Co nam strukrura magnetyczna może powiedzieć, i czego nie może jeszcze powiedzieć o oddziaływaniach

wymiennych spin-spin w krysztale?

Hamiltonian dla oddzialywania spin - spin :

H 2Jklˆ S k ˆ S l , gdzie Jkl to stala wymiany dla spinow k, l;

Energia oddzialywania spinu k w sieci ze spinami - sasiadami :

E 2 ˆ S k Jklˆ S l

istotnisasiedzi

W polprzewodnikach i izolatorach o strukturze FCC "istotni

sasiedzi" to prawie zawsze tylko 12 najblizszych sasiadow

i 6 "drugich" sasiadow. Stale wymiany dla nich oznaczymy

jako, odpowiednio, J1 i J2.

Przy "kolinearnym" ustawieniu spinow (tylko "gora", "dol")

iloczyny skalarne ˆ S k ˆ S l dla roznych par sasiadow roznia sie

tylko znakiem, co upraszcza obliczenia energii. Struktury

z "niekolineranym" ustawieniem spinow sa bardzo rzadkie!

Z rysunkow nietrudno obliczyc,

ze na kazdy spin w strukturach

Typu I, II i III przypada energia :

E I 8J1 12J2;

E II 12J2;

E III 8J1 4J2;

Zas w przypadku uporzadkowania

ferromagnetycznego, energia

wyniesie :

EFM 24J1 12J2.Uporządkowanie IV Typu wymaga istotnego wkładu od TRZECICH SĄSIADÓW

Majac energie, nietrudno z kolei obliczyc, w jakim

zakresie wartosci J1 i J2 dana struktura ma najnizsza

energie ze wszystkich - innymi slowy, mozemy

otrzymac diagram fazowy dla antyferromagnetykow

o strukturze FCC w plaszczyznie ( J1 , J2 ) :

Fakt, ze w Cd1 xMnxTe wystepuje

struktura Typu III oznaczal wiec,

ze w tym materiale dominujacym

oddzialywaniem jest antyferroma -

gnetyczne sprzezenie pomiedzy

najblizszymi sasiadami (J1), zas

oddzialywanie pomiedzy drugimi

sasiadami (J2) jest slabsze,

ale tez antyferromagnetyczne.

Na przelomie lat 1970-tych i 1980-tych rozwinela sie intensywna debata, ktorej celem bylo ustalenie mecha-nizmu fizycznego, odpowiedzialnego za to oddzialy-wanie. Bardzo wiele uwagi skupilo sie wtedy na tzw.“mechanizmie Rowlanda-Blombergena” (nieco podobnydo znanego oddzialywania RKKY -- z tym, ze w gre wchodza nie rzeczywiste nosniki, a wirtualne, zatem sprzezenie R-B moze zachodzic w materialach ubogichw nosniki, czyli w polprzewodnikach).Dla rozstzygniecia watpliwosci bardzo pomocna bylabydokladniejsza znajomosc wartosci calek wymiany -- tymczasem, wyniki neutronowych badan strukturalnychdostarczyly jedynie informacji o znaku calek J1 i J2 oraz o tym, ze J2/J1<0,5.Ale mozliwosc bardzo dokladnego wyznaczenia J1 stworzyly znow neutrony!!! -- tyle, ze przy uzyciu nie pomiarow dyfrakcyjnych, a spektometrii rozpraszania nieelastycznego.

Spektrometria nieelastycznego rozpraszania neutronów wzastosowaniu do badania izolowanych par najbliższych

sąsiadów magnetycznych w materiałach z „rodziny”półprzewodników półmagnetycznych.

Poziomy energetyczne dla pary antyferromagnetyczniesprzężonych spinów 5/2, idozwolone przejścia przy

nieelastycznym rozpraszaniu neutronów:

Schematic of a time-of-flight inelastic neutron scatteringspectrometer − a perfect tool for investigating

dispersionless excitations in solid:

Disk chopper

This is one of the instruments we use:

The idea worked, and even though the FM coupling induced byholes was weak, the information the measurement yielded wasreally valuable

People are trying to figure out the rules that govern the Mn-Mn NN exchange in dilute alloys derived from the II-VI compound family (ZnO, ZnS, ZnSe, ZnTe,CdS, CdSe, CdTe, HgS, HgSe, HgTe).

Zn1-xMnxTe (x = 2%)=4.8Å, T= 40K

• J1atm= -0.795 meV

• J4Katm=-0.836 meV

• 5% Energy Shift• 0.49% Lattice

Compression

E=0.158 meV • |2|1 Transition

Our data show that the “tendency line” based on the entire family definitely is not a universal characteristic, because the J(r) depen-dence in individual compounds from the (II,Mn)-VI family is deci-dedly steeper than it predicts (individuals showing lack of respectfor family values?)

WRACAMY DO STRUKTUR!!

Ale teraz będzie o strukturach innego rodzaju – mianowicie, o takich, co się tworzą w supersieciach złożonych z bardzo cieńkich warstw magnetycznych półprzewodników.

Tutaj używamy technik reflektometrycznych, czyli badamy odbicie neutronów od płaskiej próbki pod bardzo małymi kątami.

Neutron diffraction and inelastic scattering (suchas, e.g., phonon scattering) are often termed as

“wide-angle scattering processes”.

They are described quite well in the framework ofBorn Approximation.

However, the Born Approximation is no longer goodin the region of very small scattering angles. In particular, for describing small-angle neutron

scattering from flat surfaces, or neutron reflectivity, one has to switch to the

formalism of neutron optics.

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Różnica między współcz.załamania dla światła i dla cząstek kwantowych:

Odwrotnie niż dla światła!

Another major neutron scattering tool: reflectometry

Research profile of the OSUneutron scattering team:

Magnetic Semiconductors – primarily, in the context of

spintronics research

About spintronics: a milestone event that “gave birth” to spintronics, I would risk*

to say, was the discovery of Giant Magnetoresistance

*why risk? Well, you never know all them who thinkthey should be given credit for a discovery

Nobel Prize in Physics, 2007: Discovery of the phenomenon of

Albert Fert (France)

Peter Grunberg(Germany)

Giant Magnetoresistance

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.

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!

The aforementioned 5 billions of spintronics devices were all made of metals.

What are neutrons good for in

spintronics-oriented reseach? One such topic is certainly interlayer exchange

coupling (IEC) in multilayered structures.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.

METALS: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. But most agree that it is “a version” of 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

Experts still argue about details of the IECmechanism in metallic multilayers.

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, it mayimply that in analogous systems made of semiconductors there

is no chance of seeing IEC.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.

30-60 Å

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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

Neutron reflectivity experiments onthe EuS/PbS system

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

Situation corresponding to red data points:

Situation corresponding to blue data points

Situat. corresponding to green data points:

Neutron reflectivity experiments on EuS/PbS & EuS/YbSe systems

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