Question sessions

The purpose of a research School is to provide young scientists the basics in a working field. With this respect interactivity between students and lecturers should be promoted. Thus a key aspect of ESM is the possibility to raise questions during the course of the lectures, interrupting the lecturer. Questions can also be posted anonymously in 'question boxes'. Special sessions for answering these questions are organized, typically 1h every day or two days, during which the lecturers or voluntary students present in more detail issues raised by the students during the lectures or anonymously through a question-box. Questions raised and explained in 2018 will be listed below on a regular basis. See also the questions raised in 2017, questions raised in 2015, questions raised in 2013.

1. [S. Blügel] Can you discuss when one measures S_z=1/2 or S=√(3/4) in an experiment?
2. [S. Blügel] How can you compute electronic properties of liquids or amorphous materials by DFT, while they lack translationnel symmetry?
3. [R. Evans] How is M_s defined, measured, determined. Why use Ms instead of M?
4. [R Evans] Please summarize the use of H, M, B, B=μ₀M, B=μ₀(H+M), demagnetizing field.
5. [R. Evans] Why is a loop of current equivalent to a magnet?
6. [R. Evans] The demagnetization field is a response of the sample to an applied field. Usually, the sample response (magnetized region ) is captured in the (local) magnetization. Why do we include the demagnetizing field in H rather than M, then?
7. [S. Blügel] Particle physicists tell me that the electron is a point particle (because they typically consider it in free space). In atoms and solids the electrons are always delocalized to some degree (they are bound within the finite solid). Is it still useful (or realistic) to understand the electron as a point particle? Why?
8. [S. Blügel] Does trhe introduction of spin wave functions imply that the spin of a quantum state should be interpreted as a local (position-dependent) quantity? I understand wavefunctions as the expression of a state to the position basis.
9. [J.V. Kim] Can we have time to introduce magnetic excitations? Such as: spin wave excitation (magnons), Stoner excitation, behavior of spin under thermal or electrical effect.
10. [S. Blügel] What does the Berry phase tell you? Why is it usefull?
11. [P. Oppeneer] In the case of very few spins (down so single spin) the LLG equation is probably wrong because of the quantum nature of spin. Is there a quantized LLG equation (or, Schrödinger with precession?). Do we know the dynamics of the systems?
12. [A. Kalashnikova] Magnons well describe low-temperature range. Mean-field approximation well describes near-critical-temperature range. What kind of spin behavior does happen between the two ranges?
13. [P. Vavassori] Pr. Oppeneer said that off-diagonal components of dielectric permittivity are given by Hall conductivity. Does it mean that Hall effect and magneto-optic effect are connected? What are the microscopic parameters which determine the strength of these effects?
14. [S. Blügel] You said that Born-Oppenheimer approximation neglects two terms, one of them being the Berry phase. What are the consequences of neglecting the other term?
15. [O. Fruchart] In atomistic models spins are always assumed to be localised on lattice sites. In reality, electron wave function has a distribution and in non-tight-bonding models can be completely delocalised. Does this need to be taken into account, and if so, how?
16. [O. Fruchart] In a thin film heavy metal/ferromagnetic (ex: Pt/Co), how are the sign of the spin Hall angle and the sign of the DMI related? Is it possible to derive one from the other?
17. [O. Fruchart] For ferromagnets, there are minority and minority spins. In the band structure there are there are gaps (splitting) between majority and minority spins. What does determine the gap (splitting); Exchange intercation? Then, for antiferromagnets, there are degenerate state. In this case, why does it show a degenerate band structure.
18. [O. Isnard] When a material has an easy plane anisotropy, how to determine the direction of magnetic moment? The energy difference between hard axis and easy axis is very small.
19. [O. Isnard] When temperature increases, why do spins form spin waves instead of making totally random orientation?
20. [O. Fruchart] What is biaxial strain, and how does it affect the magnetic anisotropy of thin films?
21. [A. Kalashnikova] What is the microscopic mechanism behind spin-lattice energy transfer? What determines the strenghth of the coupling ?
22. [J-V Kim] It is not necessary true that T1=2*T2 or T2<<T1. What happens then ? Can we model this (since only T1=2*T2 agrees in micromagentics and T2<<T1 can be done by Gilbert/LL equations)?
23. [J-V Kim] How DMI is measured using Brillouin Light Scattering Microscopy?
24. [S. Blügel] What is the Berry phase ?
25. [A. Kalashnikova] Which Processes lead to proximity induced Magnetization ? How can it be measured ?
26. [J.V. Kim] Are there many different kinds of spin wave mode? (like phonon it has transverse/longitudinal mode and acoustic/optical mode)
27. [J.V. Kim] Is there any way to describe an antiferromagnetic spin wave? (spin wave in antiferromagnetism)
28. [A. Kalashnikova] One of the source of anisotropy is a deformation. Can we describe magneto-elastic coupling using anisotropy description?
29. [A. Kalashnikova]Since the Curie temperature depends on the number of neighbors, is there a known dependence from the particule size? Does it also Hold true for antiferromagnets ?
30. [O. Isnard] What is a spin glass? Is it usfefull for anything ? Where do they appear ?
31. [O. Isnard] What is meant by itinerant electron systems ?
32. [J.V. Kim] What is the Berry phase ?
33. [A. Kalashnikova] Which process lead to proximity induced magnetization ? How can it be measured
34. [A. Kalashnikova] Superdiffusion theory for ultra fast demagnetization. If polarized electrons are transported away quickly after excitation by the pulse through the sp-band, why would they not simply diffuse back after sometime (let's say µs)? In this theory, the total magnetization (sum inside and outside laser spot) does not change.
35. [O. Isnard] What is a spin glass.? is it usefull for anything ? Where does they appear ?
36. [O. Isnard] What is meant by itinerant electron systems.
37. [O. Isnard] Can antiferromagnets be a half metal ?
38. [O. Isnard] Kim has explained how magnetism can be manipulated by spin-transfer torques (by applying current). In multiferroics we can also control magnetism by electric field (magneto electric coupling effect!) vice versa. Question idid you see any system which spin-transfer torques and magneto-electric coupling effect contribute in the same time? If yes how could you seperate them?
39. [E. Georing] In XMCD, why we talk about only pure spin moment <Sz> and pure orbital moment <Lz>, despite knowing that electron possess combine quantum number, good quantum number, J ?
40. [E. Goering] What happens at the K absorption edge in XMCD ? How does it differs from L edges?
41. [E. Goering] Can you explain if XMCD is appropriate for AF systems?
42. [O. Isnard] Can we do XMCD or similar technique to distinguish not only different atoms but atoms on different atomic positions?