-insulator-ferromagnetic junctions is obviously extremely tedious due to the crucial control of two barrier interfaces. Indeed, for junctions employing one or two half-metallic Heusleralloy electrodes such as NiMnSb (Tanaka et al. 1997, 1999) and related MnSb (Panchula et al., 2003), Co 2 MnSi (Kammerer et al., 2004; Schmallhorst et al., 2004; Nakajima et al., 2005), Co 2 MnAl (Kubota et al., 2004), Co 2 Cr 0.6 Fe 0.4 Al (Inomata et al., 2004), and Co 2 FeAl (Okamura et al., 2005), the TMR remains relatively low and may result from oxidation at the Heusler-barrier interfaces or from sitedisordering and structural defects close to the barrier. More promising, Sakuraba et al. (2005a, 2005b) observe magnetoresistances of up to 70% at room temperature and 159% at T = 2 K in UHV-sputtered Co 2 MnSi-Al 2 O 3 -Co 75 Fe 25,which corresponds to a low-temperature tunneling spin polarization of +89%, closely approaching the theoretical prediction of half-metallicity. CrO 2 -based junctions are not successful in terms of high TMR. As an example, Gupta et al. (2001) have grown CrO 2 -Cr 2 O 3 -Co(Ni 81 Fe 19 ) tunnel junctions in which CrO 2 is epitaxially grown on top of TiO 2,andCr 2 O 3 (or a composition close to this) is stabilized by exposing the bottom electrode to an oxygen plasma. In this case, a TMR of only 8% and 2.3% has been achieved at T = 4 K for Co and permalloy, respectively. Magnetite (Fe 3 O 4 ) is also predicted to be half-metallic due to a gap for the majority band at the Fermi level. Junctions consisting of Fe 3 O 4 -MgO-Fe 3 O 4 show, however, only a very small TMR for all temperatures (Li et al., 1998), maybe related to a combination of spin scattering in a magnetically dead interface layer, a distorted spin structure due to a specific interface termination, or due to a reduced oxide such as antiferromagnetic Fe 1 δ O present at the interface with the MgO barrier. Also in junctions consisting basically of NiFe-Al 2 O 3 -Fe 3 O 4 (with the magnetite fabricated by plasma oxidizing a thin Fe film) only a very small TMR has been reported (Park et al., 2005). The authors suggest that the observed negative sign of TMR is consistent with the expected gap for majority Fermi electrons in Fe 3 O 4. On the other hand, Seneor et al. (1999) have reported a positive TMR of +43% at low temperature and +13% at room temperature in sputtered Co-Al 2 O 3 -Fe 3 δ O 4 -Al junctions where the iron oxide is sputtered from a Fe 2 O 3 target. This relatively large TMR is ascribed to the presence of a phase close to magnetite, although the data suggest that the TMR originates predominantly from conduction channels active only above and below the Fermi level. In epitaxial La 0.7 Sr 0.3 MnO 3 -CoCr 2 O 4 -Fe 3 O 4 junctions a negative TMR of up to 25% is in qualitative agreement with the theoretically 22 44

84 84 H.J.M. Swagten predicted negative spin polarization of Fe 3 O 4 (Hu and Suzuki, 2002). The observed maximum TMR at T 60 K is attributed to the paramagnetic to ferrimagnetic transition in the CoCr 2 O 4 barrier. Zhang et al. (2001a) have introduced an Feoxide layer at the barrier interface of CoFe-Al 2 O 3 -CoFe junctions to improve the thermal stability when annealing up to temperatures of around 400 C (see section 2.3.4). The large TMR measured after annealing is attributed to the formation of Fe 3 O 4 in the interfacial region, which is confirmed by a follow-up study using transmission electron microscopy combined with electron-energy-loss spectroscopy (Snoeck et al., 2004). In Co-Al 2 O 3 -NiFe junctions, it is shown that TMR is enhanced by roughly a factor of 1.25 due to δ doping the oxide barrier with an Fe layer with a thickness of less than 2 Å (Jansen and Moodera, 1999). Apart from other explanations, also in this case the possibility of half-metallic Fe 3 O 4 formation is hypothesized by the authors. Especially in the perovskite materials, a lot of progress has been witnessed as described in the review paper by Ziese (2002). Pioneering experiments are done by Sun et al. (1996, 1997, 1998) on junctions with La 2/3 Ca 1/3 MnO 3 (LCMO) and La 2/3 Sr 1/3 MnO 3 (LSMO) electrodes and SrTiO 3 barriers, later also combined with ferromagnetic 3d transition metals (Sun et al., 2000b). Jo et al. (2000a, 2000b) use La 2/3 Ca 1/3 MnO 3 as electrodes in LCMO-NdGaO 3 -LCMO junctions reaching TMR magnitudes of more than 500% at low T.La 2/3 Sr 1/3 MnO 3 is used by Lu et al. (1996) and Viret et al. (1997) in combination with oxide barriers such as SrTiO 3, yielding low-temperature magnetoresistances of more than 400%. This reasonably well corresponds to the measured La 2/3 Sr 1/3 MnO 3 tunneling spin polarization of approximately +72% (Worledge and Geballe, 2000b) as discussed before (see also section 3). In the latter superconducting-tunneling-spectroscopy experiment, the authors use La 2/3 Sr 1/3 MnO 3 -SrTiO 3 -Al junctions with a thick layer of YBa 2 Cu 3 O 7 grown as a buffer layer on the SrTiO 3 substrate to prevent current crowding in the bottom electrode (see section 2.1.3). Junctions consisting of La 0.7 Ce 0.3 MnO 3 -SrTiO 3 -La 0.7 Ca 0.3 MnO 3 exhibit a large positive TMR at low temperatures, whereas at intermediate temperatures below T C the sign of the observed TMR is dependent on the bias voltage, suggesting a high degree of tunneling spin polarization dominated by minority spins (Mitra et al., 2003). In the doubleperovskite Sr 2 FeMoO 6 the predicted half-metallicity has triggered spin-tunneling experiments in Sr 2 FeMoO 6 -SrTiO 3 -Co junctions (Bibes et al., 2003a). The authors have reported a TMR of 50% at low temperature that corresponds to a tunneling spin polarization of more than 85% at the Sr 2 FeMoO 6 -SrTiO 3 interface (see also section 4.5). To a great extent, this confirms the half-metallic character of this double-perovskite compound. Bowen et al. (2003) have convincingly demonstrated the impact of half-metals in MTJs. Epitaxial LSMO-SrTiO 3 -LSMO junctions have been grown by pulsed laser deposition and careful post-deposition lithographic processing, yielding a TMR of 1850% at T = 4 K (see Fig. 1.42). This corresponds to a tunneling spin polarization of 95% when both LSMO-SrTiO 3 interfaces are assumed to be equal. At higher temperatures though, the TMR is gradually suppressed and disappears at 280 K, below the Curie temperature of bulk LSMO, which is related to the interface structure and specifically the LSMO termination at the barrier interface 22 44

85 Spin-Dependent Tunneling in Magnetic Junctions 85 Figure 1.42 Magnetoresistance measurements of 350 Å La 16 2/3 Sr 1/3 MnO 3 /28 Å SrTiO 3 /100 Å 16 La 2/3 Sr 1/3 MnO 3 epitaxial junctions using SrTiO 3 substrates. On top a 150 Å Co layer is 17 deposited and subsequently oxidized for magnetically pinning the top electrode. (a) Relative change in resistance ([R R P ]/R P )versusappliedmagneticfieldh at T = 4.2 K and a bias voltage of 1 mv. In (b) and (c) the temperature dependence of TMR is shown for two junctions with different area, using V = 10 mv. Solid curves are guides to the eye. After Bowen et al. 20 (2003). 22 (Pailloux et al., 2002). In a follow-up study by Garcia et al. (2004), the relatively low value of T C in LSMO could be exploited to measure how the temperature dependence of tunneling spin polarization is related to M(T), a similar approach as followed by Hindmarch et al. (2005a) for ferromagnetic Cu 38 Ni 62.Insection2.1.4 such a relation between tunneling spin polarization and the (surface) magnetic moment has been suggested to describe the temperature dependence of TMR for regular Al 2 O 3 -based MTJs. In Figs. 1.43a and 1.43b the TMR of La 2/3 Sr 1/3 MnO 3 - TiO 2 -La 2/3 Sr 1/3 MnO 3 and La 2/3 Sr 1/3 MnO 3 -LaAlO 3 -La 2/3 Sr 1/3 MnO 3 junctions is plotted versus temperature. From this the tunneling spin polarization is calculated via the Julliere formula P(T) =[TMR(T )/(2 + TMR(T ))] 1/2 and is plotted in Fig. 1.43c andfig. 1.43d for TiO 2 and LaAlO 3, respectively. A close resemblance with the bulk magnetization of separately grown trilayers is observed, although the Curie temperature deduced from P(T)is roughly 60 K lower than the temperature where M(T) vanishes (350 K). Apparently, the magnetism of interfacial LSMO is well preserved at the interfaces with TiO 2, LaAlO 3, and SrTiO 3 (not shown), certainly when comparing it with the polarization of a free surface of LSMO measured with spin-polarized photoemission (Park et al., 1998b). In that case a much stronger decay with temperature is observed, evidencing that free surfaces and embedded interfaces have strongly different properties in manganites (Garcia et al., 2004). The use of half-metallic electrodes is particularly attractive for directly extracting density-of-states or band-structure features from the bias dependence of the tunneling transport. When only majority electrons are tunneling from half-metallic LSMO, one is able to the directly probe the majority (minority) density-of-states 44

86 86 H.J.M. Swagten 14 Figure 1.43 Temperature dependence of TMR of epitaxial junctions containing (a) 350 Å La /3 Sr 1/3 MnO 3 /32 Å TiO 2 /100 Å La 2/3 Sr 1/3 MnO 3 and (b) Å La 2/3 Sr 1/3 MnO 3 /28 Å LaAlO 3 /100 Å La 2/3 Sr 1/3 MnO 3. The lines in (a) and 16 (b) are guides to the eye. The normalized tunneling spin polarization deduced from TMR is shown as a function of temperature normalized to T C for the junction with TiO 2 (c) and LaAlO 3 (d). The solid line in (c) and (d) is the normalized magnetization measured on similar trilayers. After Garcia et al. (2004). 19 of the counter electrode when the magnetizations are (anti)parallel oriented. This idea is exploited by Bowen et al. (2005a, 2005b). They find a quantitative confirmation of the half-metallic band structure of La 2/3 Sr 1/3 MnO 3 by measuring the conductance and TMR of LSMO-SrTiO 3 -LSMO junctions for variable bias voltages. First of all, it is observed that the conductance di/dv in one bias direction for parallel oriented magnetization of the LSMO layers shows a dramatic collapse at V 0.82 V, whereas the antiparallel conductance continues to increase; see Fig. 1.44a. The collapse in parallel conductance proves that no minority band is available at E F from which electrons can tunnel into the minority t 2g band, demonstrating the half-metallic nature of the LSMO. It also proves that the majority electrons available at E F do not find any empty majority states at E F + E g with E g 0.82 ev; see the schematic diagram in Fig. 1.44c. This is consistent with a pseudo-gap in the majority density-of-states of the e g bands as predicted by Pickett and Singh (1998) for a distorted oxygen environment of Mn ions in manganites. In a related paper, the energy difference δ between the Fermi energy and the bottom of the minority t 2g band is accurately extracted from TMR, conductance and conductance derivative measurements in these junctions (Bowen et al., 2005a). In Fig. 1.44b, d 2 I/dV 2 reveals a sudden upturn of the antiparallel conductance at V 0.34 V. This marks the onset of a conduction channel for majority electrons tunneling into the minority t 2g band at E F + δ (see the schematics in Fig. 1.44d). This turns out to be in good agreement with data obtained from spin polarized inverse photoemission experiments, yielding δ = 0.38 ± 0.05 ev. When the bias voltage across these LSMO junctions exceeds δ/e, the low-temperature TMR is seen to rapidly decrease with voltage (not shown). This is again due to the opening of a new conduction channel in the antiparallel orientation, corroborating 22 44

87 Spin-Dependent Tunneling in Magnetic Junctions 87 Figure 1.44 (a) Conductance di/dv versus applied bias voltage V of 350 Å La 2/3 Sr 1/3 MnO 3 /28 Å SrTiO 3 /100 Å La 2/3 Sr 1/3 MnO 3 epitaxial junctions, for both antiparallel and parallel oriented magnetization. The conductance collapse in the parallel 15 case (at 0.82 V) is due to the absence of conduction channels at V = E g /e, asshownin the schematic band diagram in (c); adapted from Bowen et al. (2005b). In (b) the derivative of the conductance d 2 I/dV 2 is shown for bias voltages below 0.5 V. The increase of d 2 I/dV in antiparallel orientation observed at V = δ/e 0.34 V marks the onset of tunneling into 18 the minority (t 19 2g ) spin band; see the schematics in (d). The lines in (b) are added to better 19 visualize the conductance upturn. Adapted from Bowen et al. (2005a). the predictions by Bratkovsky (1997) for the bias-voltage dependence of magnetic tunnel junctions with half-metallic electrodes Role of the barrier for TMR Now that we have seen that TMR may be tuned towards very large numbers by a proper choice of ferromagnetic materials, one should realize that the combined system of (magnetic) electrodes and barrier material is decisive for the magnitude of TMR and tunneling spin polarization (section 3). In a series of remarkable experiments on La 0.7 Sr 0.3 MnO 3 -insulator-co junctions, de Teresa et al. (1999a, 1999b) have used the full polarization of the half-metallic LSMO as a detector of the spin polarization of Co adjacent to tunnel barriers of a different character. When using traditional alumina in LSMO-Al 2 O 3 -Co, a positive TMR is found at temperatures well below room temperature, which, via the Julliere formula, reflects a positive spin polarization of Co. Although this is contrary to what is expected from the smaller density-of-states at E F for the Co majority spin channel, this is believed to reflect the positive polarization of s electrons that dominate the tunneling process (see also the more elaborate discussions in section 3). A striking sign reversal of TMR is observed when replacing the alumina by SrTiO 3 or Ce 0.69 La 0.31 O In this case, it appears that electrons with a d-like character are now preferentially transmitted at the Co-SrTiO 3 or Co-Ce 0.69 La 0.31 O interfaces (Fig. 1.45a). Moreover, when using a double barrier in a LSMO-SrTiO 3 - Al 2 O 3 -Co junction the TMR is positive again, see Fig. 1.45b. Apparently, the electronic structure and chemical bonding at the Co-insulator interface is decisive for the tunneling spin polarization rather than the electron tunneling processes 44

88 88 H.J.M. Swagten Figure 1.45 (a) TMR as a function of bias voltage V of SrTiO 16 3 (001)/350 Å 16 La 2/3 Sr 1/3 MnO 3 /25 Å SrTiO 3 /300 Å Co measured at T = 5 KandT = 30 K. The inset 17 shows the resistance change ([R R P ]/R P ) with applied magnetic field H at 5 K using a bias of 0.4 V. (b) TMR versus bias voltage at T = 40 K as in (a), but now with a composite barrier: SrTiO 3 (001)/350 Å La 2/3 Sr 1/3 MnO 3 /10 Å SrTiO 3 /15 Å Al 2 O 3 /300 Å Co. (c) Relative 19 position of the DOS in La 2/3 Sr 1/3 MnO 3 and the d DOS at an fcc Co(001) surface for a bias around zero. The arrow indicates the high tunneling probability between the majority band of LSMO and the minority band of Co, when the magnetization is antiparallel (AP). For 22 V <0electrons tunnel into the empty states of Co above the Fermi level E F.Afterde Teresa et al. (1999a, 1999b); note that in these papers TMR is alternatively defined as ([R P R AP ]/R AP ). 23 in the full barrier. The dependence of TMR on bias voltage is another interesting aspect of the LSMO-SrTiO 3 -Co junctions; see Fig. 1.45a. Since the conductance is determined by only one spin channel, the variations with bias are found to be easily correlated with the d-character density-of-states of a Co(001) surface; see Fig. 1.45c. At a negative bias voltage of around 0.4 V, the majority electrons are tunneling into the predicted peak in the (unoccupied) minority density-of-states of Co above the Fermi level, leading to a maximum in negative TMR. In a more general perspective, these experiments show that the interfacial bonding is of critical relevance for spin-dependent tunneling of electrons. When d d bonding is allowed by using barriers with d-orbitals such as in SrTiO 3, it is possible to observe the d-dominated spin polarization of Co (P <0). In the opposite case of alumina barriers, the absence of d orbitals apparently favors an s-dominated tunneling current (P >0). Although these arguments are helpful to qualitatively understand the role of the barrier for tunneling spin polarization, it is evident that a more solid theoretical basis is required to substantiate this. This will be further discussed later on in this section. Thomas et al. (2005) have directly measured the tunneling spin polarization of Co-SrTiO 3 -Al by superconducting tunneling spectroscopy, yielding a positive spin polarization of +31% (see Table 1.2) in striking contrast to the aforementioned results at low bias voltage (de Teresa et al., 1999a, 1999b). This could be explained by the thermal evaporation of the barrier on polycrystalline Co, leading 44

89 Spin-Dependent Tunneling in Magnetic Junctions 89 to an amorphous SrTiO 3 layer as seen by high-resolution transmission electron microscopy (instead of the epitaxial barriers in the work of de Teresa et al. (1999a, 1999b)). Correspondingly, also a rather small (positive) TMR of around +1% has been measured at low temperatures in Co-SrTiO 3 -Co, Co-SrTiO 3 -Ni 80 Fe 20,and Co-TiO 2 -Co-Ni 80 Fe 20 junctions (Thomas et al., 2005). The tedious role of the chemical structure of the barrier and the interfaces with the ferromagnetic layers in these LSMO-based junctions is also recognized in other experimental studies, see for example Sun et al. (2000b) and Hayakawa et al. (2002), showing both negative and positive TMR in CoFe-SrTiO 3 -LSMO and Fe-SrTiO 3 -LSMO, strongly and asymmetrically dependent on the bias voltage. In the work of Oleinik et al. (2002), first-principles density-functional calculations of the atomic and electronic structure of Co-SrTiO 3 -Co(001) MTJs have established the key importance of the atomic arrangement at the barrier interfaces. It is found that the most stable structure represents the TiO 2 -terminated interface with the O atoms lying on top of the Co atoms. At the interface with Co, an induced magnetic moment of 0.25 μ B on the interfacial Ti atoms is aligned antiparallel to the magnetic moment of the Co layer, which may indeed lead to a negative tunneling spin polarization of the Co-SrTiO 3 barrier (Oleinik et al., 2002; Oleynik and Tsymbal, 2003). Using ab initio transport calculations including firstprinciples band structure methods, Velev et al. (2005) predict a very large TMR (1000% and more) in Co-SrTiO 3 -Co junctions with bcc Co(001) electrodes and barriers typically 7 to 11 monolayers in thickness. The complex band structure of SrTiO 3 enables an extremely efficient tunneling of minority d electrons from the Co, causing the tunneling spin polarization to be negative. From the calculations it is estimated that a single Co-SrTiO 3 interface carries a tunneling spin polarization of 50% that is rather independent of the barrier thickness. This is roughly a factor of 2 higher than P derived from the experiments of de Teresa et al. (1999a), and may be explained by effects of interface disorder, e.g. locally affecting the structure of bcc Co. It should be emphasized that these results show that a spin-polarized tunneling current across SrTiO 3 is carried by minority d electrons. This is essentially different as compared to sp-bonded insulators such as Al 2 O 3 (sections 2and3)and MgO (section 4.6), where tunneling is dominated by electrons from majority bands. The argument of interface (chemical) bonding has also been used to explain the sign reversal of TMR observed in junctions with others barriers containing d- type ions. Experiments by Sharma et al. (1999) on Ta 2 O 5 are already discussed in section Bibes et al. (2003b) have investigated junctions with TiO 2 barriers. La 2/3 Sr 1/3 MnO 3 -TiO 2 -Co shows a negative TMR of around 3% at low temperature. Regarding the positive spin polarization of La 2/3 Sr 1/3 MnO 3 (though against asrtio 3 barrier by Worledge and Geballe (2000b)), the tunneling spin polarization of Co-TiO 2 is negative, similar to the experiments by de Teresa et al. (1999a, 1999b) for interfaces of Co-SrTiO 3 or Co-Ce 0.69 La 0.31 O AlsoCo-Cr 2 O 3 and Ni 81 Fe 19 -Cr 2 O 3 interfaces display a negative spin polarization as determined from TMR in junctions with one half-metallic CrO 2 electrode, the other electrode being Co or NiFe (Gupta et al., 2001). A related experiment has been performed using the ferromagnetic double perovskite Sr 2 FeMoO 6 with a T C of 415 K, and a predicted half-metallicity (Kobayashi et al., 1998). Bibes et al. (2003a) have obtained a +50% 22 44

90 90 H.J.M. Swagten TMR at low temperature in a junction consisting of SrTiO 3 -Sr 2 FeMoO 6 -SrTiO 3 - Co. Using Julliere s formula and the experimental fact that the epitaxial SrTiO 3 -Co interface carries a spin polarization at low bias voltage of about 25% (de Teresa et al., 1999a), this yields a very strong tunneling spin polarization of 80%. It is important to be aware of the fact that only in a few systems the presence of negative tunneling spin polarization has been confirmed straightforwardly by superconducting tunneling spectroscopy (section 3). In the case of Co-SrTiO 3 interfaces, the expected negative tunneling spin polarization of around 25% (as deduced from the low-bias TMR data of de Teresa et al. (1999a) using LSMO as the second electrode) may be directly tested by STS on Co-SrTiO 3 -Al superconducting junctions. However, the system of Co-SrTiO 3 -Al is extremely difficult to realize epitaxially and may suffer from oxidation of either Al or Co, both reducing the spin polarization. STS data obtained by Thomas et al. (2005) using amorphous SrTiO 3 indeed did not yield the anticipated negative spin polarization as mentioned earlier. A negative tunneling spin polarization of 9.5% is for the first time measured by Worledge and Geballe (2000c) using ferromagnetic SrRuO 3 in a superconducting junction consisting of SrTiO 3 (100)-YBa 2 Cu 3 O 7 -SrRuO 3 -SrTiO 3 -Al (see Table 1.2). This negative sign is supported by theoretical calculations and emphasizes the crucial role of weighting the density-of-states factors in Eq. (20) with transmission probabilities for the tunneling processes. Also in Co 1 x Gd x ferrimagnetic alloys for 0.2 x 0.75 a negative tunneling spin polarization has been observed directly from STS (Kaiser et al., 2005a). This is explained by the relative contribution of independent spin-polarized tunneling currents from the two sublattice magnetizations (see section 3.3.2). Via the Julliere formula TMR = 2P L P R /[1 P L P R ],the negative tunneling spin polarization is in agreement with a negative magnetoresistance in junctions with one electrode of Co 1 x Gd x (P <0) and a counter electrode of Co 70 Fe 30 (P >0) Coherent tunneling in MgO junctions In the previous sections, it is emphasized that TMR is certainly not determined by the spin polarization of the individual ferromagnetic electrodes. Instead, it is sensitively dependent on the full system of ferromagnetic electrodes and the adjacent barrier, in which the electronic structure modifications at the barrier-electrode interface and the symmetry and matching of the electron wave functions are playing a crucial role. Based on this, it could be conceivable that certain electrode-barrier material combinations would allow for a highly efficient polarization of the spin currents, even with a bulk density-of-states displaying only a modest spin polarization. In Fe-ZnSe-Fe(001) junctions (MacLaren et al., 1999), it is theoretically shown that for thick enough barriers the conductance is dominated by slowly decaying s-states at k = 0 as provided by a 1 -band at the Fermi level of Fe(001). Together with the absence of a minority 1 -band at E F, this leads to a very strong asymmetry in the conductance and hence a large TMR. Experimentally, however, no such dramatic pseudo-half-metallic effects have been observed for ZnSe barriers. Gustavsson et al. (2003) report on a lowtemperature TMR of only 16% in a Fe-ZnSe-Co 0.15 Fe 0.85 junction, disappearing 44

91 Spin-Dependent Tunneling in Magnetic Junctions 91 above T 50 K. Jiang et al. (2003b) have found magnetoresistance of less than 25% at low temperature and 10% at room temperature in ZnSe-based MTJ s, which, although potentially relevant for low RA product MTJs, is again not in agreement with the promises given by theory. Although the interfaces between ZnSe(001) and Fe are reported to be very sharp without magnetically dead or modified interfacial regions even after annealing up to 300 C (Marangolo et al., 2002), it could be that significant modifications of the Fe spin-polarized band structure near E F as determined from spin-polarized inverse photoemission lead to a suppression of TMR (Bertacco et al., 2004). Also the presence of mid-gap localized states in the ZnSe barrier due to a small amount of disorder is shown to significantly suppress or even change the sign of TMR in epitaxial Fe-ZnSe-Fe junctions (Varalda et al., 2005). Similarly, the use of the II-VI compound ZnS has yielded magnetoresistances of only 5% at room temperature (Guth et al., 2001b; Guth et al., 2001a). In this case, it is suggested that the observation of an indirect ferromagnetic interaction across the insulating ZnS is mediated by the tunneling electrons (Dinia et al., 2003). Later on in this section, we will return to interlayer coupling across insulating spacers. Now we will concentrate on the spin-dependent transport properties when MgO barriers are employed. The experimental use of these barriers has also been triggered by theoretical predictions of pseudo-half-metallic behavior in Fe-MgO- Fe(001), and has resulted in a number of intriguing new observations, which will be extensively discussed below TMR of MgO-based junctions Using different theoretical approaches, both Butler et al. (2001b, 2005) and Mathon and Umerski (2001) come basically to the same conclusion for coherent tunneling in an Fe-MgO-Fe(001) magnetic tunnel junction, i.e. for electrons tunneling normal to the barrier in the [001] direction. For majority electrons, there are four Bloch states of different symmetry present around the Fermi level for k = 0, viz. a double-degenerate 5 state compatible with pd symmetry, 2 with d symmetry, and a 1 state with spd symmetry. However, for the minority spins the 1 state is replaced by a d-type 2 state. Due to its s-type character, only the Bloch states of 1 symmetry are able to effectively couple with the evanescent sp states in the MgO barrier region, which, at the Fermi level, is only available for majority electrons. This pseudo-half-metallicity of the band structure in the [001] direction is schematically shown in Fig and Fig. 1.47a by the absence of 1 minority states for tunneling electrons. For thick enough barriers, the majority conductance in parallel alignment of magnetization becomes fully dominated by these 1 -band contributions, and, correspondingly, extremely large TMR in these junctions (of 1000% and more) are expected to show up experimentally (Butler et al., 2001b; Mathon and Umerski, 2001). Early experiments using MgO as a barrier have only been partially successful. First of all, when the electrodes are polycrystalline and the MgO is amorphous (Moodera and Kinder, 1996; Platt et al., 1997; Smith et al., 1998; Kant et al., 2004c), only a modest TMR or tunneling spin polarization is found. In fully epitaxial systems grown by molecular beam epitaxy combined with pulsed 44

92 92 H.J.M. Swagten 13 Figure 1.46 Layer-resolved tunneling density-of-states (DOS) for k = 0 in Fe(100)/ 8 monolayers MgO/Fe(100) for majority electrons (a) and minority electrons (b) when the magne tization of the Fe layers is parallel oriented. Each curve is labelled by the symmetry of the 15 incident Bloch state in the left Fe electrode, showing, for example, the absence of minority states with 1 symmetry, whereas the majority 1 states decay only very slowly in the MgO 17 barrier. After Butler et al. (2001b) Figure 1.47 (a) Calculated band dispersion of Fe in the [001] (Ɣ-H) direction. Solid and dotted curves represent majority and minority-spin subbands, respectively; as indicated, thicker lines are the 1 subbands. Adapted from Yuasa et al. (2004a). (b) Calculated local 37 spin-polarized density-of-states for Fe at the bottom interface with MgO in Fe/MgO/Fe (grey) and Fe/FeO/MgO/Fe (black), the latter representing the presence of one complete O layer between Fe and MgO. E F is the Fermi level. Top panel is for majority electrons, bottom 40 panel for minority electrons. After Tiusan et al. (2004). 40 laser deposition, the TMR is reported to be quenched by defects in the epitaxial MgO barrier (Klaua et al., 2001; Wulfhekel et al., 2001). Bowen et al. (2001) have reported a TMR of 60% at 30 K and 27% at room temperature in Fe-MgO-FeCo(001) by combining laser ablation and sputtering. In the case of 44

93 Spin-Dependent Tunneling in Magnetic Junctions Figure 1.48 Inner-loop resistance when switching the free magnetic layer, expressed as (R R P )/R P, versus magnetic field strength H for MTJs with a crystalline MgO barrier (a) Junctions consisting of 100 Å TaN/250 Å IrMn/8 Å Co 84 Fe 16 /30 Å Co 70 Fe 30 /29 Å 15 MgO/150 Å Co Fe 16 /100 Å Mg, annealed at T A = 120 C and 380 C (after Parkin 16 et al. (2004)). (b) Junctions of 100 Å Ta/150 Å PtMn/25 Å Co 70 Fe 30 /8.5 Å Ru/30 Å Co 60 Fe 20 B 20 /18 Å MgO/30 Å Co 60 Fe 20 B 20 /100 Å Ta/70 Å Ru measured at T = 20 K 18 and T = 300 K, after an anneal at 360 C (afterdjayaprawira et al. (2005)). 18 Fe-MgO-Fe-Co grown by molecular beam epitaxy (Faure-Vincent et al., 2003), a TMR of 67% has been observed at room temperature, increasing up to around 100% at low T (see also the earlier work of Popova et al., 2002). Since the TMR is still far from the existing theoretical predictions, the authors attribute this to the growth-induced difference in topology of the two interfaces by which the required symmetric matching of the wave functions is affected. By first-principle calculations of the electronic structure of Fe-FeO-MgO-Fe, it is theoretically demonstrated that the chemical bonding between Fe and O strongly reduces the conductance in parallel orientation (Zhang et al., 2003a). The corresponding reduction in TMR could suggest that oxide formation at the barrier interfaces may be a common problem for epitaxial MgO-based junctions; see also the surface X-ray diffraction experiments by Meyerheim et al. (2001). On the other hand, Tusche et al. (2005) have shown that oxygen at the barrier interfaces may promote a fully coherent growth of Fe on top of the MgO spacer, leading to a coherent and symmetric MTJ structure characterized by FeO layers at both Fe-MgO interfaces. A considerably improved room-temperature magnetoresistance in MgO junctions has been found by Parkin et al. (2004). They have observed giant TMR values up to 220%, whereas at low T it rises towards 300%. In their approach, exchange-biased CoFe-MgO-CoFe(001) junctions are fabricated with regular sputtering deposition, the MgO being reactively magnetron-sputtered in an Ar-O 2 mixture, and the full stack subsequently annealed at relatively high temperature (up to 380 C). In Fig. 1.48a an example curve for these junctions is displayed. Obviously, these films are not epitaxial but polycrystalline and (001)-textured (including the MgO barrier), which suggests that especially the well-defined crystalline orientation of the barrier and electrodes is key to the strong tunneling spin polarization. Separately, STS measurements on CoFe-MgO-Al junctions are used to directly 22 44

94 94 H.J.M. Swagten measure the tunneling spin polarization. A positive P of 85% is found in optimized junctions in accordance with the dominance of majority electrons with 1 symmetry as indicated above. Via the Julliere formula TMR = 2P L P R /(1 P L P R ) this relates to a magnetoresistance of 520% at low T, corresponding to a TMR effect of around 260% at room temperature when correcting for the T dependence of TMR, which is in close agreement with the magnetoresistance data (Parkin et al., 2004). An even higher TMR at room temperature is found when MgO is sandwiched between amorphous CoFeB ferromagnetic electrodes. Djayaprawira et al. (2005) have compared the magnetoresistance of magnetron-sputtered structures containing either Co 70 Fe 30 -MgO-Co 70 Fe 30 or CoFeB-MgO-CoFeB, where CoFeB is sputtered from a Co 60 Fe 20 B 20 target. The barriers are deposited using rf sputtering directly from a MgO target. All junctions are annealed at 360 C. As shown by transmission electron microscopy, the structural quality of MgO in the CoFe junctions is very poor and the interfaces are rough. In that case, a TMR of only 62% at room temperature is observed. When growing MgO on top of the CoFeB, it shows after the anneal a good crystallinity with a preferred (001) orientation, most probably due to the amorphous nature of the underlying CoFeB (although some parts are crystallized). In the CoFeB-MgO-CoFeB junctions, the TMR ratio is now 230% at room temperature increasing to 294% at T = 20 K; see Fig 1.48b. It seems that for obtaining this very high TMR the correct structural symmetry of the MgO(001) barrier is crucial, although it is presently not clear how an amorphous magnetic electrode can give rise to giant TMR in view of the importance of the electrode band structure along the k = 0 direction. In this respect, the authors do not exclude the possibility that the annealed junctions show local (re)crystallization of a few monolayers of CoFeB at the electrode-barrier interfaces, beyond the detection limit of transmission electron microscopy. Indeed, Yuasa et al. (2005b) have shown that a sputtered, amorphous Co 60 Fe 20 B 20 layer grown on top of e-beam evaporated Mg(001) crystallizes in a bcc structure with (001) orientation, after annealing at temperatures of around 360 C. In this reflective high-energy electron diffraction study, it is also demonstrated that a MgO layer grown on amorphous CoFeB initially has an amorphous structure as well, and begins to crystallize in the (001) orientation only when t MgO exceeds 5 monolayers. Hayakawa et al. (2005b) have shown from transmission-electron-microscopy images that annealing of Co 40 Fe 40 B 20 -MgO-Co 40 Fe 40 B 20 junctions at sufficiently high temperature results in the formation of highly-oriented crystalline CoFeB electrodes that were initially amorphous in the as-deposited state, a crystallization process that is initiated at both the interfaces with MgO. When annealing a junction with a MgO thickness of around 20 Å at 375 C, this yields an optimal TMR of 260% at room temperature and 403% at T = 5 K. By varying the Ar pressure for sputter deposition of MgO (Ikeda et al., 2005), their optimized annealed junctions exhibit a room-temperature TMR of 355%, and 578% at T = 5 K. An additional improvement of the crystalline orientation of the Mg(001) layer may be achieved by introducing an ultrathin 4 Å Mg layer between the bottom CoFeB electrode and MgO (Tsunekawa et al., 2005). Especially for MgO(001) layers in the range between 7 Å and 11 Å the addition of Mg during growth as suggested by Linn and 22 44

95 Spin-Dependent Tunneling in Magnetic Junctions 95 Figure 1.49 TMR at T = 20 KandT = 293 K at low bias voltage as a function of the thickness of the MgO barrier t MgO. (b) Cross-sectional transmission electron microscopy of an MTJ with t MgO = 18 Å, using two different magnifications showing the excellent 17 crystallinity of the layers. The junction stack consists of MgO(001)/MgO seed/1000 Å Fe/t MgO/100 Å Fe/100 Å IrMn. After Yuasa et al. (2004b). 18 Mauri (2005) leads to a considerable enhancement of TMR and is typically well above 100% (Tsunekawa et al., 2005). These huge TMR values are accompanied by extremely small RA products of only a few µm 2, an attractive combination never reached in alumina-based junctions (see section 2.2, Fig. 1.17). Comparable giant TMR values (180% at room temperature, 250% at T = 20 K) have been demonstrated in single-crystalline (001)-oriented Fe-MgO-Fe-IrMn junctions grown by molecular beam epitaxy (Yuasa et al., 2004a, 2004b). Apart from the large magnetoresistances, the variation of TMR with the thickness of the MgO barrier shows a number of interesting features, see Fig Tostart with, on the average the TMR increases with the thickness of the MgO, and saturates beyond t MgO 20 Å. This is in qualitative agreement with the experiments by Hayakawa et al. (2005b) on sputtered junctions, and is also in line with the aforementioned predictions (MacLaren et al., 1999; Butler et al., 2001b; Mathon and Umerski, 2001). When the barrier is thick, the conductance is dominated by electrons with the momentum vector normal to the barrier (k 0), by which electrons in the highly polarized Fe- 1 band lead to giant TMR values. For thinner barriers the TMR effect is suppressed by the increasing probability of electrons tunneling with off-normal momentum vector. It is shown by Belashchenko et al. (2005b) from first principles that for small MgO thickness the minority spin bands at the interfaces make a significant contribution to the tunneling conductance in the antiparallel orientation of the Fe layers. In agreement with the data, this efficiently reduces the TMR in Fe-MgO-Fe for small t MgO. Additionally, in the experiments of Yuasa et al. (2004b) TMR versus t MgO is shown to clearly oscillate with a period of 3.0 Å over the full spectrum of barrier thicknesses (Fig. 1.49), not dependent on temperature or bias voltage. It is emphasized that the period is not corresponding to the thickness of one monolayer of MgO(001), i.e. 2.2 Å