"Interlayer exchange coupling in metallic and all-semiconductor multilayered structures"

Transcription

1 "Interlayer exchange coupling in metallic and all-semiconductor multilayered structures" OUTLINE Why are interlayer coupling phenomena interesting and Important? The explanation will be in the form of a longer story about magnetoresistance and GMR, a Nobel Prize effect. Why should one study interlayer coupling effects in all-semiconductor systems? Why should we use neutron scattering tools for this purpose? What we have found so far in the course of our studies of EuS-based all-semiconductor superlattices.

2 There is much written text on some slides Let me explain why. My plan is to post this Power Point Presentation on the Web. For some people it will perhaps be a useful tutorial. And, I hope, after doing some more work on it, it may also serve as sort of propaganda movie for informing people e.g., prospective students -- about research conducted in our Department (we will then need a whole package of such slide shows, of course).

3 To begin, we have to go back to In 1857 Scottish scientist William Thomson, who later becomes Lord Kelvin, discovers that the application of external magnetic field to a nickel (Ni) wire increases its electric resistance. The term magnetoresistance is introduced for this new phenomenon. The picture shows Lord and Lady Kelvin chairing the ceremony of coronation of King Edward II in Scientist at that time were given all respect they deserved in sharp contrast with the present situation!

4 After the original Kelvin s discovery......physicists rushed to study other metals. Essentially, it was found that MR effects occur in any metal. For the non-magnetic ones, those findings can be summarized as a simlpe rule of thumb : the worse conductor the metal is, the stronger the MR effects are manifested. Bismuth (which is not even classified as a metal, but a semimetal ) was found to be the recordholder in strong magnetic fields its resistance could increase by as much as 50%. But in copper or gold the resistance changed only by a small fraction of 1%, even in very strong fields. Not surprisingly, the MR phenomena did not find too many practical applications

5 Soon it was realized that magnetoresistance is not an effect standing by itself, but it belongs to a larger class of phenomena, called galvanomagnetic effects, or magnetotransport effects, which can be all described in the framework of the same theory. Another member of this class is the well-known Hall Effect.

6 The theory of ordinary magnetoresitance (OMR) and the Hall Effect for a simple non-magnetic metal By taking the equation of motion for electrons: And intoducing the cyclotron frequency: One obtains a solution in a matrix form, where the diagonal elements represent magnetoresistance, and the off-diagonal the Hall effect: Standard Hall Effect Geometry:

7 The above theory was found to work pretty well for non-magnetic metals and semiconductords In ferromagnets (FMs), B is a non-linear function of applied field an T, showing hysteresis. However, this function can be readily determined from experiments. It was therefore expected that if experimental B values were used, the same theory would work well for FMs. But it did not work!! Both Hall Effect and magnetoresistance in FMs were found to behave in a highly unpredictable way. New terms were coined for them: Anomalous Hall Effect (AHE) and Anomalous MagnetoResistance (AMR).

8 It turned out that the AHE and AMR in FM metals can only be explained on the grounds of quantum theory. The first successful theory of AHE and AMR was created by another British scientist-aristocrat, the famous Sir Nevil Mott (Nobel 1977). He asked himself: why certain transition metals Ni, Pd, Pt are much poorer conductors than their immediate neighbors in the Periodic Table, Cu, Ag and Au?

9 Here is the answer: in transition metals the current is conducted by electrons from the d-bands and s-bands (or hybrydized s+p bands) Electron in the d-bands are more tightly bound and less mobile. But the s-band electrons may be scattered by defects (always present) or by phonons, and may end up in the d-band, losing mobility and increasing the resistance. Schematic representation of the bands in a transition metal with a partially filled d-band (the bands for spin-up and spindown electrons are shown separately).

10 In copper, however, the 3d band is completely filled, so such scattering cannot occur therefore, copper is an excellent conductor!

11 However, in nickel, copper s next-door neighbor, the situation is different The d-band is not completely filled, so that s d scattering may occur, making Ni a poorer conductor There is one more important aspect: in the FM state, the situation is no longer symmetric the 3d sub-band for only one of the spin states is now incompletely filled. This fact, it turns out, has far-reaching consequences!

12 MMM (Mott s Motel Model) From the Mott s picure, it follows that there are two currents: For spin-up current the resistance is low (no scattering). For spin-down current the resistance is high because such electrons may be scattered into the 3d sub-band

13 According to Mott s theory, an FM conductor can be thought of as two parallel sets of resistors. By applying an external magnetic field, one can re-orient the domains, and thus change the specimen resistance as had been originally observed by Lord Kelvin. In bulk specimens the effect is not particularly strong, though, which makes practical applications difficult

14 W. Reed & E. Fawcett s 1964 experiment on single-crystal iron (Fe) whiskers The result was a beautiful confimantion of the Mott model yet, whiskers are technologically unfriendly

15 Everything grows giant these days: Pumpkins, pandas, schnauzers. Magnetoresistance is NOT an exception! The credit for introducing the term Giant Magnetoresistance should be given to Dr. S. von Molnar, who used it in a 1967 paper reporting unusually strong magnetoresistance effects seen in EuSe crystals doped with Gadolinium (Gd).

16 However, what we call GMR now is not exactly the same effect as that observed in bulk specimens by von Molnar et al.. Today, GMR refers to an effect occurring in nanometer-thick multilayered structures, discovered by A.Fert (France) and P. Grünberg (Germany), for which they were awarded a Nobel Prize in 2007.

17

18 Joseph Haydn, composer of The German National Anthem

19

20 GMR in a Fe/Cr/Fe sandwich

21 Electron states in a non-magnetic metal (left) and in a ferromafnetic metal (right)

22 More detailed explanation of the GMR mechanism

23

24 Spin valves: sophisticated GMR-based sensors The application of such sensors in the reading heads of hard-drives made it possible to increase their capacity by nearly two orders of magnitude Since 1997, about 5 billions of such reading heads have been produced.

25 More spin valves

26 But the reign of GMR-based reading heads did not last long Recently, they have been dethroned by even more efficient sensors utilizing another magnetoresistance effect namely, Tunnel MagnetoResistance (TMR)

27 Outwardly, a TMR system is similar to a GMR one but now the two FM conducting layers are separated by a thin (~ 1 nm) insulating layer (e.g., MgO) Ferromagnetic coupling: High tunneling probability Antiferromagnetic coupling: Low tunneling probability

28 However, no matter whether the sensors utilize GMR, or TMR, they always have one thing in common: Zero magnetic field Applied field In the initial state, the magnetization vectors in the two FM layers must be antiparallel...because only then the applied field will change their mutual orientation. If the magnetization vectors were initially parallel then the applied field would not change their mutual orientation, and such system would not be sensitive to the field.

29 In other words in all types of thin film magnetoresistance sensors there has to be an interaction that couples the FM films antiferromagnetically acros the intervening non-magnetic spacer: This interaction also assures that the system returns to its initial configuration after the field is removed.

30 But how can one obtain a coupling of a desired sign between two FM films? Well, the whole GMR saga started when one day in 1986 Peter Grunberg prepared a trilayer consisting of two iron films, with a wedge-shaped non-magnetic chromium metal layer in between. He observed that a domain pattern with alternating magnetization directions formed in the top layer, meaning that the sign of the interaction between the Fe layers was an oscillating function of the Cr layer thickness. So, Grunberg s discovery showed that the desired configuration can be obtained by choosing an appropriate spacer thickness.

31 What is the origin of the interlayer interaction with oscillating sign? There is still no consensus among researchers ragarding this issue. Some argue that it is simply the old RKKY interaction (known since 1950s). It couples magnetic atoms embedded in non-magnetic metals, and its sign oscillates with distance r. It is mediated by Fermi electrons RKKY r

32 Other researchers are of the opinion that Quantum Well States (QWS) play a crucial role In this model, the non-magnetic spacer is though of as a quantum well, in which electrons are confined between two walls, with the magnetized layers playing such a role. There are discrete E levels in such a well (recall particle in a box ). When the well expands, these energies decrease. Each time a consecutive E level cuts through the Fermi level, the sign of the coupling changes:

33 But no matter who is right, there is no doubt about one point: namely, it is the conduction electrons that play a crucial role in interlayer coupling effects seen in multilayered metallic GMR systems. In semiconductors, in contrast, the concenttration of conduction electrons is orders of magnitude lower than in metals. Some of them are nearly-insulating. So, the above may imply that in analogous systems made of semiconductors there is no chance of seeing interlayer coupling effects. RIGHT?!

34 NOT RIGHT! We have been conducting neutron scattering studies on all-semiconductor multilayered systems consisting of alternating magnetic and nonmagnetic layers, and in many of them we observed pronounced interlayer magnetic coupling effects.

35 Is it important to investigate all-semiconductor system? The existing all-metal GMR sensors are the first generation of spintronics systems. But in the opinion of many experts the future belongs to semiconductor spintronics. Such devices can be more easily integrated with existing electronics. Also, semiconductors have many highly interesting optical properties. Semiconductor spintronics may become an ideal partner for photonics!

36 There is one big problem, though. For building practical spintronics devices one would need semicondutors that are ferromagnetic at room temperature. And God did not make them. Rather, God left it as a challenge for us to create such materials synthetically. Material technogists in many labs worldwide continue to work hard on this problem

37 Room-temperature FM semiconductors: present situation The record-holder now is epitaxially prepared Ga(Mn)As alloy, with about 10% of Mn. It stays FM up to 175 K still more than 100K below the target value. What can be done in such situation? Well, there are some fundamental problems that need to be studied. For instance what is the mechanism giving rise to interlayer coupling effects in systems with low concentration of mobile electrons? We decided to do such studies on multilayers containing EuS, a well-known prototypical FM semiconductor (with Curie T of only 16 K, though).

38 Ferromagnetic EuS/PbS and EuS/YbSe SL s EuS Heisenberg ferromagnet T C = 16.6 K (bulk), E g =1.5 ev PbS narrow-gap (E g =0.3 ev) semiconductor (n cm -3 ) YbSe wide-gap (E g =1.6 ev) semiconductor (semiinsulator) all NaCl-type structure with lattice constants: Ǻ Ǻ Ǻ (lattice mismatch 0.5%) Ǻ Å number of repetitions (001) a=6.29 Å

39

40

41 Neutron reflectivity experiments onthe EuS/PbS system (NG-1 reflectometer, NIST Center for Neutron Research) Situation corresponding to red data points: Situat. corresponding to green data points: Situation corresponding to blue data points

42 Unpolarized neutron reflectivity experiments on the EuS/PbS system (NG-1 reflectometer, NIST Center for Neutron Research)

43 Our collaborators

44 Electronic band structure in EuS

45

46

47 Alternative explanations... PbS is a narrow-gap material. At low T the concentrations of carriers may be still pretty high. Perhaps the effect seen in EuS/PbS is a carrier-mediated coupling? Crucial test: make a EuS/XY system, in which XY is a wide-gap semiconductor or an insulator An ideal material, YbSe was found for that purpose.

48 Interlayer exchange coupling mediated by valence band electrons J.Blinowski & P.Kacman, Phys. Rev. B 64 (2001) P.Sankowski & P.Kacman, Acta Phys. Polon. A 103 (2003) 621

49 Unpolarized neutron reflectivity experiments on the EuS/YbSe system (NG-1 reflectometer, NIST Center for Neutron Research)

50 CLOSING REMARKS It is good to inspiration from the work of others. If these people got a Nobel Prize, it would add prestige to your work! Now, more seriously: Metal-based spintronics has a bright future. One new application that is emerging is generating GHz signals, which may lead to further progress in cellullar phone technology. Semiconductor spintronics will more likely utilize TMR than GMR. Note that in a TMR device the FM films are separated by an insulating spacer. From that standpoint, our work makes much sense essentially, what we are doing, is studying interlayer coupling between FM films across insulating spacers. Las fall, for example, we made measurements on system in which EuS layers are separated with barriers of SrS, which has energy gap width 4.6 ev, making it a perfect insulator. And we saw pronounced antiferromagnetic interlayer coupling in those systems.