Question sessions

Below is a list of questions raised by the participants of ESM2019, and addressed by the lecturers during the 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 2019 will be listed below on a regular basis. See also the questions raised in 2018, 2017, 2015, 2013.

  1. [L. Abelmann] How can you explain the spiky shape of ferrofluids when subjected to an external magnetic field?
  2. [V. Simonet] Why is spin-orbit coupling large in platinum?
  3. [O. Fruchart] What does an electron feel: H or B?
  4. [V. Simonet] Will you discuss the gyromagnetic ratio for band magnetism, where the appareng gJ may take crazy values. [Answers to questions 4-7]
  5. [V. Simonet] Can you explain again Van Vleck paramagnetism?
  6. [V. Simonet] Why do we still have a degeneracy of the multiplet since we have filled te e- in the boxes?
  7. [V. Simonet] Using Hund's rules, I put all the electrons into their boxes and with their spin, so I know ml and ms fro each electron. From that I obtain J (with 3rd Hund's rule), and I know the degeneracy (values of mJ). Where does this degeneracy come from? What are the different states of teh electrons (for each value of mJ). Fomr the rules it seems that each electron has already a defined state (l,ml,s,ms).
  8. [O. Fruchart] Following Chikazumi fig.7.12, there are different ferrimagnetic ordering: N, P, Q and R type. Same thing for the various types of antiferromagnetic ordering. Is there some technological (application) where one needs to know this ordering type?
  9. [O. Fruchart] Classical energy consideration. Magnetic field does not do any work. What is the energy behind a magnet pulling a ferromagnetic body? Where it comes from and where it goes?
  10. [O. Fruchart] Concerning the "loop" model of ferromagnet. What about superconductors? It seems more physical in such a system...
  11. [V. Simonet] Why there are three valence electrons for all actinoids and lanthanoids?
  12. [V. Simonet] Is these a general and unambiguous definition or experimental proof for a quantum spin liquid? If yes, what is it?
  13. [O. Fruchart] Can you explain the Dzyaloshinskii-Moriya interaction with a suitable example? If yes, please explain it. How can the Dzyaloshinskii-Moriya interaction Dij be measured?
  14. [O. Fruchart] How one can obtain angle-dependent M-H curves using a VSM magnetometer? (as discussed in switching field versus coercive field slide).
  15. [L. Abelmann] Why are magnets often colored blue (S) and red (N).
  16. [O. Fruchart] Why in magnetism flux = quantity x area (e.g. magnetic flux = B tiems area), but in other branches of sciences, it is flux :quantity/area (ex: heat flux, mass flux, momentum flux).
  17. [L. Abelmann] How is spin-orbit coupling responsible for magneto-crystalline anisotropy?
  18. [V. Simonet] Is the Dzyaloshinskii-Moriya interaction, which promotes spiral and skyrmion systems, is also responsible for helimagnetism?
  19. [O. Fruchart] Can you comment more on the difference between saturation and spontaneous magnetization Ms? Is spontaneous magnetization an intrinsic property and saturation saturaiton an extrinsic property, or is it more complex?
  20. [T. Jungwirth] What is the difference between spin transfer torque and spin-orbit torque?
  21. [T. Jungwirth] What are field-like and damping-like torques? What is the physical meaning of damping paramater alpha?
  22. [T. Rasing] Consider the various configurations of ferromagnets, ferrimagnets, antiferromagnets (in-plane, out-of-plane, non-collinear), which of them are in theory the best candidates for all-optical switching in terms of: 1/ Faster magnetization reversal 2/ Deterministic reversal 3/ Lower threshold energy of magnetization reversal
  23. [T. Jungwirth] In the spin-Hall effect, the electrical current, spin current and spin polarization axis are all orthogonal to each other. If you have an in-plane electrical current, what decides if the resulting spin current is in-plane or out-of-plane? DO you have both? Does the thin film geometry play a role (magnetostatics)? And the structure of the heavt metal (crystallographic axis).
  24. [T. Jungwirth] If Néel came to life today, and you had 30 seconds to talk to him about antiferromagnets, what would you say?
  25. [T. Jungwirth] Could you give some words about magnetic symmetry groups in the context of ferromagnets and antiferromagnets?
  26. [T. Jungwirth] A ferromagnet forms domains to reduce the stray field. Why are there domain walls in antiferromagnets?
  27. [T. Jungwirth] How can the antiferromagnetic order parameter be measured? Which methods can visualize it, or how to distinguish the varous types of domains, and antiphases.
  28. [O. Fruchart] Could you give a more pictorial explanation about the domain wall configuration before and after the Walker breakdown in ferromagnets? Which is the critical difference between ferromagnets and antiferromagnets to not experience the Walker breakdown in the second one? I mean, the dynamics of magnetic textures in the second one is governed by special relativity and not the first one. Some way to tune the occurrence of the Walker breakdown?
  29. [O. Fruchart] Is damping parameter alpha isotropic? What is the microscopic origin of alpha? Could alpha be chiral?
  30. [O. Fruchart] Why are the dipolar fields in low-dimensional systems short-ranged? How does the M-dependence change when dimensionality reduces from 2D to 1D and OD?
  31. [O. Fruchart] How is a recoil loop different from a first-order reversal curve?
  32. [O. Fruchart] How does the Dzyaloshinskii-Moriya interaction lead to the formation of a skyrmion?
  33. [O. Fruchart] How could one vizualize (domain wall reversal) / (switching of magnetization direction across a domain wall) with respect to the crystalline structure? For example, in a material with a Néel wall does the magnetization at each atomic site turn or is it the moment per unit call that changes across the DW? [Question requires clarification/reformulation - not provided = not answered]
  34. [O. Fruchart] Why isn't an antiferromagnetic material ferromagnetic at the surface?
  35. [T. Rasing] How do you predict/explain the typical oscillation period of the RKKY interaction when the thickness of the material changes? (for exemple in ruthenium between two magnetic layers)
  36. [O. Fruchart] I consider an antiferromagnet coupled to a ferromagnetic layer (thin film configuration), and apply a field to rotate the magnetization of the ferromagnet. 1/ What happens to the antiferromagnet? Can this be monitored experimentally? What technique would you use?
  37. [L. Ranno] Can you relate skyrmion dynamics to Magnus force?
  38. [L. Ranno] Concerning HAMR hard drives … We lower the magnetic volume we need to switch… But Ku (aniso.) is higher. Thus the energy to switch is ~ the same… But we need to come with plasmonic system with low efficiency… Is the overall energy balance (E/bit) of HAMR worse than regular hard drive?
  39. [L. Ranno] How to distinguish bubble domains and skyrmions if one considers both diameters are same? [Both having the same diameter].
  40. [L. Ranno] Does W give the perpendicular anisotropy to Fe (Pt gives perpendicular anisotropy to Co, Pt/Co) if Fe is deposited on top of W? Could you please explain it?
  41. [L. Ranno] What is relation between spin Hall angle and iDMI of Heavy metal (W, Pt, Au, Ta, Ir)? If spin Hall angle is negative (in the case of W), the iDMI should be negative or Positive or it depends on the interface between Heavy metal / Ferromagnet? Could you please explain with examples? W/Fe, W/Co, Pt/Fe, Au/Fe.
  42. [?] What is the maximum storage capacity of human brain? (in terms of GB) [not relevant to magnetism - sorry :)]
  43. [L. Ranno] Could you explain the origin of TAMR* when using STM techniques? *TAMR=Tunneling Anisotropic Magneto-resistance.
  44. [R. Shaefer] While enhancing the Ak (Kerr amplitude) using interfacial [interference] layers how canwe select the material and its thickness?
  45. [R. Shaefer] Domains in the paramagnetic state? The unpublished [work] from konstantin Skokov and Oliver Gutfleisch: Do you think it can be spin canting in paramagnetic state?
  46. [R. Shaefer] Having a magnetostrictive sample (ex.ribbon): What is a suitable experimental approach to have stably positioned [fixed] sample (for Kerr microscopy) without having concerns that magnetostriction-induced stress modifies the domain structure?
  47. [R. Shaefer] What are the benefits and disadvantages of using laser source for MOKE magnetometry?
  48. [R. Shaefer] Is there some maximum thickness for the sample to see the domains with the MOKE microscope? If there is, what is it?
  49. [R. Shaefer] If we measure in reflexion a nanoparticles thin film deposited on a dielectric transparent substrate, could we have any undesirable Faraday effect? Is there any way to avoid it?
  50. [R. Shaefer] Is the surface rugosity [roughness] an inconvenient to measure MOKE?
  51. [J. Lüning] Is it possible to make X-ray waveguide?
  52. [J. Lüning] How do you select the energy? [synchrotron X-rays] How do you select (create) a polarization of the X-ray beam? What is a difference between undolator and wiggler?
  53. [J. Lüning] Consider two situations: (1) Antiferromagnetic ordering of two different elements' magnetic moments. (2) Antiferromagnetic ordering due to just one element. - In case (2) one can apply XMLD to get antiferromagnet structure but not quantitative description. What are possible time-resolved Xray methods to do it quantitatively? In case (1) one can't do XMLD as both elements have different resonance edges. In this case, will it be possible to get info about AFM order by doing XMCD at two resonance edges and then compare the [illegible: "signs"?]. How to do quantitative analysis in this case?
  54. [L. Ranno] Maybe it's a particular situation. How can affect to see the domains in a ferromagnetic material above a diamagnetic material? [improve/clarify the question if you want a different answer].
    Rephrased: Can a ferromagnet be "screened" by a diamagnetic layer? Addition: How about superconductor instead of a common diamagnet?
  55. [A. Lubk] Does DPC work in diffraction?
  56. [A. Lubk] How does SEMPA detector work?
  57. [O. Isnard] ...[After Industry perspectives by a corporate member] Do you think it is OK to keep promoting such a way of working [corporates]? [Participants had a chance to see presentations and discuss with 3 people "from industry": start-up, side business of a reasearcher, corporate]
  58. [V. Basso/A. Garcia Arribas] Could you explain the Josephson effect?
  59. [V. Basso] What information can be obtained from minor loop?
  60. [V. Basso] Which unit weshould use in cae ofexternal field acting on sytem/material, A/m or T? Or maybe A/m for magnetizing field H and T for mu_0 H?
  61. [A. Lubk] What exactly mean large scale variation in TIE? << 1 micron, or more?
  62. [A. Lubk] Are there other ways how to separate electric and magnetic contributions to the phase shift in electron holography (off-axis)?
  63. [V. Basso] What can we learn from FORC?
  64. [V. Basso] Why is it impossible to achieve saturation [hysteresis loop]? Why did scientists came up with something unattainable again?
  65. [M. Urbánek] Where will be the banquet? How will I get there?
  66. [L. Ranno] FMR measurement: sometimes (in some papers) referred to as the dispersion of resonance. Why not discussed likethis in the lecture? Is there a difference?
    What kind of materials are used for circulators (Faraday effect) in the Q, s, ... bands?
  67. [L. Ranno] Why the damping alpha estimated from FMR is different from the one estimated from DW motion? [picture attachment] alpha_FMR << alpha_DW typically. Why??
  68. [L. Ranno] VNA-FMR, how can we fit the FMR spectra (S_21 or S_12) to calculate H_res & linewidth? Could you please explain? ... Which type of lorentzian equation is suitable for perpedicular measurement?
  69. [L. Ranno] How to determine the relaxation time of whole system? Can we calculate [it] from properties of the material or only estimate it from experiments? How this time depends on magnetizing field?