Co nam strukrura magnetyczna może powiedzieć, i czego

1. Co nam strukrura magnetyczna może powiedzieć, i czego nie może jeszcze powiedzieć o oddziaływaniach wymiennych spin-spin w krysztale?

3. Uporządkowanie IV Typu wymaga istotnego wkładu od TRZECICH SĄSIADÓW

5. 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 podobny do znanego oddzialywania RKKY -- z tym, ze w gre wchodza nie rzeczywiste nosniki, a wirtualne, zatem sprzezenie R-B moze zachodzic w materialach ubogich w nosniki, czyli w polprzewodnikach). Dla rozstzygniecia watpliwosci bardzo pomocna bylaby dokladniejsza znajomosc wartosci calek wymiany -- tymczasem, wyniki neutronowych badan strukturalnych dostarczyly 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.

6. Spektrometria nieelastycznego rozpraszania neutronów w zastosowaniu do badania izolowanych par najbliższych sąsiadów magnetycznych w materiałach z „rodziny” półprzewodników półmagnetycznych.

8. Poziomy energetyczne dla pary antyferromagnetycznie sprzężonych spinów 5/2, idozwolone przejścia przy nieelastycznym rozpraszaniu neutronów:

9. Schematic of a time-of-flight inelastic neutron scattering spectrometer − a perfect tool for investigating dispersionless excitations in solid: Disk chopper

10. This is one of the instruments we use:

12. The idea worked, and even though the FM coupling induced by holes was weak, the information the measurement yielded was really valuable

13. 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).

14. E=0.158 meV Zn1-xMnxTe (x = 2%)=4.8Å, T= 40K • J1atm= -0.795 meV • J4Katm=-0.836 meV • 5% Energy Shift • 0.49% Lattice Compression • |2|1 Transition

17. 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 respect for family values?)

18. 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.

19. Neutron diffraction and inelastic scattering (such as, e.g., phonon scattering) are often termed as “wide-angle scattering processes”. They are described quite well in the framework of Born Approximation. However, the Born Approximation is no longer good in 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.

20. n0 k0 k0x a Różnica między współcz. załamania dla światła i dla cząstek kwantowych: k0y k’y n’ b k’x k’ Odwrotnie niż dla światła!

21. Another major neutron scattering tool: reflectometry

23. Research profile of the OSU neutron 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 think they should be given credit for a discovery

24. Nobel Prize in Physics, 2007: Discovery of the phenomenon of Giant Magnetoresistance GMR in a Fe/Cr/Fe “sandwich”: Peter Grunberg (Germany) Albert Fert (France)

26. 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.

27. The aforementioned 5 billions of spintronics devices were all made of metals. 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. Semicon- ductor spintronics may become an ideal partner for photonics!

28. What are neutrons good for inspintronics-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 antiferromagnetically acros the intervening non-magnetic spacer: This interaction also assures that the system returns to its initial configuration after the field is removed.

29. METALS: 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 prepa- red a “trilayer” consisting of two iron films, with a wedge-shaped non-magne- tic 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 be- tween the Fe layers was an oscillating function of the Cr layer thickness. So, Grunberg’s discovery sho- wed that the desired con- figuration can be obtained by choosing an approp- riate spacer thickness.

30. What is the origin of the interlayer interaction with oscillating sign? There is still no consensus among researchers ragarding this issue. But most agree that it is “a version” of RKKY interaction (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 r

31. Experts still argue about details of the IEC mechanism in metallic multilayers. 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 metal- lic GMR systems. In semiconductors, in contrast, the concent- tration of conduction electrons is orders of magnitude lower than in metals. Some of them are nearly-insulating. So, it may imply that in analogous systems made of semiconductors there is no chance of seeing IEC. RIGHT?!

32. 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.

33. FerromagneticEuS/PbSandEuS/YbSeSL’s EuS – 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: 5.968Ǻ 5.936 Ǻ 5.932Ǻ (lattice mismatch ≈ 0.5%) 4-200 Ǻ 30-60 Å number of repetitions 10-20 (001) a=6.29 Å

35. Neutron reflectivity experiments onthe EuS/PbS system (NG-1 reflectometer, NIST Center for Neutron Research) Situation corresponding tored data points: Situat. corresponding togreen data points: Situation corresponding toblue data points

36. Neutron reflectivity experiments on EuS/PbS & EuS/YbSe systems (NG-1 reflectometer, NIST Center for Neutron Research)