Magnetically Engineered Spintronic Sensors and Memory

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1 Magnetically Engineered Spintronic Sensors and Memory STUART PARKIN, SENIOR MEMBER, IEEE, XIN JIANG, CHRISTIAN KAISER, ALEX PANCHULA, KEVIN ROCHE, AND MAHESH SAMANT Invited Paper The discovery of enhanced magnetoresistance and oscillatory interlayer exchange coupling in transition metal multilayers just over a decade ago has enabled the development of new classes of magnetically engineered magnetic thin-film materials suitable for advanced magnetic sensors and magnetic random access memories. Magnetic sensors based on spin-valve giant magnetoresistive (GMR) sandwiches with artificial antiferromagnetic reference layers have resulted in enormous increases in the storage capacity of magnetic hard disk drives. The unique properties of magnetic tunnel junction (MTJ) devices has led to the development of an advanced high performance nonvolatile magnet random access memory with density approaching that of dynamic random-access memory (RAM) and read-write speeds comparable to static RAM. Both GMR and MTJ devices are examples of spintronic materials in which the flow of spin-polarized electrons is manipulated by controlling, via magnetic fields, the orientation of magnetic moments in inhomogeneous magnetic thin film systems. More complex devices, including three-terminal hot electron magnetic tunnel transistors, suggest that there are many other applications of spintronic materials. Keywords Field sensor, giant magnetoresistance (GMR), magnetic engineering, magnetic random-access memory (MRAM), magnetic recording, magnetic tunneling junction (MTJ), magnetic tunneling, magnetoelectronics, magnetoresistance, oscillatory interlayer coupling, read head, spin-dependent transport, spin valve, spintronics. I. INTRODUCTION Conventional magnetic materials have long been used for various near-ubiquitous applications including electric motors and magnetic compasses and sensors [1]. In recent years a new class of magnetic materials has emerged based on the microscopic generation and manipulation of spin-polarized electrons in magnetic multilayered thin-film structures [2]. Manuscript received February 15, 2003; revised March 10, This work was supported in part by the Defense Advanced Research Projects Agency. The authors are with the IBM Almaden Research Center, San Jose, CA USA ( parkin@almaden.ibm.com). Digital Object Identifier /JPROC In particular, these materials can act as extremely sensitive magnetic field sensors, because their electrical resistance can change in the presence of magnetic fields at room temperature by factors much larger than are possible with conventional magnetic materials. In this article we will focus on two classes of novel materials, metallic magnetic multilayered structures and magnetic tunnel junctions, and their applications to magnetic information storage in the form of magnetic hard disk drives and magnetic random access memory [3]. We will also briefly discuss more complex devices based on these materials, in particular the magnetic tunnel transistor (MTT). In order to make technologically useful devices, these materials have to be magnetically engineered so as to control and tune their response to magnetic fields. The discovery of oscillatory interlayer coupling in 1989 in transition metal based magnetic multilayers [4] plus the phenomenon of exchange biasing discovered much earlier in 1959 [5] together form the basis of the magnetic engineering of many of today s most useful magnetic nanostructures. II. SPIN-VALVE MAGNETIC RECORDING READ HEADS In a magnetic recording hard disk drive information is stored by magnetizing regions within a magnetic thin film (see Fig. 1). The transitions between these regions represent bits which are detected, via their fringing magnetic fields, by the read sensor. The read sensor is part of a merged read write recording head which has a separate writing element. The recording head is attached to a small ceramic slider which is flown on an air bearing above the recording medium at a height of just a few nanometers. The number of magnetic bits per unit area, the areal density, has increased at compound growth rates (CGRs) exceeding 100% for the past several years (see Fig. 2). These astounding increases in storage capacity have been driven in large part by a new generation of magnetic recording read heads based on the phenomenon of giant magnetoresistance (GMR), which were /03$ IEEE PROCEEDINGS OF THE IEEE, VOL. 91, NO. 5, MAY

2 Fig. 1. Fundamentals of magnetic recording. Schematic diagram of a hard disk drive. (a) Information is stored by magnetizing regions of a thin magnetic film on the surface of a disk. Bits are detected by sensing the magnetic fringing fields of the transitions between adjacent regions as the disk is rotated beneath a magnetic sensor. As the area of the magnetized region has decreased, the read element has had to scale down in size accordingly. (b) The read element is incorporated into a merged read/write head, which is mounted on the rear edge of a ceramic slider flown above the surface of the rapidly spinning disk via a cantilevered suspension. A hard drive unit usually consists of a stack of several such head disk assemblies plus all the motors and control electronics required for operation (for more details see, for example, [7]). first developed and introduced by IBM in late 1997 [2]. GMR heads are now found in virtually all hard disk drives produced today. From the earliest days of magnetic recording more than 50 years ago [6] there have been a succession of predictions of the ultimate achievable recording density but so far these have always been surpassed. As the areal density has increased and the bit size correspondingly decreased many technological challenges have been met. These challenges [7] range from mechanical issues such as reduced flying height and contamination sensitivity, servoing and head stiction; read issues such as magnetic shielding and sensor sensitivity; write issues concerning field strength and speed; media issues including noise and data stability as bit size approaches the super-paramagnetic limit. One of the most important technological hurdles has been and continues to be 662 PROCEEDINGS OF THE IEEE, VOL. 91, NO. 5, MAY 2003

3 Fig. 2. Increases in areal density and shipped capacity of magnetic storage over time. Since the invention of magnetic disk recording in the 1950s, the areal density (bits stored per square inch) of disk drives has increased rapidly. Fueled by the development of the anisotropic magnetoresistance (AMR) and GMR read sensors, in recent years its compound growth rate (CGR) has exceeded 100%. Developments in the magnetic media have kept pace with those of the read sensor and the write head. For example, antiferromagnetically coupled (AFC) recording media allow the writing of narrower tracks on media which would otherwise be susceptible to the super-paramagnetic effect. The inset plot shows the total capacity of hard drives shipped per year; in 2002 that shipped capacity was 10 EB worth of data. [Data provided by Ed Grochowski (private communication)]. the need to extract ever more read signal from diminishing volumes of magnetic material. The read signal from a GMR recording head is one to two orders of magnitude bigger than that from prior generation state-of-the-art read heads [8] which were based on the phenomenon of anisotropic magnetoresistance (AMR) [9]. The resistance of conventional ferromagnetic metals varies as approximately where is the angle between the direction of the magnetic moment of the ferromagnet and the direction of current flow. The fringing fields from transitions in the magnetic media are then detected by changes in resistance of the read sensor element as the media is rotated at great speed under the sensor. These fields are quite small in the range of Oe. The active component of an AMR sensor is essentially an ultrathin magnetoresistive ferromagnetic layer (typically permalloy, ) whose plane is arranged to be orthogonal to that of the magnetic media. The sensing layer is sandwiched between and electrically isolated from two much thicker ultrasoft magnetic layers which act as shields to ensure that the sense layer measures flux from only one transition. The separation of these shields plus the thickness of the sense layer determines the spatial resolution of the read head. AMR, usually quite small just a few percent at room temperature, is a consequence of bulk scattering, and moreover, in thin films typically decreases with decreasing film thickness as scattering from the surfaces of the film becomes more important [10]. This modest magnitude of AMR led to predictions that areal densities would be limited to Gb/in. In contrast to AMR, GMR is a much larger effect of up to more than 100% at room temperature [11] and is dominated by interface scattering [12], [13]. The GMR effect was originally discovered in MBE (molecular beam epitaxy) grown epitaxial (100) oriented Fe/Cr/Fe sandwiches [14] and Fe/Cr multilayers [15] but the effects were quite modest at room temperature. Shortly afterwards it was discovered that similar effects could be found in polycrystalline sputtered Fe/Cr multilayers [4] and subsequently very large room temperature magnetoresistance was found in Co/Cu and related multilayers [11], [16]. These latter materials form the basis of GMR sensors and storage devices today. AMR and GMR are compared with other classes of structures and materials which display significant magnetoresistance in Fig. 3. GMR is a result of spin-dependent scattering in inhomogeneous magnetic metallic systems (for recent reviews see, for example, [17] and [18]). As originally discussed by Mott in the 1930s [19], [20] current in 3d transition ferromagnetic metals is carried independently by spin-up and spin-down electrons. According to this two-current model [21], [22] the scattering rates can be quite different for electrons within these two channels. In the most naïve model the current is considered to be carried predominantly by the low-mass electrons but the heavier electrons, which PARKIN et al.: MAGNETICALLY ENGINEERED SPINTRONIC SENSORS AND MEMORY 663

4 Fig. 3. Types of magnetoresistance. (a) AMR results from bulk spin-polarized scattering within a ferromagnetic metal; it is manifested as a dependence of the resistance on the angle between an applied external field and the direction of current flowing through the material. (b) Colossal magnetoresistance (CMR) results from interactions predominantly between adjacent atoms in certain crystalline perovskites. (c) GMR results from interfacial spin-polarized scattering between ferromagnets separated by conducting spacers in a heterogeneous magnetic material, such as a magnetic multilayer or granular alloy. (d) Tunneling magnetoresistance (TMR) in magnetic tunnel junctions results from spin filtering as spin-polarized electrons tunnel across an insulating barrier from one ferromagnet to another. (e) Anomalous MR from domain wall effects has been observed in single-crystal ferromagnetic Fe whiskers and patterned magnetic wires. Spin-polarized scattering of electrons as they cross from one domain to another leads to increased resistance in a GMR-like model, although both increased and decreased resistance has been observed in lithographically patterned magnetic elements in the presence of domain walls. (f) Ballistic MR (BMR) is another type of domain wall effect in the limit of very narrow constrictions where the conductance may be quantized. Very large BMR effects have been observed but their origin, whether from spin-polarized transport, or from magnetostrictive effects is controversial. give rise to the ferromagnetism, provide a spin-dependent reservoir of empty states into which these sp electrons can be scattered. Since the density of states (DOS) of and electrons at the Fermi energy are quite distinct this rationalizes the spin-dependent scattering rates. In an inhomogeneous magnetic system where the magnetic moment direction varies spatially the scattering rates for and electrons will also vary in space. In particular, as shown schematically in 664 PROCEEDINGS OF THE IEEE, VOL. 91, NO. 5, MAY 2003

5 Fig. 3(c), magnetic multilayers will typically have a lower resistance when the magnetic moments of the individual layers are parallel (P) than when antiparallel (AP) [23]. Defining the magnetoresistance as the largest GMR effects at room temperature of 110% have been found in sputter deposited Co/Cu multilayers [11]. For GMR the MR varies as the cosine of the angle between the moments of adjacent ferromagnetic layers. The largest GMR effects in magnetic multilayers are observed for structures containing the thinnest possible magnetic and nonmagnetic layers. This is because the GMR effect is dominated by spin-dependent scattering at the magnetic/nonmagnetic interfaces [12], [13] and, especially for current flow in the plane of the multilayer, thickening these layers largely results in shunting of current away from these interfacial regions [24]. In the early days of GMR the preponderance of groups argued that GMR was largely a consequence of spin-dependent bulk scattering within the interior of the ferromagnetic layers [25] and the dominant role of the interfacial contribution was not appreciated. Of course in structures with very thick ( 100) ferromagnetic layers, when the GMR effects are quite small, contributions from bulk scattering may compare with those from interface scattering. However, for technological applications the interfacial origin of GMR makes the effect much more useful because only very thin ferromagnetic layers are needed to support large GMR effects. In particular, demagnetizing fields associated with the magnetic moments of the ferromagnetic layers within these devices will increase with the magnitude of these magnetic moments so it is usually very important to minimize the volume of magnetic material. For devices of microscopic dimensions the self demagnetizing fields, which depend in detail on the shape of the device, will increase the magnetic field required to change the magnetic state of such devices. For sensors this will eventually limit the smallest fields which such devices can detect. For memory applications demagnetizing fields may lead to interactions between closely packed neighboring devices which will eventually limit the density of such memories. The understanding and control of demagnetizing fields in magnetic nanostructures is key to the fabrication of useful devices. III. MAGNETIC ENGINEERING Magnetic multilayers such as those shown schematically in Fig. 3(c) are not by themselves useful for most sensor or memory applications because the fields required to generate large changes in the resistance of such multilayers are typically large and, moreover, are very sensitive to the thickness of the nonmagnetic spacer layer. In addition, especially for sensing applications, the maximum sensitivity to small magnetic fields is obtained for particular orientations of the magnetic moments within the sensor. For AMR read sensors [see Fig. 4(a)] the moment of the magnetic layer must be set at an angle of from the direction of the current flowing through the layer. When the moment is parallel or perpendicular to the current direction, for small changes in the moment direction, there will be little change in resistance. Various ingenious schemes were developed to maximize the sensitivity of AMR sensors by controlling the current flow through the device and the quiescent moment direction [26]. Usually demagnetizing fields from a second soft ferromagnetic layer separated from the sense layer by a nonmagnetic conducting layer (e.g., Ta) would be used to help bias the magnetic moment of the sense layer at the correct angle. The moment of the bias layer was itself set by the self-field of the sense current passing through the device. The moment direction of the sense layer was then determined by a combination of the bias field, the magnetic anisotropy and shape demagnetizing fields of the sense layer and the self-field of the sense current [see Fig. 4(b)]. The magnitude of the magnetic fields required to saturate the change in resistance of magnetic multilayers with thin layers are very high and are determined by the intrinsic exchange coupling mediated through the nonmagnetic spacer layers [27]. The coupling strength was discovered [4] to oscillate between ferromagnetic (F) and antiferromagnetic (AF) coupling with increasing spacer layer thickness for almost all nonmagnetic transition metal spacer layers [28]. Companion oscillations in the magnitude of the GMR effect were also found [4]. These latter oscillations simply reflect the fact that for ferromagnetically coupled magnetic layers in symmetric multilayer structures no relative change in magnetic orientation of adjacent magnetic layers will result with the application of magnetic field. By taking advantage of the oscillatory interlayer coupling it was shown that multilayer structures could be spin engineered [29] or constructed in such a way that ferromagnetically coupled neighboring magnetic layers could be arranged to become antiparallel in intermediate magnetic fields. This allows both the measurement of the ferromagnetic coupling strength between these layers and the observation of a GMR effect from these layers [29]. For antiferromagnetically coupled magnetic multilayers the moments of adjacent magnetic layers naturally lie antiparallel in zero magnetic field. In a strong enough magnetic field these moments will eventually align with the field becoming parallel to one another and so resulting in a change in the resistance of the multilayer. The saturation field is directly related to the strength of the antiferromagnetic interlayer exchange coupling energy. The magnitude of the oscillatory interlayer coupling energy was found to vary systematically for magnetic multilayers with spacer layers formed from the 3d, 4d, and 5d transition metals in the periodic table, increasing in strength with d band filling and from the 5d to 4d to 3d elements [28]. For technological applications one of the most useful elements is Ru which not only displays relatively high interlayer oscillatory exchange energy but also, for certain ferromagnetic materials, displays antiferromagnetic coupling in the limit of ultrathin Ru layers as thin as 2 to 3 [4], [29]. For GMR sensors, where additional metallic layers will shunt and so dilute [24] the magnitude of the effect, using a very thin AF coupling layer is important. Moreover, Ru has other useful properties including excellent thermal stability and growth habits. PARKIN et al.: MAGNETICALLY ENGINEERED SPINTRONIC SENSORS AND MEMORY 665

6 Fig. 4. Comparison of AMR and GMR read sensors. (a) Geometry of read sensors: the AMR sensor comprises a magnetically soft adjacent layer (sal) separated via a spacer layer (spac) from the free ferromagnet layer (ff), which is exchange biased by antiferromagnetic material (af) at the ends of the sensor element only. Current is passed parallel to the layers via a pair of current leads. The GMR sensor comprises a free ferromagnet layer spin-coupled through a spacer layer (spac) to a pinned ferromagnet layer (pf) which is exchange biased by an entire layer of antiferromagnetic material (af). As in the AMR sensor, current is passed parallel to the layers via current leads. (b) Mode of operation of AMR sensor. The rate of change of resistance with respect to field in an AMR structure is greatest when the moment of the sense layer ff is at an angle of 45 to the direction of the sense current in the absence of any applied external field. This angle is achieved by careful engineering of the combined effects of shape anisotropy of the element, exchange bias from the partial antiferromagnet layer af, and the demagnetization field from the moment of the soft adjacent layer sal induced by the self field of the sense current flowing through the sensor. Introducing an external field such as the fringing fields at a bit transition in the recording medium perturbs the angle of the moment of the sense angle away from the balanced 45 angle; the resistance of the sense element changes due to AMR and, hence, the voltage drop across the sensor changes in response to the field change. Resistance of the AMR sensor varies as cos (). Deconvoluting the voltage changes with respect to time yields the desired data stream. (c) Mode of operation of a GMR sensor. The initial angle of the moment in the GMR sense layer ff is the vector sum of the effects of shape anisotropy, magnetostatic coupling to the pinned layer pf and the self field of the sense current. The angle of the moment of the pinned layer pf is set by exchange bias from the antiferromagnetic layer af. As with the AMR sensor, introducing an external field perturbs the angle of the moment and changes the resistance of the sense element, which is detectable as a voltage change and, hence, enables one to read the recorded data. Note that the GMR effect, however, changes as cos() and has a maximum 1R=R much greater than the AMR effect, making the GMR sensor sensitive to much smaller field changes. Hence, using GMR enables the detection of smaller recorded bit patterns than the AMR sensor. In order to make useful magnetic field sensors the magnetic moments of the sensor must respond in a highly reproducible and nonhysteretic manner to magnetic field. It is not easy to control the magnetic field response of the many magnetic layers in a multilayer structure, and for magnetic recording read heads, in particular, the thickness of such multilayers precludes their use for high density recording applications because of the narrow gap between magnetic shields. For sensing large magnetic fields over relatively large areas, such as for position sensors or rotation sensors for automobile antilock brakes, magnetic multilayers may be useful [30]. For the detection of the comparatively small magnetic fields in magnetic recording read heads the most useful GMR device is a spin-valve sandwich structure [31]. The simplest form of this device, which is shown schematically in Fig. 5(c), is a sandwich of two ferromagnetic layers separated by a thin Cu layer, where the Cu layer thickness is chosen to be sufficiently thick that the oscillatory interlayer exchange coupling field is weak. This field, which varies inversely with the ferromagnetic layer thickness, can be made smaller by increasing the thicknesses of these layers, and also by engineering the thickness of Cu to be interme- 666 PROCEEDINGS OF THE IEEE, VOL. 91, NO. 5, MAY 2003

7 Fig. 5. Evolution of magnetically engineered multilayers (a), (b) The easy axis of the free ferromagnetic layer in a magnetoresistive device is oriented based on the purpose for which it is engineered. Field sensor devices such as read heads rely on a free layer with an easy axis at right angles to the moment of the pinned layer. Impinging magnetic fields will rotate the moment away from this position and the sensor resistance changes. On the other hand, MR devices designed for use in memory applications will have a free layer easy axis parallel or antiparallel to that of the pinned layer. (c) A very basic GMR/TMR (tunneling magnetoresistance) stack consisting of a pinned ferromagnetic layer magnetically locked by exchange-bias to the interfacial field of an antiferromagnetic layer, and a simple ferromagnetic free layer. The spin valve is such a stack using a conducting spacer layer between the ferromagnetic layers. (d) In this case the pinned layer is an element consisting of a pair of ferromagnetic layers antiferromagnetically coupled through a ruthenium (Ru) spacer layer; the lower layer in this artificial antiferromagnet is pinned via exchange bias as in (c). This flux closure increases the pinned layer magnetic stability and reduces coupling to the free layer. (e) Pinned element consists of an AF-coupled pair of ferromagnetic layers acting as a single hard layer. No exchange bias layer to discourage rotation of the pinned element. (f) Both the pinned and free elements consist of AF-coupled pairs. (g) A double tunnel junction. All ferromagnetic elements consist of AF-coupled pairs. There are two pinned ferromagnets, both exchange biased by antiferromagnetic layers. Spin filtering occurs both as current tunnels from the first pinned layer to the free element and again as it tunnels from the free element to the second pinned element. diate between those which give rise to AF and F coupling [2]. At these Cu thicknesses, corresponding to nodes in the oscillatory coupling, another form of interlayer exchange coupling, biquadratic coupling, is sometimes observed in which the magnetic layers want to align themselves perpendicularly to one another [32]. Biquadratic coupling was first PARKIN et al.: MAGNETICALLY ENGINEERED SPINTRONIC SENSORS AND MEMORY 667

8 observed in Fe/Cr/Fe (100) sandwiches [33]. Several models have been proposed to account for this effect including models in which interface roughness leads to competing F and AF interactions so favoring an orthogonal orientation of adjacent magnetic moments [34], [35]. The magnetics of both AMR and GMR sensors is often engineered by means of the phenomenon of exchange bias [36], [37]. Exchange bias, first documented in the 1950s [5], involves the stabilization of the moment of a ferromagnetic layer by coupling it to an antiferromagnetic (AF) layer. The AF layer gives rise to an internal exchange field at the interface with the adjacent F layer whose direction is fixed by cooling the F/AF couple from above the Néel temperature of the AF layer in a magnetic field sufficiently large to magnetize the F layer in one direction. When the bilayer is cooled below the ordering temperature of the AF layer, it is now believed that the spins in the AF layer are blocked in AF domains with very small uncompensated net moments in the direction of the F layer so giving rise to a net magnetic bias field [38] [40]. The exchange coupling of these moments with that of the adjacent F layer means that the F layer exhibits a unidirectional magnetic anisotropy so that its magnetic moment direction is fixed in one direction. Since the exchange coupling is at the F/AF interface, the external magnetic field required to rotate the moment of the F layer increases inversely with the magnetic moment (i.e. thickness) of the F layer. Exchange bias has been used in magnetic recording read heads for many years as a means of stabilizing the magnetization within the sensing layer so as to reduce Barkhausen noise [41]. The use of exchange bias in AMR heads is illustrated in Fig. 4(a) and (b). Both ends of the ferromagnetic sensing layer are covered with an AF exchange bias layer which prevents the ends of the sense layer from forming flux closure domains. The magnetic structure of these regions may otherwise change irreversibly in the presence of field or due to thermal fluctuations which can introduce significant noise during the read process. In a spin-valve device one of the magnetic layers, the reference layer, is pinned by exchange bias with an antiferromagnetic layer [42]. Provided that the external field is much weaker than the exchange bias field, the magnetic moment of the reference F layer is essentially fixed so that only the free or sensing layer in the sandwich responds to external fields. Optimizing the sensitivity of GMR sensors for the detection of small magnetic fields is actually somewhat more straightforward than that of AMR sensors. The optimal sensitivity (maximum relative change in resistance per unit field) is realized when the magnetic moments of the adjacent layers in a spin-valve device are orthogonal to one another in zero external field as illustrated in Fig. 4(c). In a magnetic recording disk drive the reference F layer is exchange biased in a direction perpendicular to the surface of the magnetic medium and the sense layer moment is designed to be parallel to the medium. In advanced recording read heads today, which are deep submicrometer in size, there will be significant magnetic coupling of the reference and sense layers through the demagnetizing fields generated from magnetic poles at the edges of the magnetic layers. These fields favor antiparallel coupling of these layers. In addition, because of roughness, especially correlated roughness, of the layers in the spin-valve sandwich, dipolar fields derived from microscopic troughs and valleys at the reference F/Cu interface will couple to valleys and troughs in the opposing Cu/sense F interface, giving rise to a Néel dipolar coupling field which favors parallel alignment of the F layers [43], [44]. Finally, the sense current itself will generate self-magnetic fields which couple the sense and reference magnetic layers. In spin-valve read heads these three sources of magnetic interactions are carefully balanced so that the sense layer moment is optimally arranged with respect to that of the reference layer moment. For areal densities in today s magnetic recording disk drives of close to Gb/in the read sensor element has a width of only 100 nm and a height about times smaller than its width of 70 nm. These tiny dimensions make the manufacture of spin-valve sensors a tour de force. Moreover, these tiny dimensions lead to significant magnetic shape anisotropies resulting from the self-demagnetizing fields of the magnetic sense layer (and reference layer) which must also be taken into account in engineering the optimal operating point of the spin-valve sensor. As mentioned above, magnetostatic interactions within a spin-valve device are very significant and, as these devices are shrunk further to enable the detection of ever smaller magnetic bits at ever higher recording densities, these interactions become ever larger (typically these interactions will scale with the inverse of length and width of the sensor). In the simplest spin-valve sensors [Fig. 5(c)], even for thicknesses of the sense and reference layers of as little as 2 4 nm, magnetostatic interactions will give rise to such significant magnetic coupling between these layers that the device is inoperable. The solution to this problem proposed by Parkin [45] is to replace the reference layer with an artificial antiferromagnetic (AAF) layer comprised of a sandwich of two F layers antiferromagetically coupled via an ultra thin Ru layer as shown in Fig. 5(d). The net moment of the reference layer can be set to an arbitrarily small value by varying the relative thicknesses of the two F layers in the AAF reference layer. The AF coupling strength through Ru layers as thin as is very large and is about 1% of the ferromagnetic exchange coupling within the F layers themselves. Thus, the magnetic field required to overcome the AF coupling through Ru can be many tesla depending on the thicknesses of the F layers. A second most important benefit of the AAF reference layer is that the exchange bias field of the reference layer is significantly increased, roughly in proportion to the reduced moment of the reference layer. This makes the reference moment considerably more magnetically stable. All spin-valve read sensors have used this AAF structure since their first introduction by IBM in magnetic disk drives. As shown in Fig. 5(e) one can envisage building spinvalve devices without an AF exchange bias layer at all by using an AAF reference layer. For ultrahigh-density magnetic recording where the gap between the magnetic shields 668 PROCEEDINGS OF THE IEEE, VOL. 91, NO. 5, MAY 2003

9 Fig. 6. Standard versus AFC media. (a) Standard disk recording media. Single layer of ferromagnetic material (one section is depicted as granular in this illustration). Demagnetization fields at bit transitions can be strong enough to spontaneously remagnetize domains if the bits are small enough for thermal effects to destabilize them (the so-called superparamagnetic limit) (b) AFC media. Two layers of ferromagnetic material separated by a thin ruthenium coupling layer. Domains in the top layer are stabilized by mirror-image domains in the bottom layer, allowing smaller bit patterns to be thermally stable. must be further reduced, thinning the spin-valve sensor is extremely important. Since the exchange bias layer is usually quite thick relative to the active portion of the device (AAF/Cu/F) eliminating the AF layer would be a significant advantage. Moreover, the AF layer, which is typically comprised of a Mn alloy such as Pt Mn or Ir Mn, may limit the thermal stability of the spin valve device and so limit the maximum processing temperature during the manufacture of the magnetic recording head. The AAF structure may also be used for the sense layer itself as shown in Fig. 5(f). Another important application of magnetic engineering based on oscillatory interlayer exchange coupling through Ru is for high-density magnetic recording media. As shown schematically in Fig. 6 conventional longitudinal magnetic media is comprised of a finely grained high coercivity magnetic alloy typically based on CoPtCr with additional additives such as B and Ta [46], [47]. The Pt is added to Co to increase its magnetic anisotropy. The seed and underlayers on which the media is deposited are very important and control the detailed microstructure of the magnetic storage layer. One magnetic bit is designed to be comprised of many, typically 1000, individual grains whose magnetocrystalline anisotropy easy axes will vary randomly in direction within the plane of the media. If the number of grains is too small per magnetic bit (whose typical bit length is 100 nm) this will lead to significant variations in signal from bit to bit. Thus, the grains must be very small compared to the bit length. As the bit length is shrunk the grains must correspondingly shrink. Similarly, the thickness of the magnetic media must also be reduced in order to properly scale the magnetic recording system and allow for smaller magnetic bits [7]. This means that the net volume of magnetic material per grain and, thus, the magnetic anisotropy energy will also be reduced. Thermal fluctuations will eventually cause destabilization of the magnetic moment of individual grains when the energy barrier to the rotation of these moments provided by the magnetocrystalline anisotropy energy becomes too small [47]. The magnetic anisotropy energy can be increased by modifying the composition and microstructure of the magnetic media but this will require higher writing fields. However, the maximum possible writing fields provided by the recording write head are limited by the magnitude of the magnetization of suitable magnetic materials. To overcome the super-paramagnetic limit the magnetic media can be formed from an AAF structure [47] which allows for a smaller net magnetic moment per grain while allowing for a greater volume of magnetic material and consequently magnetic anisotropy in the upper magnetic layer of the AAF structured grain [see Fig. 6(b)]. IV. MAGNETIC TUNNEL JUNCTIONS The areal density of magnetic recording disk drives continues to increase at such a high rate that alternatives to GMR spin valve sensors may be needed. The maximum useful magnetoresistance provided by spin-valve recording read sensors today is about 16% 18%. The very largest GMR so far observed in a spin-valve device at room temperature is 24% [48]. Much larger MR values are found in magnetic tunneling junctions (MTJs) [3]. A MTJ is similar to a spin-valve device but the metallic Cu spacer is replaced by a very thin insulating tunnel barrier and the sense current is passed perpendicularly through the device [49] [51]. Tunneling magnetoresistance (TMR) values as high as 50% or more have been found at room temperature in devices with alumina tunnel barriers [3], [52], [53]. The perpendicular current flow through a tunnel junction device is extremely attractive for ultra high density magnetic recording since this means that the sensor can be directly attached to the magnetic shields which can then be used as the electrical contacts to the device [54]. By contrast, in conventional GMR sensors where the current flows parallel to the layers in the device the sensor has to be electrically isolated from the conducting magnetic shields. The isolation layers occupy space between the shields thereby limiting the ultimate recording density which such sensors can support. As the sensor size is further reduced it may eventually be possible to use metallic GMR sensors in a CPP (current perpendicular to the sensor plane) geometry since the resistance of such devices increases inversely with the cross-sectional area of the device. The resistance of CPP GMR sensors with areas of m corresponding to Gb/in is perhaps times too low to provide enough read signal today. However, the resistance of conventional CPP GMR devices can be increased by, for example, partially oxidizing layers within the structure [55]. It may also be possible to replace the metals currently used in GMR spin-valve device PARKIN et al.: MAGNETICALLY ENGINEERED SPINTRONIC SENSORS AND MEMORY 669

10 with less conducting materials whilst maintaining high GMR values. TMR sensors have some advantages over GMR sensors, namely much larger changes in resistance with magnetic field and improved compatibility with ultra high density recording read heads since they are naturally CPP devices. However, by contrast with CPP GMR devices the resistance of TMR devices is currently too high to allow them to be useful for high performance, high data rate disk drives. The lowest resistance TMR junctions have resistance-area (RA) values of as little as m which means that such sensors may be suitable for disk drives with areal densities of Gb/in. Whereas scaling to even higher areal densities makes CPP GMR sensors more attractive, MTJ devices become less attractive as their resistance, for otherwise the same RA, is further increased. V. SPIN POLARIZATION OF TUNNELING CURRENT Both GMR and MTJ devices may be considered as spintronic devices but whereas GMR is based on spin-dependent elastic scattering of electrons at the Fermi energy, magnetic tunneling truly concerns the generation, manipulation and detection of spin polarized current. In the simplest model of spin-dependent tunneling, the TMR of an MTJ depends on the magnitude of the spin polarization of the current tunneling through the device [49]. The spin polarization P is related to the density of states (DOS) of majority and minority spin-polarized electrons in the ferromagnet so that. If one assumes that the tunneling probability is simply proportional to the corresponding DOS, as proposed by Julière, then the TMR, defined as, is given by where and are the polarizations of the two ferromagnetic layers in the MTJ [49], [56]. In this simple model the TMR is bigger when the spin polarization is larger. What is not entirely clear is the meaning of the term spin polarization. Clearly, tunneling matrix elements, which depend on the symmetry of the electronic states in the ferromagnetic metals and the detailed band structure of the insulating tunnel barrier, should influence the tunneling probability and this will depend on the spin state of the tunneling electrons [57], [58]. Similarly, the dependence of the tunneling probability on the thickness of the tunnel barrier might be expected to depend on the symmetry of the electronic states in the ferromagnetic layers so that not only the magnitude but even the sign of the polarization of the tunneling electrons might vary with tunnel barrier thickness [59]. Another important question is the extent to which the spin polarization is determined by the F/ tunnel barrier interface electronic states. None of these questions have been satisfactorily answered so far. One problem with tunnel junctions is the degree to which the formation of the tunnel barrier affects the properties of the underlying ferromagnetic electrode. For example, the most common and useful tunnel barrier is Al O which is typically formed by first depositing a thin Al layer and subsequently oxidizing this layer [60]. Oxidation can be carried out using a wide variety of methods ranging from energetic plasmas to natural oxidation by exposure to air or oxygen. However, all of these methods may result in partial or even complete oxidation of the underlying ferromagnetic electrode which will strongly influence the spin polarization and TMR of such devices. For example, it is predicted that an Fe vacuum interface will have an opposite spin polarization to an Fe O interface [61]. The spin polarization can be probed by a variety of techniques including photoemission [62], point contact Andreev reflection (PCAR) [63] and superconducting tunneling spectroscopy (STS) [60]. Photoemission measures the overall spin polarization integrated over the DOS at the surface of a ferromagnet. Andreev reflection occurs at the interface between a metal and a superconductor (SC) and results in an increased conductance for bias voltages within the superconducting energy gap as compared to the conductance for bias voltages exceeding the superconducting energy gap [64]. When the metal is ferromagnetic and the current is spin-polarized the increased conductance is suppressed and the degree to which it is suppressed is a measure of the spin polarization of the ferromagnet [63]. Somewhat related is the technique of tunneling from a F metal into a SC, whereby the tunneling conductance is measured in a ferromagnet insulator superconductor (FIS) structure where the ferromagnet and superconductor are separated by a thin insulating layer. This technique probes the polarization at the Fermi Energy with a contribution from the various bands that take part in the tunneling conduction weighted by the corresponding tunneling matrix elements [65]. Although each of these techniques is a measure of the spin polarization of the ferromagnet s electronic states, each is related to the polarization in different ways so it cannot be expected that their results can be directly compared to each other. From a practical standpoint, the spin polarization inferred from an STS measurement, which we will call the tunneling spin polarization (TSP), is most closely related to the TMR of an MTJ because of the similarity of the structures involved. Hence the TSP is most relevant to spin dependent tunneling and can be used to calculate the TMR in related MTJ s using Juliére s model mentioned above [49]. Spin dependent tunneling from a ferromagnet into a superconductor was first studied by Meservey and Tedrow in the early 1970s using Al superconducting electrodes [60]. The superconducting transition temperature of Al must be sufficiently high that the Al layer remains superconducting with a well defined energy gap in the presence of large magnetic fields of the order of 1 2 T. The SC transition temperature of Al can be increased by depositing Al layers in a poor vacuum [60] or, alternatively, by adding small amounts of solutes, such as Cu or Si, to the Al [66]. The latter is preferable because a poor vacuum will likely contaminate the F Al O interface affecting the spin polarization associated with this interface. When the Al layer is sufficiently thin it will remain superconducting when uniform magnetic fields of the order of 1 T are applied exactly in the plane of the film. The field Zeeman spin-splits the quasiparticle DOS in the SC so that the SC can be used as an analyzer for the spin polarized tunneling current from the F layer in the FIS junction. Typical STS data are shown in Fig. 7 for which the tunneling 670 PROCEEDINGS OF THE IEEE, VOL. 91, NO. 5, MAY 2003

11 Fig. 8. TSP (lines) and saturation magnetization (symbols) for various alloys of Fe, Co, and Ni. Magnetization data from [116]. Fig. 7. Conductance curves (open symbols) and corresponding fits (lines) for Co Fe in normal (A) and inverted structure (B), in an applied field of 1 T. In the normal structure the SC forms the bottom electrode whereas in the inverted structure the ferromagnet (FM) is on the bottom. The TSP extracted from fits to the data are included in the figure and are the same for both structures within the experimental uncertainty. conductance is measured as a function of bias voltage for voltages up to mv. In zero magnetic field, the conductance curves are symmetric with respect to positive and negative bias voltage and show typical BCS like peaks in conductance on either side of the superconducting tunneling energy gap [67]. In a magnetic field the conductance curves become highly asymmetric (see Fig. 7) and display two sets of peaks corresponding to largely independent tunneling of majority and minority electrons. The larger the asymmetry, the larger is the spin polarization of the ferromagnetic metal. The magnitude and sign of the TSP are extracted by fitting the conductance curves using the superconducting DOS derived by Maki which takes into account depairing due to the magnetic field and mixing of the spin channels due to spin orbit scattering in the superconductor [68]. The polarization thus can only be measured using this technique at temperatures well below the Al superconducting transition temperature and in high fields T. In this regard MTJs are useful devices to explore spin polarization values over a much wider temperature range. Fig. 8 shows a summary of TSP values obtained from STS studies for several Co Fe and Ni Fe alloys using an Al O tunnel barrier [68]. The TSP is large and is about 45 55% for all alloys except for those with high Ni content. The sign of the TSP is consistent with majority spin polarized tunneling current in all cases. Also plotted in Fig. 8 are the corresponding saturation magnetic moments per atom. Interest- ingly, these recent results differ significantly from the original results of Meservey and Tedrow who found an approximately linear relationship between the TSP and the corresponding magnetic moment for various families of Ni-based ferromagnetic alloys [60]. Such a relationship is difficult to understand given that the TSP and magnetic moment have quite different physical origins. While the TSP depends on the spin-polarized DOS very near the Fermi energy, weighted by appropriate tunneling matrix elements, the magnetic moment depends on the integrated DOS below. Indeed, recent measurements suggest that this proposed relationship between TSP and magnetic moment is accidental and that the decrease of the TSP for alloys with high fractions of Ni is most likely due to the difficulty of forming a clean interface with Al O barriers [69]. The extreme sensitivity of TSP and TMR to the F/barrier interface means that it is very important to prepare high-quality junctions with well-defined interfaces. In addition to alloys of Co, Fe, and Ni, Meservey and Tedrow also measured the spin polarization of various ferromagnetic rare-earth metals using an Al O barrier [70]. They derived small positive spin polarizations for Gd ( 13%), Tb ( 6%), Dy ( 6%), Ho ( 7%), Er ( 5%), and Tm ( 3%). A promising class of materials that are of great interest for use in tunneling devices are the so-called half-metallic ferromagnets. These ferromagnets are characterized by having an absence of either majority or minority states at the Fermi energy E so that the tunneling current should be 100% spin polarized at least at low bias voltages. Thus, MTJs formed with two half-metallic electrodes should display an infinite resistance in the antiparallel state. Among those materials that are predicted to be half metals are CrO [71], Fe O [72], [73], Heusler alloys, such as NiMnSb and PtMnSb [74], and the manganites, for example, La Sr MnO [75]. There have been efforts to measure the spin polarization of many of these materials using all of the various techniques mentioned above. For STS studies the tunneling current will depend sensitively on the ferromagnet/barrier interface [76], [77] so that it is clearly very important that the interface reflect the half-metallic property predicted for the bulk material. For PARKIN et al.: MAGNETICALLY ENGINEERED SPINTRONIC SENSORS AND MEMORY 671

12 Fig. 9. Transistor-switched MTJ memory. In this cross section, one element of an array of transistor-switched memory cells is depicted. The Write Word line and Read Word (Control) line extend in and out of the plane of the figure. The MTJ element of the cell is above the word line, connected to the bit line. The switching transistor is embedded in the substrate of the cell. example, Tanaka et al. measured a TSP of only 28% for NiMnSb [78], which is lower than most conventional ferromagnets. However, it is reasonable to suppose that the electronic structure of the interface may well be very different from that of the bulk, or that there is a change in stoichiometry or crystal structure at the interface. Worledge et al. investigated the TSP of La Sr MnO using an SrTiO (STO) barrier [79] and found a value of 72%. Indeed, several years ago very high TMR values were reported at low temperatures in MTJ s with manganite electrodes and an STO or barrier [80] [82] with TMR values of more than 970% which corresponds to a TSP of 91% [82]. More recently, Parker et al. reported TSP measurements on CrO which showed nearly complete spin polarization with no evidence for minority spin states [83] although MTJs with CrO electrodes and Co counterelectrodes have shown only small TMR effects ( 25%) corresponding to modest TSP values. These results clearly indicate that there is no fundamental limit to the magnitude of tunneling spin polarization and tunneling magnetoresistance. This is an important distinction from GMR for which the maximum MR is limited by shunting and other similarly prosaic effects. However, the practical use of many of these complex half-metallic materials in MTJ devices is a significant challenge, as their preparation is typically difficult and their Curie temperatures are quite low in most cases. Moreover, typically it is found that the spin polarization and corresponding TMR decrease with increasing temperature to very small values well below the corresponding Curie temperature. In addition to measuring and comparing TSP values for different ferromagnetic materials, the STS technique can help in other ways to improve our understanding of spin dependent tunneling. In particular, by placing the superconducting electrode either above or below the tunnel barrier, the STS technique can yield information about the nature of each of the ferromagnet/barrier interfaces independently. Since, for example, Al O tunnel insulating barriers are usually formed by oxidation of a thin Al layer it is crucial to avoid over- or under-oxidation of the Al as this will likely otherwise lead to a reduction of TMR. Fig. 7 shows conductance curves for FIS structures with Co Fe as either the top or bottom electrode. These data show that the measured TSP is the same for both interfaces within experimental uncertainty, which means that for Co Fe the bottom interface can be prepared with the same quality as the top interface. By contrast for Ni based alloys the TSP is slightly lower when it is deposited as the bottom electrode. This supports the aforementioned view, that it is difficult to prepare a good interface between Al O and Ni-based alloys. VI. MTJS CONTAINING AN Fe O ELECTRODE Fe O (magnetite) is predicted theoretically to be a negatively spin polarized half-metal [84] and there is evidence from photoemission that Fe O has a high degree of spin polarization [85]. Among the half-metallic ferromagnets is attractive for applications due to its high curie temperature of 858 K, and reasonable switching characteristics. The material belong to the ferrite spinel class of compounds sometimes written as FeO Fe O. Within this structure, there are two coordination sites for Fe ions, octahedral and tetrahedral. The tetrahedral ions are Fe while the octahedral ions are half filled with Fe and half with Fe. Only the octahedral coordinated ions are involved in the conduction process. Magnetite is, in fact not a ferromagnet, but rather is ferrimagnetic with the magnetic moment of the octahedral and tetrahedral sites opposite to one other. 672 PROCEEDINGS OF THE IEEE, VOL. 91, NO. 5, MAY 2003

13 As mentioned above most elemental ferromagnets show majority spin polarization. Intrinsically negative spin polarization is rare in magnetic materials but is potentially useful for building interesting spintronic devices which require both positive and negative spin polarizations, thereby simplifying the magnetic engineering required. The negative spin polarization in magnetite is understood by recognizing the fact that due to Hund s rule the addition of an electron to the half filled d band of the Fe will have the opposite spin polarization to the underlying t and e electrons. It is this additional electron which is involved in transport in bulk magnetite, and predominantly involved in tunneling in MTJ s. The transport mechanism within the magnetite is one of thermally activated hopping from an Fe site to an Fe site. As the temperature is lowered below 120 K in bulk material, the conductivity decreases by two orders of magnitude, concomitant with a lattice distortion. It has been proposed that this phase change, the Verwey transition, is due to charge ordering of the spin down t electrons. Previous work on integrating magnetite into tunnel junctions has met with limited success. Li [86] explored MTJs with Fe O electrodes but, however, observed tiny MR effects. Seneor et al. [87] claim to have grown MTJ s with an Fe O electrode with a Co counterelectrode and an amorphous aluminum oxide barrier. However, they measure positive MR which is contrary to theoretical predictions. One major obstacle to the integration of magnetite into devices is that other phases of iron oxide readily form. In particular, Fe O is easily formed, which in the gamma phase (maghemite) is ferromagnetic, and in the alpha (hematite) is antiferromagnetic. Similarly, a mixed phase iron oxide can also readily form, for which the polarization is likely to be positive and/or very small. MTJs containing magnetite can be formed by first depositing a series of metals such that the exposed surface is bcc Fe (100). Then, by reactively depositing Fe in an oxygen atmosphere, or by reactively depositing Al by depositing Al in an oxygen-containing atmosphere directly onto this exposed surface, (100) Fe O is formed. An MTJ is then completed when a barrier and ferromagnetic counter-electrode are deposited on top of the magnetite layer: negative TMR values of more than 15% have been seen at room temperature. This is the first observation of a significant inverse tunneling magnetoresistance effect in a magnetoelectronic device at room temperature [88]. MTJs with magnetite electrodes exhibit very large increases in resistance upon cooling of a factor of twenty or more, which is consistent with the loss of thermally activated conduction in the Fe O electrode at the Verwey transition. This is in stark contrast with typical MTJ s where the resistance only increases by about 20% 50% between room temperature and 4.2 K. VII. MTJ AND GMR MAGNETIC RANDOM ACCESS MEMORY (MRAM) Another potential application of magnetically engineered structures is for nonvolatile MRAMs based on magnetic storage elements. In recent years MRAM technologies have favored arrays of individually patterned magnetic storage cells or bits where each bit comprises a magnetic thin-film multilayered structure. The magnetic bit is designed to have two stable magnetic states in zero and small magnetic fields which, usually, exhibit two different resistance values representing 0 and 1. Until recently such bits involved the use of the comparatively small AMR effect. While some of these structures are very ingenious [89], these memories have been, not only of comparatively poor performance, but very expensive, and thus limited in their application. Replacing AMR bit structures with GMR spin-valve bit structures [90] suggests some obvious advantages. Firstly, just as for magnetic field sensor applications, the magnetic states of the GMR bit cells are much simpler (see Fig. 5). Secondly, the larger GMR effect should give rise to larger signals. Since, in a first approximation, the time required to read the state of the bit cell depends on the magnitude of the difference in signal between its two possible states, this is clearly an advantage. Moreover, the signal from a GMR cell in an appropriately designed MRAM can be sufficiently large that the bit may be read nondestructively that is, without changing its magnetic state. This offers, in addition to the speed advantage, lower power consumption because the bit does not have to be rewritten every time it is read. This is a definite advantage, especially for mobile applications where battery life is a critical consideration. However, even the larger signals available from GMR structures do not make GMR MRAM attractive for mainstream RAM applications. The conductive nature of a GMR structure necessitates that they be wired in series with one another. In order to achieve reasonable memory array densities many GMR cells (of number N) would have to be electrically connected in that series; the actual signal available when reading any one particular cell is MR/N. This signal is not sufficient to make GMR MRAM competitive with conventional dynamic random access memory (DRAM) and static random access memory (SRAM) [91]. On the other hand, the fundamentally highly resistive nature of the barrier in a magnetic tunnel junction allows one to fully utilize the high MR signal from individual MTJ storage cells via a novel cross-point MRAM architecture [3], [92]. By connecting each MTJ element in series to a switch, for example, a silicon diode, current only passes through a single MTJ cell in such an arrangement and the available signal when reading that cell is MR/1. With reasonable MR values, such an MRAM architecture has the potential to rival that of DRAM in density, and SRAM in speed. A key advantage of an MTJ over a GMR device in memory applications is that current is passed perpendicularly through the MTJ. The electrical contacts to the MTJ cell thus essentially occupy the same space as the MTJ device itself making the cell very small. Just as for the spin-valve structure, the free layer is likely to be comprised of a bilayer where the interface layer is chosen to give maximum tunneling magnetoresistance and the remainder of the free layer chosen for small magnetorestriction or other properties for optimal magnetic switching characteristics of the free layer or op- PARKIN et al.: MAGNETICALLY ENGINEERED SPINTRONIC SENSORS AND MEMORY 673

14 Fig. 10. Reading and writing from MTJ memory array. (a) Reading a bit. Voltage is applied to the desired Read Word line (to enable the transistors in that word) and voltage at the desired Bit line is measured. The transistor only turns on to provide current (with a resultant voltage) if the MTJ in the cell is in the low-resistance (parallel moments) state. (b) Writing bits. Current is passed through the desired Write Word line and appropriate Bit lines. Superposition of the fields generated by the two currents orients the moment of the free layer in the MTJ in the desired direction. The polarity of the moment is determined by the direction of the current in the Bit line; current flowing one direction will flip it one way, and current in the opposite direction will flip it the other way. timal characteristics for processing of the MTJ memory cell. Again, just as for the spin-valve GMR structure discussed above there are many advantages to using an AAF reference layer. In addition, the switching characteristics of the cell can be further tuned by using an AF-coupled structure for the free layer, or even by building a double tunnel junction utilizing AF-coupled elements for both pinned ferromagnetic elements and the free ferromagnet (see Fig. 5). Although, in principle no switch is needed, and a MTJ memory could be constructed with all the MTJ cells in the array connected in parallel, this leads to sneak currents passing through all the MTJ cells in the array which consumes more power and reduces the available signal [91], [93]. In Figs. 9 and 10, a more sophisticated design is shown where an array of MTJ memory elements are shown, connected in parallel to a set of upper bit lines. Directly below but electrically isolated from the MTJ is a write word line orthogonal to the bit line. The other electrode of the MTJ is connected to the source of a transistor element in a layer below the write lines; the gate of this transistor is connected to a read word line that is parallel to the write word line above. The metal bit and word lines are conductors through which electrical current can be passed. A unique combination of orthogonal bit and write word lines [94] can be selected in order to be able to individually address one magnetic memory cell to set or write its magnetic state, by passing currents simultaneously along the corresponding write word and bit lines (only one of the word and bit line currents need be bipolar). The vector combination of the orthogonal magnetic self-fields of the currents or current pulses passed through these lines is arranged such that the magnetic state of the selected memory element at the intersection of the chosen bit and word lines can be appropriately set. The polarity of the written state is selected by the direction of the current in the bipolar line. It is important that the self-fields of these same currents must be such that the magnetic state of the half-selected devices along the same bit and word lines is not altered; these latter cells will, however, be magnetically disturbed and it is very important that even after many such disturbances the magnetic state of these cells does not creep either to some intermediate state or completely reverse. The half-selected devices are also more sensitive to thermal fluctuations through the superparamagnetic effect. Careful engineering of both the exchange bias and the shape anisotropy of the MTJ cells is necessary to eliminate such instabilities. Reading memory cells within the array is similar to writing in that a particular cell is addressed by selecting the appropriate bit line and read word line. In reading, however, the read word line, which is the gate terminal of the transistor connected to the MTJ, is simply raised to a voltage sufficient to enable the transistor to switch. If the MTJ is in the on (low resistance) state, the transistor will switch on and current will flow. Since the resistance of the MTJ element can readily be varied by many orders of magnitude the behavior of the cell can be optimized for reduced power and/or for maximum reading speed. A nonvolatile magnetic random access memory using magnetic tunnel junction storage cells in either the simple cross-point architecture or with a transistor or other switch per MTJ element, as originally proposed by Gallagher, Parkin, and Scheinfein at IBM Research [3], [92], [95], is now being seriously considered by numerous other companies (see, for example, the contribution by Slaughter et al. in this issue which describes the very similar MRAM to IBM s original proposal currently being pursued by Motorola). VIII. MAGNETIC TUNNEL TRANSISTOR In conventional magnetoelectronic devices such as the spin-valve and the MTJ described above, electron transport occurs at energies near the Fermi level. It is also possible to utilize electrons with much higher energies, or so called hot electrons, to make interesting spintronic devices. One such example is the spin-valve transistor (SVT) [96], [97]. The SVT integrates a spin valve base with two semiconductor substrates, serving as the emitter and the collector respectively. The collector current of the SVT is very sensitive to the alignment of the magnetic moments within the spin-valve base, resulting in very high magnetic field sensitivity. However, the hot electron energy in the SVT is limited by the emitter Schottky barrier height to 0.9 ev. 674 PROCEEDINGS OF THE IEEE, VOL. 91, NO. 5, MAY 2003

15 The small difference between the emitter and the collector Shottky barrier heights leads to a low collector current ( 20 na). To sandwich a spin valve base between two semiconductor substrates also makes the fabrication process very complex. In the magnetic tunnel transistor (MTT), a tunnel barrier is used as the emitter [98] [101]. The hot electron energy can be easily adjusted by varying the emitter/base bias voltage across the tunnel barrier, enabling the MTT to explore electron transport over a wide energy range. Moreover, large collector currents can be obtained by applying large bias voltages [100]. The schematic band diagram of one form of the MTT is shown in Fig. 11(a). A ferromagnetic (FM) emitter injects spin polarized hot electrons across a tunnel barrier into a FM base layer. Scattering in the base layer causes the electrons to lose energy and/or change momentum. Only those electrons that maintain energy larger than the collector Schottky barrier height and that can be transmitted into the collector conduction band states contribute to. Spin-dependent hot electron scattering in the base layer preferentially scatters electrons whose spins are antiparallel with the majority spins in the base layer and, thus, makes them less likely to be collected. As a result, depends critically on the relative orientation of the magnetic moments of the emitter and the base. This dependence can be quantified by the magnetocurrent (MC), defined as (a) where and are collector current for parallel (P) and antiparallel (AP) alignment of the emitter and base magnetic moments respectively. The collector current also depends on the conduction band structure of the semiconductor collector. Here we present experimental results on MTT s with a GaAs collector. GaAs has a direct conduction band minimum at the Brillouin zone center ( point) and higher energy indirect minima at L and X points in the Brillouin zone. The energy separation between the point and the L and X minima is 0.29 and 0.48 ev, respectively [102]. All the conduction valleys contribute to the collection of hot electrons. The MTT is formed by dc magnetron sputtering at room temperature. Three shadow masks are used to form the base layer, the emitter isolation pads, and the emitter layer, respectively [Fig. 11(b)]. A typical MTT structure is given by GaAs Co Fe Al O Co Fe Ir Mn Ta, where is the thickness of the base layer. The barrier is formed by plasma oxidation of a thin Al film. The antiferromagnetic IrMn layer pins the emitter magnetic moment and therefore allows independent switching of the base magnetic moment in external magnetic fields. The active area of the tunnel junction and the total area of the base layer are mm and mm, respectively. To reduce the leakage current of the Schottky barrier due to the large base area, all transport measurements are conducted at 77 K, but in principle the MTT can operate at higher temperatures, even well above room temperature, if the base area is made significantly smaller. In Fig. 11(c), (c) Fig. 11. Schematic energy diagram for (a) an MTT with a single FM base layer and (b) picture of the MTT fabricated with magnetron sputtering. (c) The collector current of a typical MTT as a function of magnetic field at 77 K. (b) is plotted as a function of magnetic field for an MTT with. The relative change in for parallel and antiparallel alignments gives rise to an MC of 73% at. The MTT is a powerful tool to study spin dependent hot electron transport and to probe electronic structures in metals and semiconductors. One such example is to measure spin PARKIN et al.: MAGNETICALLY ENGINEERED SPINTRONIC SENSORS AND MEMORY 675

16 Fig. 13. base. Schematic energy diagram of an MTT with a spin-valve Fig. 12. (a) The majority (solid circles) and minority (open circles) electron attenuation length and (b) the ratio of the two as a function of the emitter/base bias voltage. dependent hot electron attenuation lengths in thin FM films within the base layer of the MTT [101]. The collector current of the MTT can be modeled by the following formula: where is the tunnel current, is the emitter polarization, is the base layer thickness, is the attenuation length for majority (minority) electrons within the FM base layer, and is the electron collection efficiency at the base/collector interface. A series of MTTs is fabricated with base layer thicknesses varying from 18 to 120. The collector current of each MTT is measured at a given bias voltage (i.e., hot electron energy) as a function of magnetic field. By fitting the data to the formula given above, the hot electron attenuation lengths can be extracted at this given energy. The same measurement is then conducted at various electron energies. Typical results are summarized in Fig. 12(a). A large spin asymmetry in attenuation length is observed for majority and minority electrons. The majority electron attenuation length decreases with the electron energy. This is mainly due to the strong energy dependence of electron-electron scattering rate, which is the most important scattering mechanism for majority spin electrons [103] [108]. On the other hand, minority electrons are subject to more efficient scattering because of the abundant available states to scatter into in the d-band near the Fermi level and additional scattering mechanisms such as spontaneous spin wave scattering [109] [111]. As a result, the minority electron attenuation length is much smaller than that of the majority electrons. The ratio between the majority and minority attenuation length is plotted in Fig. 12(b), which decreases slowly from 6.4 to 4.8 in the experiment energy range. The large spin asymmetry in attenuation length implies that the MTT is a very effective spin filter and has the potential to be a highly spin polarized electron source for spintronic applications. A different form of the MTT is shown in Fig. 13, where a spin-valve structure is used as the base. A typical spin valve is formed by Co Fe Cu or Au Ni Fe. The emitter layer is Cu. The CoFe and NiFe layers have different coercivities. In large magnetic fields, the magnetic moments in the base are aligned parallel. When the field is reversed, the magnetic moment of the NiFe layer first switches at 20 Oe, while the magnetic moment of the CoFe layer remains in the same orientation up to 120 Oe and 50 Oe for Cu and Au spacer layers, respectively. The switching from parallel to antiparallel alignment gives rise to a giant MC exceeding 1200% for both structures at (Fig. 14) [112], which is nearly two orders of magnitude larger than the TMR of typical MTJ devices and also much larger than the MC of the MTT with a single FM base layer. The MTT operates in the electron energy range from 1 ev up to a few electron volts, within the energy range in which a maximum scattering asymmetry between majority and minority electrons is expected [106], [108] [110], [113]. The high field sensitivity and large output current make it an intriguing spintronic device. Moreover, spin-dependent scattering in the base layer gives rise to a nearly 100% spinpolarized current at the base/collector interface and the use of a tunnel barrier makes the MTT free from the conductivity mismatch problem [114], [115]. As a result, the MTT promises to be a source of highly spin-polarized electron current that may be of importance for future spintronic applications. IX. SUMMARY In the past decade the field of spintronics has blossomed with the development and application of magnetically engineered thin-film spintronic magnetic field sensors. It is clear that this field has been energized by two main developments, the discovery of giant magnetoresistance in epitaxial antiferromagnetically coupled Fe/Cr [15] and subsequently polycrystalline Fe/Cr [4] and Co/Cu [16] magnetic multilayers, 676 PROCEEDINGS OF THE IEEE, VOL. 91, NO. 5, MAY 2003

17 interfaces, which have been proven to be of paramount importance. ACKNOWLEDGMENT The authors thank J. Harris and G. Solomon for their contributions to our work on the magnetic tunneling transistor. REFERENCES Fig. 14. Collector current measured as a function of the magnetic field at V = 1:6 V for an MTT with a 50 A CoFe=40 A Cu=50 A NiFe (upper panel) and a 50 A CoFe=40 A Au=50 A NiFe (lower panel) spin-valve base. A giant MC of more than 1200% is measured for both MTTs. and the discovery of oscillatory interlayer coupling in many transition metal magnetic multilayers [4], [28]. The latter has led to the magnetic engineering of magnetic thin film structures, allowing the development of artificial antiferromagnets [29], [45] and other related structures. Spintronic devices in the form of spin valves have already had an enormous impact on magnetic recording hard disk drives, allowing increases in capacity of such drives by almost two orders of magnitude in just five years since their introduction by IBM. The subsequent development of magnetically engineered magnetic tunneling junctions, whose basic principle had been realized more than two decades earlier, seems destined to allow for a novel magnetic random access memory with high speed and density and nonvolatility, thereby realizing the dream of a high performance magnetic nonvolatile memory whose quest began more than 50 years ago. Indeed, the field of spintronics has its roots dating back to the 1930s when it was first recognized that electrical transport in ferromagnetic metals is comprised of largely independent currents of majority and minority spin electrons. Generating, manipulating and detecting such spin-polarized current is the substance of spintronics. To date the most important applications of spintronics have involved all-metal and tunneling devices. These devices involve thin film materials with layers as thin as just a few angstroms. A decade ago many would have thought this inconceivable. Spintronics has been driven by the development of thin-film magnetic materials and, in particular, the understanding and control of their [1] J. D. Livingston, Driving Force: The Natural Magic of Magnets. Cambridge, MA: Harvard Univ. Press, [2] S. S. P. Parkin, Giant magnetoresistance in magnetic nanostructures, in Annual Review of Materials Science, B. W. Wessels, Ed. Palo Alto, CA: Annual Reviews Inc., 1995, vol. 25, pp [3] S. S. P. Parkin, K. P. Roche, M. G. Samant, P. M. Rice, R. B. Beyers, R. E. Scheuerlein, E. J. O Sullivan, S. L. Brown, J. Bucchigano, D. W. Abraham, Y. Lu, M. Rooks, P. L. Trouilloud, R. A. Wanner, and W. J. Gallagher, Exchange-biased magnetic tunnel junctions and application to nonvolatile magnetic random access memory, J. Appl. Phys., vol. 85, pp , [4] S. S. P. Parkin, N. More, and K. P. Roche, Oscillations in exchange coupling and magnetoresistance in metallic superlattice structures: Co/Ru, Co/Cr and Fe/Cr, Phys. Rev. Lett., vol. 64, pp , [5] W. H. Meiklejohn and C. P. Bean, New magnetic anisotropy?, Phys. Rev. B, vol. 102, pp , [6] F. Jorgensen, The Complete Handbook of Magnetic Recording. New York: TAB, [7] K. G. Ashar, Magnetic Disk Drive Technology: Heads, Media, Channel, Interfaces, and Integration. New York: IEEE Press, [8] C. Tsang, T. Lin, S. A. MacDonald, M. Pinarbasi, N. Robertson, H. Santini, M. Doerner, R. T. L. Vo, T. Diola, and P. Arnett, 5 Gb/in recording demonstration with conventional AMR dual element heads & thin film disks, IEEE Trans. Magn., vol. 33, pp , Sept [9] T. R. McGuire and R. I. Potter, Anisotropic magnetoresistance in ferromagnetic 3d alloys, IEEE Trans. Magn., vol. MAG-11, pp , July [10] A. F. Mayadas, J. F. Janak, and A. Gangulee, Resistivity of permalloy films, J. Appl. Phys., vol. 45, pp , [11] S. S. P. Parkin, Z. G. Li, and D. J. Smith, Giant magnetoresistance in antiferromagnetic Co/Cu multilayers, Appl. Phys. Lett., vol. 58, pp , [12] S. S. P. Parkin, Dramatic enhancement of interlayer exchange coupling and giant magnetoresistance in Ni Fe =Cu multilayers by addition of thin Co interface layers, Appl. Phys. Lett., vol. 61, pp , [13], Origin of enhanced magnetoresistance of magnetic multilayers-spin-dependent scattering from magnetic interface states, Phys. Rev. Lett., vol. 71, pp , [14] G. Binasch, P. Grünberg, F. Saurenbach, and W. Zinn, Enhanced magnetoresistance in layered magnetic structures with antiferromagnetic interlayer exchange, Phys. Rev. B, vol. 39, pp , [15] M. N. Baibich, J. M. Broto, A. Fert, F. N. v. Dau, F. Petroff, P. Etienne, G. Creuzet, A. Friederich, and J. Chazelas, Giant magnetoresistance of (001) Fe/(001)Cr magnetic superlattices, Phys. Rev. Lett., vol. 61, pp , [16] S. S. P. Parkin, R. Bhadra, and K. P. Roche, Oscillatory magnetic exchange coupling through thin copper layers, Phys. Rev. Lett., vol. 66, pp , [17] M. A. M. Gijs and G. E. W. Bauer, Perpendicular giant magnetoresistance of magnetic multilayers, Adv. Phys., vol. 46, pp , [18] E. Y. Tsymbal and D. G. Pettifor, Perspectives of giant magnetoresistance, in Solid State Physics, H. Ehrenreich and F. Spaepen, Eds. New York: Academic, 2001, vol. 56. [19] N. Mott, Electrons in transition metals, Adv. Phys., vol. 13, pp , [20] N. F. Mott and H. Jones, Theory of the Properties of Metals and Alloys. London, U.K.: Oxford Univ. Press, PARKIN et al.: MAGNETICALLY ENGINEERED SPINTRONIC SENSORS AND MEMORY 677

18 [21] J. W. F. Dorleijn, Electrical conduction in ferromagnetic metals, Philips Res. Rep., vol. 31, pp , [22] I. A. Campbell and A. Fert, Transport properties of ferromagnets, in Ferromagnetic Materials, E. P. Wohlfarth, Ed. Amsterdam, The Netherlands: North-Holland, 1982, vol. 3, p [23] D. M. Edwards, J. Mathon, R. B. Muniz, and M. S. Phan, A resistor network theory of the giant magnetoresistance in magnetic superlattices, IEEE. Trans. Magn., vol. 27, pp , July [24] S. S. P. Parkin, A. Modak, and D. J. Smith, Dependence of giant magnetoresistance on Cu layer thickness in Co/Cu multilayers: a simple dilution effect, Phys. Rev. B, vol. 47, pp , [25] J. Bass and J. W. P. Pratt, Current-perpendicular (CPP) magnetoresistance in magnetic metallic multilayers, J. Magn. Magn. Mater., vol. 200, pp , [26] P. Ciureanu, Magnetoresistive sensors, in Thin Film Resistive Sensors, P. Ciureanu and S. Middelhoek, Eds. Bristol, U.K.: Inst. of Physics, 1992, pp [27] S. S. P. Parkin, Giant magnetoresistance and oscillatory interlayer coupling in polycrystalline transition metal multilayers, in Ultrathin Magnetic Structures, B. Heinrich and J. A. C. Bland, Eds. Berlin, Germany: Spinger-Verlag, 1994, vol. II, p [28], Systematic variation of strength and oscillation period of indirect magnetic exchange coupling through the 3d, 4d and 5d transition metals, Phys. Rev. Lett., vol. 67, pp >, [29] S. S. P. Parkin and D. Mauri, Spin-engineering: direct determination of the RKKY far field range function in Ruthenium, Phys. Rev. B, vol. 44, pp , [30] J. M. Daughton, GMR and SDT sensor applications, IEEE Trans. Magn., vol. 36, pp , Sept [31] B. Dieny, V. S. Speriosu, S. S. P. Parkin, B. A. Gurney, D. R. Wilhoit, and D. Mauri, Giant magnetoresistance in soft ferromagnetic multilayers, Phys. Rev. B, vol. 43, pp , [32] K. Pettit, S. Gider, M. B. Salamon, and S. S. P. Parkin, Strong biquadratic coupling and antiferromagnetic-ferromagnetic crossover in NiFe/Cu multilayers, Phys. Rev. B, vol. 56, pp , [33] M. Rührig, R. Schäfer, A. Hubert, R. Mosler, J. A. Wolf, S. Demokritov, and P. Grünberg, Domain observations in Fe-Cr-Fe layered structures, Phys. Stat. Solidi A. Appl. Res., vol. 125, pp , [34] J. C. Slonczewski, Fluctuation mechanism for biquadratic exchange coupling in magnetic multilayers, Phys. Rev. Lett., vol. 67, pp , [35] S. O. Demokritov, E. Tsymbal, P. Grünberg, W. Zinn, and I. K. Schuller, Magnetic-dipole mechanism for biquadratic interlayer coupling, Phys. Rev. B, vol. 49, pp , [36] A. E. Berkowitz and K. Takano, Exchange anisotropy, J. Magn. Magn. Mater., vol. 200, pp , [37] J. Nogués and I. K. Schuller, Exchange bias, J. Magn. Magn. Mater., vol. 192, pp , [38] H. Ohldag, A. Scholl, F. Nolting, S. Anders, F. U. Hillebrecht, and J. Stöhr, Spin reorientation at the antiferromagnetic NiO(001) surface in response to an adjacent ferromagnet, Phys. Rev. Lett., vol. 86, pp , [39] H. Ohldag, T. J. Regan, J. Stöhr, A. Scholl, F. Nolting, J. Lüning, C. Stamm, S. Anders, and R. L. White, Spectroscopic identification and direct imaging of interfacial magnetic spins, Phys. Rev. Lett., vol. 87, pp , [40] W. Zhu, L. Seve, R. Sears, B. Sinkovic, and S. S. P. Parkin, Field cooling induced changes in the antiferromagnetic structure of NiO films, Phys. Rev. Lett., vol. 86, pp , [41] C. Tsang, Magnetics of small magnetoresistive sensors, J. Appl. Phys., vol. 55, pp , [42] S. S. P. Parkin, Applications of magnetic nanostructures, in Spin Dependent Transport in Magnetic Nanostructures, S. Maekawa and T. Shinjo, Eds. London, U.K.: Taylor & Francis, [43] L. Néel, Étude théorique du couplage ferro-antiferromagnétique dans les couches minces, Ann. Phys., vol. 2, p. 61, [44] J. C. S. Kools, Exchange-biased spin-valves for magnetic storage, IEEE Trans. Magn., vol. 32, pp , July [45] S. S. P. Parkin and D. E. Heim, Magnetoresistive spin valve sensor with improved ferromagnetic layer and magnetic recording system using the sensor, U.S. Patent , [46] B. Lu and D. E. Laughlin, Microstructure of longitudinal media, in The Physics of Ultra-High-Density Magnetic Recording, M.L. Plumer, J. van Ek, and D. Weller, Eds. Berlin, Germany: Springer- Verlag, [47] A. Moser, K. Takano, D. T. Margulies, M. Albrecht, Y. Sonobe, Y. Ikeda, S. Sun, and E. E. Fullerton, Magnetic recording: advancing into the future, J. Phys. D, Appl. Phys., vol. 35, pp. R157 R167, [48] W. F. Egelhoff, P. J. Chen, C. J. Powell, M. D. Stiles, R. D. McMichael, C.-L. Lin, J. M. Sivertsen, J. H. Judy, K. Takano, A. E. Berkowitz, T. C. Anthony, and J. A. Brug, Optimizing the giant magnetoresistance of symmetric and bottom spin valves, J. Appl. Phys., vol. 79, pp , [49] M. Julliére, Tunneling between ferromagnetic films, Phys. Lett., vol. 54A, pp , [50] T. Miyazaki and N. Tezuka, Giant magnetic tunneling effect in Fe=Al O =Fe junction, J. Magn. Magn. Mater., vol. 139, pp. L231 L234, [51] J. S. Moodera, L. R. Kinder, T. M. Wong, and R. Meservey, Large magnetoresistance at room temperature in ferromagnetic thin film tunnel junctions, Phys. Rev. Lett., vol. 74, pp , [52] X.-F. Han, T. Daibou, M. Kamijo, K. Yaoita, H. Kubota, Y. Ando, and T. Miyazaki, High-magnetoresistance tunnel junctions using Co75Fe25 ferromagnetic electrodes, Jpn. J. Appl. Phys., vol. 39, pp. L439 L441, [53] M. Tsunoda, K. Nishikawa, S. Ogata, and M. Takahashi, 60% magnetoresistance at room temperature in Co-Fe/Al-O/Co-Fe tunnel junctions oxidized with Kr-O2 plasma, Appl. Phys. Lett., vol. 80, pp , [54] R. Dill, R. E. Fontana, S. S. P. Parkin, and C. Tsang, Shielded magnetic tunnel junction magnetoresistive read head, U.S. Patent , [55] K. Nagasaka, Y. Seyama, L. Varga, Y. Shimizu, and A. Tanaka, Giant magnetroresistance properties of specular spin valve films in a current perpendicular to plane structure, J. Appl. Phys., vol. 89, pp , [56] J. C. Slonczewski, Conductance and exchange coupling of two ferromagnets separated by a tunnel barrier, Phys. Rev. B, vol. 39, pp , [57] P. Mavropoulos, N. Papanikolaou, and P. H. Dederichs, Complex band structure and tunneling through ferromagnet/insulator/ferromagnet junctions, Phys. Rev. Lett., vol. 85, pp , [58] O. Wunnicke, N. Papanikolaou, R. Zeller, P. H. Dederichs, V. Drchal, and J. Kudrnovský, Effects of resonant interface states on tunneling magnetoresistance, Phys. Rev. B, vol. 65, pp , [59] J. Mathon, Tight-binding theory of tunneling giant magnetoresistance, Phys. Rev. B, vol. 56, pp , [60] R. Meservey and P. M. Tedrow, Spin-polarized electron tunneling, Phys. Rep., vol. 238, pp , [61] E. Y. Tsymbal, I. I. Oleinik, and D. G. Pettifor, Oxygen-induced positive spin polarization from Fe into the vacuum barrier, J. Appl. Phys., vol. 87, pp , [62] R. Feder, Polarized Electrons in Surface Physics, Singapore: World Scientific, [63] R. J. Soulen, J. M. Byers, M. S. Osofsky, B. Nadgorny, T. Ambrose, S. F. Cheng, P. R. Broussard, C. T. Tanaka, J. Nowak, J. S. Moodera, A. Barry, and J. M. D. Coey, Measuring the spin polarization of a metal with a superconducting point contact, Science, vol. 282, pp , [64] G. E. Blonder, M. Tinkham, and T. M. Klapwijk, Transition from metallic to tunneling regimes in superconducting microconstrictions: excess current, charge imbalance, and supercurrent, Phys. Rev. B, vol. 25, pp , [65] I. I. Mazin, How to define and calculate the degree of spin polarization in ferromagnets, Phys. Rev. Lett., vol. 83, pp , [66] D. J. Monsma and S. S. P. Parkin, Spin polarization of tunneling current from ferromagnet=al O interfaces using copper-doped aluminum superconducting films, Appl. Phys. Lett., vol. 77, pp , [67] E. L. Wolf, Principles of Electron Tunneling Spectroscopy. New York: Oxford Univ. Press, [68] D. J. Monsma and S. S. P. Parkin, Temporal evolution of spin polarization in ferromagnetic tunnel junctions, Appl. Phys. Lett., vol. 77, pp , [69] B. Nadgorny, R. J. Soulen, M. S. Osofsky, I. I. Mazin, G. Laprade, R. J. M. v. d. Veerdonk, A. A. Smits, S. F. C. F. Skelton, and S. B. Qadri, Transport spin polarization of Ni Fe : electronic kinematics and band structure, Phys. Rev. B, vol. 61, pp. R3788 R3791, PROCEEDINGS OF THE IEEE, VOL. 91, NO. 5, MAY 2003

19 [70] R. Meservey, D. Paraskevopoulos, and P. M. Tedrow, Tunneling measurements of conduction-electron-spin polarization in heavy rare-earth metals, Phys. Rev. B, vol. 22, pp , [71] K. P. Kämper, W. Schmitt, G. Güntherodt, R. J. Gambino, and R. Ruf, CrO a new half-metallic ferromagnet, Phys. Rev. Lett., vol. 59, pp , [72] A. Yanase and K. Siratori, Band structure in the high temperature phase of Fe O, J. Phys. Soc. Jpn., vol. 53, pp , [73] M. Imada, A. Fujimori, and Y. Tokura, Metal-insulator transitions, Rev. Mod. Phys., vol. 70, pp , [74] R. A. d. Groot, F. M. Mueller, P. G. v. Engen, and K. H. J. Buschow, New class of materials: half-metallic ferromagnets, Phys. Rev. Lett., vol. 50, pp , [75] M. B. Salamon and M. Jaime, The physics of manganites: structure and transport, Rev. Mod. Phys., vol. 73, pp , [76] S. S. P. Parkin, Magnetic tunnel junction device with nonferromagnetic interface layer for improved magnetic field response, U.S. Patent , June 9, [77] P. LeClair, H. J. M. Swagten, J. T. Kohlhepp, R. J. M. v. d. Veerdonk, and W. J. M. d. Jonge, Apparent spin polarization decay in Cu-dusted Co=Al O =Co tunnel junctions, Phys. Rev. Lett., vol. 84, pp , [78] C. T. Tanaka, J. Nowak, and J. S. Moodera, Spin-polarized tunneling in a half-metallic ferromagnet, J. Appl. Phys., vol. 86, pp , [79] D. C. Worledge and T. H. Geballe, Spin-polarized tunneling in La Sr MnO, Appl. Phys. Lett., vol. 76, pp , [80] J. Z. Sun, W. J. Gallagher, P. R. Duncombe, L. Krusin-Elbaum, R. A. Altman, A. Gupta, Y. Lu, G. Q. Gong, and G. Xiao, Observation of large low-field magnetoresistance in tri-layer perpendicular transport devices made using doped manganate perovskites, Appl. Phys. Lett., vol. 69, pp , [81] J. O Donnell, A. E. Andrus, S. Oh, E. V. Colla, and J. N. Eckstein, Colossal magnetoresistance magnetic tunnel junctions grown by molecular-beam epitaxy, Appl. Phys. Lett., vol. 76, pp , [82] J. Z. Sun, D. W. Abraham, K. Roche, and S. S. P. Parkin, Temperature and bias dependence of magnetoresistance in doped manganite thin film trilayer junctions, Appl. Phys. Lett., vol. 73, pp , [83] J. S. Parker, S. M. Watts, P. G. Ivanov, and P. Xiong, Spin polarization of CrO at and across an artificial barrier, Phys. Rev. Lett., vol. 88, pp , [84] Z. Zhang and S. Satpathy, Electron states, magnetism, and the Verwey transition in magnetite, Phys. Rev. B, vol. 44, pp , [85] Y. S. Dedkov, U. Rüdiger, and G. Güntherodt, Evidence for the half-metallic ferromagnetic state of Fe O by spin-resolved photoelectron spectroscopy, Phys. Rev. B, vol. 65, pp , [86] X. W. Li, A. Gupta, G. Xiao, W. Qian, and V. P. Dravid, Fabrication and properties of heteroepitaxial magnetite (Fe O ) tunnel junctions, Appl. Phys. Lett., vol. 73, pp , [87] P. Seneor, A. Fert, J.-L. Maurice, F. Montaigne, F. Petroff, and A. Vaurès, Large magnetoresistance in tunnel junctions with an iron oxide electrode, Appl. Phys. Lett., vol. 74, pp , [88] S. S. P. Parkin, A. Panchula, P. M. Rice, R. P. Sears, and B. Sinkovic, Inverse tunneling magneto-resistance in magnetic tunneling junctions containing ferrimagnetic Fe O electrodes, unpublished. [89] J. M. Daughton, Magnetoresistive memory technology, Thin Solid Films, vol. 216, pp , [90] D. D. Tang, P. K. Wang, V. S. Speriosu, S. Le, and K. K. Kung, Spin valve RAM cell, IEEE Trans. Magn., vol. 31, pp , Nov [91] R. E. Scheuerlein, Magneto-resistive IC memory limitations and architecture implications, presented at the NCE Seventh Biennial IEEE Int. Nonvolatile Memory Technology Conf., Albuquerque, NM, June 22 24, [92] W. J. Gallagher, S. S. P. Parkin, R. E. Scheuerlein, and J. Kaufman, Magnetic random access memory (MRAM) with diode-isolated circuit architecture, U.S. Patent , June 17, [93] J. M. Daughton, Magnetic tunneling applied to memory, J. Appl. Phys., vol. 81, pp , [94] R. E. Matick, Computer Storage Systems and Technology. New York: Wiley, [95] W. J. Gallagher, S. S. P. Parkin, Y. Lu, X. P. Bian, A. Marley, K. P. Roche, R. A. Altman, S. A. Rishton, C. Jahnes, T. M. Shaw, and G. Xiao, Microstructured magnetic tunnel junctions, J. Appl. Phys., vol. 81, pp , [96] D. J. Monsma, J. C. Lodder, T. J. A. Popma, and B. Dieny, Perpendicular hot electron spin-valve effect in a new magnetic field sensor: The spin-valve transistor, Phys. Rev. Lett., vol. 74, pp , [97] D. J. Monsma, R. Vlutters, and J. C. Lodder, Room temperature-operating spin-valve transistors formed by vacuum bonding, Science, vol. 281, pp , [98] K. Mizushima, T. Kinno, T. Yamauchi, and K. Tanaka, Energydependent hot electron transport across a spin-valve, IEEE Trans. Magn., vol. 33, pp , Sept [99] R. Sato and K. Mizushima, Spin-valve transistor with an Fe/Au/Fe(001) base, Appl. Phys. Lett., vol. 79, pp , [100] S. van Dijken, X. Jiang, and S. S. P. Parkin, Room temperature operation of a high output current magnetic tunnel transistor, Appl. Phys. Lett., vol. 80, pp , [101], Spin-dependent hot electron transport in Ni Fe and Co Fe films on GaAs(001), Phys. Rev. B, vol. 66, pp , [102] J. S. Blakemore, Semiconducting and other major properties of gallium arsenide, J. Appl. Phys., vol. 53, pp. R123 R181, [103] J. J. Quinn, Range of excited electrons in metals, Phys. Rev., vol. 126, pp , [104] D. P. Pappas, K. P. Kämper, B. P. Miller, H. Hopster, D. E. Fowler, C. R. Brundle, A. C. Luntz, and Z. X. Shen, Spin-dependent electron attenuation by transmission through thin ferromagnetic films, Phys. Rev. Lett., vol. 66, pp , [105] G. Schönhense and H. C. Siegmann, Transmission of electrons through ferromagnetic material and applications to detection of electron-spin polarization, Ann. Phys.-Leipzig, vol. 2, pp , [106] H.-J. Drouhin, A. J. v. d. Sluijs, Y. Lassailly, and G. Lampel, Spindependent transmission of free electrons through ultrathin cobalt layers, J. Appl. Phys., vol. 79, pp , [107] W. H. Rippard and R. A. Buhrman, Spin-dependent hot electron transport in Co/Cu thin films, Phys. Rev. Lett., vol. 84, pp , [108] H.-J. Drouhin, Spin-dependent scattering in transition metals, J. Appl. Phys., vol. 89, pp , [109] R. Knorren, K. H. Bennemann, R. Burgermeister, and M. Aeschlimann, Dynamics of excited electrons in copper and ferromagnetic transition metals: Theory and experiment, Phys. Rev. B, vol. 61, pp , [110] E. Zarate, P. Apell, and P. M. Echenique, Calculation of low-energyelectron lifetimes, Phys. Rev. B, vol. 60, pp , [111] R. Vlutters, O. M. J. v. t. Erve, S. D. Kim, R. Jansen, and J. C. Lodder, Interface, volume, and thermal attenuation of hot-electron spins in Ni Fe and Co, Phys. Rev. Lett., vol. 88, pp , [112] S. van Dijken, X. Jiang, and S. S. P. Parkin, Comparison of magnetocurrent and tansfer ratio in magnetic tunnel transistors with spinvalve bases containing Cu and Au spacer layers, Appl. Phys. Lett., vol. 82, pp , [113] M. Aeschlimann, M. Bauer, S. Pawlik, W. Weber, R. Burgermeister, D. Oberli, and H. C. Siegmann, Ultrafast spin-dependent electron dynamics in fcc Co, Phys. Rev. Lett., vol. 79, pp , [114] G. Schmidt, D. Ferrand, L. W. Molenkamp, A. T. Filip, and B. J. v. Wees, Fundamental obstacle for electrical spin injection from a ferromagnetic metal into a diffusive semiconductor, Phys. Rev. B, vol. 62, pp. R4790 R4793, [115] E. I. Rashba, Theory of electrical spin injection: tunnel contacts as a solution of the conductivity mismatch problem, Phys. Rev. B, vol. 62, pp. R16267 R16270, [116] B. D. Cullity, Introduction to Magnetic Materials. Reading, MA: Addison-Wesley, PARKIN et al.: MAGNETICALLY ENGINEERED SPINTRONIC SENSORS AND MEMORY 679

20 Stuart Parkin (Member, IEEE) is a native of the U.K, He received the B.A. degree from Trinity College, Cambridge, U.K., in 1977, and the Ph.D degree from the Cavendish Laboratory, Cambridge, U.K., in He was elected a Research Fellow in 1979 at Trinity College. He joined IBM Research in 1982 as a World Trade Post-Doctoral Fellow, becoming a permanent member of the staff the following year. Currently, he is Manager of the Magnetoelectronics Group at the IBM Almaden Research Center, San Jose, CA. His current work involves the study of magnetic tunnel junctions and the development of an advanced nonvolatile magnetic random access memory based on magnetic tunnel junction storage cells. His earlier research interests have included organic superconductors, ceramic high-temperature superconductors and, most recently, the study of magnetic thin-film structures and nanostructures exhibiting giant magnetoresistance (GMR). In 1991, he discovered oscillations in the magnitude of the interlayer exchange coupling in transition metal magnetic multilayered systems. Dr. Parkin shared both the American Physical Society s International New Materials Prize (1994) and the European Physical Society s Hewlett-Packard Europhysics Prize (1997). He has received other awards, including the Materials Research Society Outstanding Young Investigator Award (1991), the Charles Vernon Boys Prize from the Institute of Physics, London (1991), the American Institute of Physics Prize for Industrial Applications of Physics, as well as several awards from IBM. In 2001, he was named the first Innovator of the Year by R&D Magazine. He was elected a Fellow of the Royal Society in 2000 and is also a Fellow of the American Physical Society and the Institute of Physics (London). In 1997, he was elected a member of the IBM Academy of Technology and named an IBM Research Master Inventor. In 1999, he was appointed an IBM Fellow, IBM s highest technical honor. Christian Kaiser received the Masters degree in physics from the University of Aachen, Aachen, Germany, in His Masters thesis involved spin-dependent tunneling spectroscopy of ferromagnetic alloys. He is currently enrolled in the Ph.D. program at the University of Aachen, while continuing to work on his dissertation at the IBM Almaden Research Center, San Jose, CA. His research involves helping to build an advanced deposition system which will combine a wide variety of deposition techniques with in-situ analysis. This new tool will allow the fabrication of advanced spintronic devices using advanced complex thin film materials. Alex Panchula received the B.S. degree in physics and mathematics from Iowa State University, Ames, in He is currently working toward the Ph.D. degree in the Department of Applied Physics, Stanford University, Stanford, CA. In 2000, he joined the IBM Almaden Research Center, San Jose, CA, to work on his dissertation on the magnetotransport of magnetic nanostructures focusing on new materials. His undergraduate research focused on crystal growth, transport, and magnetic properties of rare-earth intermetallics. Kevin Roche received the B.A. degree in physics from the University of California, Berkeley, in He began work at the IBM Research San Jose Research Laboratory in the field of FM laser spectroscopy. He is now part of the Magnetoelectronics Group at the IBM Almaden Research Center, San Jose, CA. Xin Jiang received the B.S. degree in physics from Tsinghua University, Beijing, China, in He is currently working toward the Ph.D. degree in applied physics at Stanford University, Stanford, CA. He is carrying out work for his dissertation at the IBM Almaden Research Center, San Jose, CA. Mr. Jiang was a Stanford Graduate Fellow from 1998 to Mahesh Samant received the B.Tech. degree from the Indian Institute of Technology, Bombay, India, in 1981, and the Ph.D. degree from Stanford University, Stanford, CA, in 1986, both in chemical engineering. His Ph.D. dissertation concerned structural studies of bimetallic catalysts. He has subsequently worked in the areas of the electrochemistry of under-potentially deposited monolayer metallic films, liquid crystal materials for displays, and characterization of magnetic thin films using X-ray magnetic circular dichroism. Currently, he is a Scientist at the IBM Almaden Research Center, San Jose, CA, working on spintronic materials and devices. 680 PROCEEDINGS OF THE IEEE, VOL. 91, NO. 5, MAY 2003