"Interlayer exchange coupling in metallic and

all-semiconductor multilayered structures" OUTLINE

• Why are interlayer coupling phenomena interesting andImportant? The explanation will be in the form of a longerstory about magnetoresistance and GMR, a Nobel Prizeeffect.

• 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 studiesof EuS-based all-semiconductor superlattices .

There is much written text on some slides

Let me explain why. My plan is to post this Power PointPresentation on the Web. For some people it will perhapsbe 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 willthen need a whole “package” of such slide shows, of course).

To begin, we have to go back to 1857...

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 “magneto-resistance” is introduced for this new phenomenon.

The picture shows Lord and Lady Kelvinchairing the ceremony of coronation of King Edward II in 1902. Scientist at that time weregiven all respect they deserved – in sharpcontrast with the present situation!

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 “record-holder” – 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…

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.

The theory of “ordinary magnetoresitance” (OMR) andthe Hall Effect for a simple non-magnetic metal

By taking the equation of motion for electrons:

And intoducingthe cyclotron

frequency:

One obtains a solution in a matrixform, where the diagonal elementsrepresent magnetoresistance, and the off-diagonal – the Hall effect:

Standard Hall Effect Geometry:

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 appliedfield an T, showing hysteresis. However, this function can be readily determined from experi-ments.

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 magneto-resistance 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).

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?

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-bandsare more tightly bound and less mobile.But the s-band electrons may be scattered by de-fects (always present) orby phonons, and may end up in the d-band,losing mobility and incre-asing the resistance.

Schematic representation of the bands ina transition metal with a partially filledd-band (the bands for spin-up and spin-down electrons are shown separately).

In copper, however, the 3d band is completely filled, so such scattering cannot occur –

therefore, copper is an excellent conductor!

However, in nickel, copper’s next-door neighbor, the situation is different

The d-band is not completely filled, so that s→dscattering may occur, making Ni a poorer conductor

There is one more important aspect: in the FM state, thesituation is no longer symmetric – the 3d sub-band foronly one of the spin states is now incompletely filled.

This fact, it turns out, has far-reaching consequences!

“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

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-orientthe 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 ☹

W. Reed & E. Fawcett’s 1964 experimenton single-crystal iron (Fe) whiskers

The result was a beautiful confimantion of the Mottmodel – yet, whiskers are “technologically unfriendly”

Everything grows giant these days:Pumpkins, pandas, schnauzers….

Magnetoresistance is NOT an exception!

The credit for introducing the term Giant Magneto-resistance should be given to Dr. S. von Molnar, whoused it in a 1967 paper reporting unusually strongmagnetoresistance effectsseen in EuSe crystals dopedwith Gadolinium (Gd).

However, what we call “GMR” nowis not exactly the same effect asthat observed in bulk specimens

by von Molnar et al. .

Today, “GMR” refers to an effectoccurring in nanometer-thick multi-layered structures, discovered byA.Fert (France) and P. Grünberg (Germany), for which they were awarded a Nobel Prize in 2007.

http://urlcut.com/Vive_la_France

http://urlcut.com/German_National_Anthem

Joseph Haydn,composer ofThe GermanNational Anthem

GMR in a Fe/Cr/Fe “sandwich”

Spin valves: sophisticated GMR-based sensorsThe application of such sensors in the reading headsof hard-drives made it possible to increase their capacity by nearly two orders of magnitude…

Since 1997, about 5 billionsof such reading heads have been produced.

More spin valves

But the reign of GMR-based reading heads did not last long…

Recently, they have been “dethroned” by even more efficient sensors utilizing another magnetoresistanceeffect – namely, Tunnel MagnetoResistance (TMR)

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

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 magne-tization vectors in the two FMlayers must be antiparallel…

...because only then the appliedfield will change their mutual orientation.

If the magnetization vectors were initially parallel…

…then the applied field wouldnot change their mutual orien-tation, and such system wouldnot be sensitive to the field.

In other words……in all types of thin film magnetoresistance sensors there has to be an interaction that couples the FM films antiferromagneticallyacros the intervening non-magnetic spacer:

This interaction also assures that the system returnsto its initial configuration after the field is removed.

But how can one obtain a coupling of a desired sign between two FM films?

Well, the whole “GMR saga”started when one day in1986 Peter Grunberg prepa-red a “trilayer” consistingof two iron films, with a wedge-shaped non-magne-tic chromium metal layerin between. He observed that a domain pattern withalternating magnetization directions formed in the top layer, meaning that thesign of the interaction be-tween the Fe layers was anoscillating function of the Cr layer thickness. So, Grunberg’s discovery sho-wed that the desired con-figuration can be obtainedby choosing an approp- riate spacer thickness.

What is the origin of the interlayer interaction with oscillating sign?

r

There is still no consensus among researchers ragarding this issue. Some argue that it is simply the “old” RKKYinteraction (known since 1950s). It couples magnetic at- oms embedded in non-magnetic metals, and its sign osc- illates with distance r . It is mediated by Fermi electrons

RKKY

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 magnetizedlayers 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 thecoupling changes:

But no matter who is right, there is no doubtabout one point: namely, it is the conductionelectrons that play a crucial role in interlayercoupling effects seen in multilayered metal-lic GMR systems. In semiconductors, in contrast, the concent-tration of conduction electrons is orders ofmagnitude 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?!

NOT RIGHT!

We have been conducting neutron scat-

tering 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.

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 devicescan be more easily integrated with existing electronics. Also, semiconductors have many highly interesting optical properties. Semicon-ductor spintronics may become an idealpartner for photonics!

There is one big problem, though.

For building practical spintronics devicesone would need semicondutors that are ferromagnetic at room temperature. AndGod did not make them. Rather, God left it as a challenge for us to create suchmaterials synthetically. Material techno-gists in many labs worldwide continueto work hard on this problem…

Room-temperature FM semiconductors:present situation

The “record-holder” now is epitaxially preparedGa(Mn)As alloy, with about 10% of Mn. It staysFM up to 175 K – still more than 100K below the “target value”. What can be done in such situation? Well, thereare some fundamental problems that need to be studied. For instance – what is the mechanism giving rise to interlayer coupling effects in sys-tems with low concentration of mobile electrons? We decided to do such studies on multilayers containing EuS, a well-known “prototypical” FMsemiconductor (with Curie T of only 16 K, though).

30-60 Å

4-200 Ǻ

number of repetitions10-20

Ferromagnetic EuS/PbS and EuS/YbSe SL’sEuS – Heisenberg ferromagnet TC = 16.6 K (bulk), Eg=1.5 eV

PbS – narrow-gap (Eg=0.3 eV) semiconductor (n ≈ 1017 cm-3)YbSe – wide-gap (Eg=1.6 eV) semiconductor (semiinsulator)

all NaCl-type structure with lattice constants:5.968 Ǻ 5.936 Ǻ5.932 Ǻ(lattice mismatch ≈ 0.5%)

(001)a=6.29 Å

Unpolarized neutron reflectivity experiments on the EuS/PbS system

(NG-1 reflectometer, NIST Center for Neutron Research)

Our collaborators

Electronic band structure in EuS

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.

Interlayer exchange coupling mediated by valence band electronsJ.Blinowski & P.Kacman, Phys. Rev. B 64 (2001) 045302.P.Sankowski & P.Kacman, Acta Phys. Polon. A 103 (2003) 621

Unpolarized neutron reflectivity experiments on the EuS/YbSe system

(NG-1 reflectometer, NIST Center for Neutron Research)

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.

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

More detailed explanation of the GMR mechanism