Questions

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 2022 are listed below. See also the questions raised in 2019, 2018, 2017, 2015, 2013.

1. -answered- [S. Blundell] Is Zeeman energy a fifth Maxwell equation, or can it be derived?
2. -answered- [S. Blundell] Was the Aharanov-Bohm effect experiment the first time to give the vector potential a physical meaning?
3. -answered- [J. M. D. Coey] Could you explain in an easy way what the g factor is (Landé)?
4. -answered- [J.M.D. Coey] If the electrons hop between orbitals, at what speed do they do this?
5. -answered- [J.M.D. Coey] Why do orbits have these funny names? s, p, d etc.
6. -answered- [J.M.D. Coey] In his lecture, Prof. Coey said that 3d orbitals can be localized and delocalized, whereas 4f orbitals are only localized. However, 3d orbitals are much smaller than 4f ones. Why the spatial expansion of 4f orbitals can't be considered like delocalized orbitals?
7. -answered- [C. Donnelly] Amorphous ferromagnets are known as very soft magnetic materials. Compared with Permalloy thin films, why do amorphous ferromagnetics get more soft magnetic with increasing thickness, while the coercivity of permalloy thin films does not vary much with thickness?
8. -answered- [J. M. D. Coey] How do you define the Néel vector in antiferomagnets?
9. -answered- [J.M.D. Coey] I want to understand the difference between the echange and dipole-dipole interaction
10. -answered- [J.M.D. Coey] Mn behave antiferromagnetically on a hexagonal lattice et low temperature. Why is Mn not considered as an itinerant metal like Fe, Co, Ni?
11. -answered- [J.M.D. Coey] Fe becomes antiferromagnetic when only 1 monolayer of it is deposited on W(001). Could you comment on this? I would like to have a perspective from the Stoner criterium point of view
    • Observation and explanation in this paper: A. Kubetzka, P. Ferriani, M. Bode, S. Heinze, G. Bihlmayer, K. von Bergmann, O. Pietzsch, S. Blügel, R. Wiesendanger, Revealing Antiferromagnetic Order of the Fe Monolayer on W(001): Spin-Polarized Scanning Tunneling Microscopy and First-Principles Calculations, Phys. Rev. Lett. 94, 087204 (2005)
12. -answered- [J.M.D. Coey] Cr is a classic example of an itinerant magnetic material. Do these spin density waves move? If I were to probe locally (atom by atom), would I see a net moment with magnitude varying in space, from atom to atom?
13. -answered- [J.M.D. Coey] How is magnetism treated in amorphous materials? (terminology etc.)
14. -answered- [J.M.D. Coey] What is a crystal electric field?
15. -answered- [C. Donnelly] Could you talk about the gyromagnetic ratio in terms of applications?
16. -answered- [C. Donnelly] In the Stoner-Wohlfarth model an anisotorpy term is considered. Is it coming from dipolar energy? How does it work?
17. -answered- [C. Donnelly] What is the best technique to image domain nucleation under the application of a magnetic field?
18. -answered- [O. Fruchart] Will fixing the surface morphology of a thin film, fix its magneto-crystalline anisotropy?
19. -answered- [O. Fruchart] What are the ways to experimentally quantify magnetic anisotropy for a thin film?
20. -answered- [B. Dupé] What is the link between magnetic anisotropy and spin-orbit coupling? S·L gives a scalar, but anisotropy is a vector.
21. -answered- [B. Dupé] What is the origin of the word moment, in, say, magnetic moment.
22. -answered- [B. Dupé] When it is useful to view magnetic moments around atoms as "arrows", or points, and when is it useful to view them as electron clouds with a spatial extension?
23. -answered- [O. Isnard] Do all distortions result in lifting the degeneracy of the eg or t2g energy levels or how much should be the minimum distortion required for that?
24. -answered- [O. Isnard] Why Mn3+ ions can't give rise to Jan-Teller distortion? Why does doping, causing a mixture of Mn3+ and Mn4+, gives rise to Jan-Teller distortion?
25. -answered- [A. Manchon] Can you give us an intuitive explanation of the orbital Hall effect and orbital Hall conductivity?
26. [A. Manchon] In the case of thin film, the substrate imposes a strain on the film. Will the thin film of LSMO still have a Jan-Teller distortion even if it grows on a lattice-matched substrate?
27. -answered- [A. Manchon] How close are we in understanding the reason for colossol magnetoresistance effect? What is the problem in the realization of CMR-based devices?
28. -answered- [A. Manchon] What is the origin of the phenomùmnon of anisotropic magnetoresistance? I understand that it is due to spin-orbit interaction and J-T distortions in the crystal. Could you elaborate?
29. -answered- [A. Manchon] The Fermi arcs in Dirac semimetals are (as far as I have found) always arcs (ARPES). The "links-up" to connect charges could also be trivial, correct? Why do we only find the arc case?
30. -answered- [A. Manchon] We learned that the crystal electric field as a real operator implies that the eigenfunctions of the operator have to be a superposition of ±m (quantum numbers). Example: |Ψ>=(|+m>+|-m>)/√2. Otherwise, there would not be any real eigenfunction. The question is: what would happen if our crystal field is varying in time? The crystal-field operator would then also have an imaginary component (I guess) so that the atomic eigenstates |m,ℓ> are good eigenstates also for the crystal field again. Would the quenching be lifted in this case?
31. -answered- [C. Donnelly] What is the difference between a Néel skyrmion and an antiskyrmion? And if any, what are the corresponding anto-skyrmions fr the Bloch and Néel type?
32. -answered- [C. Donnelly] Can you define precisely what topology is?
33. -answered- [C. Donnelly] Could you re-explain the skyrmion annihilation process? I did not clearly understand why we need the Bloch point or an antiskyrmion to annihilate a skyrmion.
34. -answered- [C. Donnelly] Could you please elaborate more on topological protection? Temperature can always destroy skyrmions, right? But driving them to sample boundaries does not destroy them? It is a bit confusing that they can be by B sweeps or electric currents. There is nothing like a topological conservation law, correct?
35. -answered- [C. Donnelly] How do you cope with the high intensity, hence energy flux, provided by XFEL facilities. Do these energies effect the measurements?
36. -answered- [C. Donnelly] Can the various types of pinning walls be categorized (e.g., on an energy scale) according to how strongly they pin domain walls? Or, does this hierarchy depend on the type of material?
37. [B. Dupé] Prof. Coey mentioned that phase transitions was depending on the dimensionality (1D, 2D, 3D). In 2D, the magnetic order and phase transitions are tricky. Hoow can we theoretically understand the magnetic order in 2D materials that was found experimentally rcently?
38. [B. Dupé] Could yo uplease explain why you have Néel domain walls in thin films. Any exceptions?
39. [B. Dupé] Can we discuss in more detail the origin of magnetic domains in antiferromagnets?
40. [B. Dupé] What is the origin of shape effects in antiferromagnets?
41. [B. Dupé] In the LLG equation the Gilbert damping is written in a scaler quantity. Are the material situations leading to a tensor quantity? (symmetry or direction considerations)
42. [B. Dupé] Is a skyrmion a closed domain wall?
43. [B. Dupé] Are Bloch points defined only for 3D systems? Or, can theyr be found in 2D systems?
44. [C. Donnelly] Can skyrmions be described with a Chern number as well, or ate they a ℤ₂ topological index? If so, can you have double, triple etc. skyrmions?
45. [T. Thomson] Why is magnetic recording in HDD more favorable with perpendicular anisotropy?
46. [T. Thomson] Ru and Ir provide AF coupling to ferromagnetic blocks. Are there any other materials like these.
47. [A. Manchon] Why YIG has the lowest damping coefficient?
48. [A. Manchon] Can you please explain the Rashba and Dresselhaus effects?
49. [A. Manchon] Can you explain the chemical potential with respect to band structures? (referring to band filling)
50. [A. Manchon] In STT, when I send a spin-up current to a layer with spin down, which spin exerts a torque? How to know?
51. [A. Manchon] How do I understand the importance of k space (energy versus k) in spin-split bands, when I want to do experiments?
52. [A. Manchon] Antiferromagnets are usually said to be ultrafast, having high frequency of precession. This is also one of the motivations to study them. Could you comment, where these high frequencies come from?
53. [A. Manchon] Why do we (not) have to consider other spins than up/down fro spin transfer?
54. [A. Manchon] Is the coherence of the tunneling across an MgO barrier different for in-plane and out-of-plane configurations?
55. [A. Manchon] Intuitivly, what is the Berry phase, and how does it relate with the spin Halle effect?
56. [N. Dempsey] I have measured the magnetic hysteresis along (100) for Nd-Sr-Mn-O thin films at low-temperatures. How can I tell which one has improved magnetic anisotropy along the measured direction ?
57. [O. Isnard] Why Weiss theory did not explain Ms versus T solution exactly and how scientists thought that spin wave theory will work?
58. [O. Isnard] What is the difference between electron paramagnetic resonance (EPR° and ferromagnetic resonance (FMR)?