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 2013 are listed below.
Questions raised in 2013 (processed):
- Is Zeeman energy a postulate or can it be demonstrated?
- Are the dipolar field, demagnetizing field and magnetostatic field similar?
- 1/ What is the unity statA? 2/ Is these any link between A and statA?
- Explain bracket notation: 〈 X | ABC | Y 〉
- What is the difference between the magnetic moment m0=gJ and the effective moment Meff=sqrt(J(J+1)). Which one is measured experimentally?
- Why is Meff=3.4µB measured for Eu3+ although one expects Meff=0 as J=0?
- What is the susceptibility? What is the physical meaning of real and imaginary susceptibilities? [ Supporting slides ]
Magnetism in matter
- Why the exchange interaction in a magnetic material is described with spin interactions only and not directly with J interaction? [ Supporting slides ]
- Critical exponent σ∼(1-T/Tc)β. In the Heisenberg model β≈0.36. How is this found?
- How do the molecular field parameter λ and Heisenberg exchange strength relate?
- How can we define (in the best way) the value of the Tc from experiment [M(T) curve]: with M2→0? Or ∂H/∂T→0? or something else?..
- Usually, the deviation of the inverse susceptibility (1/χ) with respect to temperature (T) from Curie-Weiss law originates from short-range magnetic order. Questions: Why? How to observe this experimentally? [ Supporting slides ]
- Does the spin magnetic moment change for different structures of a sample (like bulk, slab, wire etc.)? [ Supporting slides ]
- 1/ Why Ho is only ferromagnetic below 20K? 2/ Do you think one could "design" other materials with even higher magnetic induction saturation than Ho? What would be your approach?
- Is there an easy explanation how RKKY interaction can mediate helical spin structures as e.g. in Ho or Eu? In addition to the answer, please see tutorial of C. Lacroix [ Supporting slides ]
- Can you talk about Dzyaloshinky-Moriya interaction [ Supporting slides ]
- What is the triangle rule?
- Why the expansion of Φ converge when the system has weak spin-orbit coupling? Where is it expressed on the expansion?
- Could you again explain in simple words the anisotropy constant? [ Supporting slides ]
- A question about magnetic anisotropy and crystal field: is there any possibility to have a less strict condition on the number of atoms, to define magnetic anisotropy also for microstructures or thin films or for these systems a completely different approach is needed? How the equations of anisotropy evolve to take into account systems with a small number of particle (∼103, 104).
- How to obtain crystal field parameters? [ Supporting slides ]
- In the lectures it was stated that the Stevens operators Cnm(J')=Cnm(θ,φ).Cn0(J). Where does it come from? Added during the reply, very good reference: Quantum theory of angular momentum, D. A. Varshalovich, A. N. Moskalev, V. K. Khersonski,World scientific, Singapore (1988).
- Could you explain the Jahn-Teller effect in La1-xCaxMnO3? [ Supporting slides ]
Magnetization patterns and processes
- What is a practical definition of magnetic saturation (induction)?
- Why do domain walls in ferromagnets move?
- Sketch of W. Wulfhekel for a domain wall inside a nanowire: how can there be positive charges in the domain wall, and how can the sum of all charges be zero?
- Could you explain a little bit more Bloch point wall? Conditions for instance.
- What is the easy axis?
- About easy/hard magnetization axis: sometimes we have an inversion of magnetization as for magnetization curves along  and. What is the physical interpretation of that? Which axis is the easy / hard magnetization axis?
- Could you explain the Preisach model for hysteresis in bulk permanent material in simple words?
- What demagnetizing factor does a powder have (a set of many small particles)? What influence does the shape of the particles have (round, rods, irregular)?..
- What is the domain structure of a sphere?
- Why does ∇.B=0 implies a constant magnetization at the domain boundaries?
- What experiments can be used to see the difference between superparamagnetism and spin glass?
- What is a magnon? What is two-magnon scattering?
- Can we use system like MOKE to measure anisotropy?
- How can I distinguish an antiferromagnet from a paramagnet experimentally: probably in the presence of a ferromagnetic phase in the same system? [ Supporting slides ]
- What is an Auger electron?
- What is the significance of mℓ/ms ratio? (usually reported in papers with XMCD measurements)
- Is it not possible to correct for chromaticity by using sextupoles?
- Can you explain how can PEEM technique provide magnetization orientation (FM and AFM)?
- Is it possible that magnetic materials have a very sharp phase transition and no (or very little) hysteresis?
- Is the metamagnetic tricritical point a mathematical trick, or is it real?
- Please give some practical example of a refrigerant, for example MnFe, some values: TC, cooling power per kg etc. How is such a fridge constructed?
- Please explain how Carnot efficiency can be understood with magnetic cooling.
- What is the difference (if any) between a magnetoelastic multiferroic and a magnetocaloric
- How do you switch M in a ferromagnet using a piezoelectric?
- How to observe a twinning structure experimentally? How to make stress versus strain measurements? How easy is it to change the structural transition temperature by eg slightly changing the composition? [ Supporting slides ]
- How and why magnetostriction influence of modify the maximum of the variation of the magnetic entropy? [ Supporting slides ]
- What is the spin transfer torque?
Questions raised in 2013 (pending):
- In the introduction to magnetism lectures Prof. Wulfhekel, Landé factor g=2.0023... was given for the electron spin. How is this value calculated?
- Can you describe in simple words what is spin?
- What is the hamiltonian? What is perturbation?
- In classical mechanics we have A×A=0 but why in quantum mechanics we have A×A≠0?
- [to O. Fruchart] To W. Wulfhekel: could you explain why we have Zeeman splitting (without equations please). Physical explanation.
Magnetism in matter
- Why Gd2O2S and other rare earth X2O2S have high specific heat (peaks) at cold temperatures (∼4-10K) and does not follow 1/T3?
- [to K. Sandeman] How does C=∂E/∂T∼τ0 relates to Debye law C∼T3?
- [M. Viret] What is the wavelength of a typical spin-wave? How about the long-wavelength case?
Magnetization patterns and processes
- [to O. Fruchart] What is the difference (if any) between exchange bias and exchange coupling?
- [to O. Fruchart] How evolves the Curie temperature in the case of a trilayer (or more complex: multilayer) such as magnetic/non-magnetic/magnetic (as a function of thickness of non-magnetic layer).
- [to O. Fruchart] Could you comment on the picture taken from Science magazine that you have showed when discussing Landau-Lifshitz-Gilbert damping (1st lecture)? What does it show? How was it taken?
- [To O. Fruchart] Does having a broad size distribution of particles influences the hysteresis loop? If so, in what way?
- [to L. Heyderman] Using MFM how can you distinguish between these two situations: thin films with either basal or perpendicular magnetization, each with two domains separated with a Bloch domain wall?
- [to O. Gutfleisch] Instead of defect dirty NdFeB, is it better to grow single crystal?
- [to O. Gutfleisch] Please explain again the practical importance of the Ha (anisotropy field) in permanent magnets, for example if a permanent magnet is used in a wind generator?
- [to O. Gutfleisch] What is the a, b, c etc/ axes in a permanent magnets?
- [to O. Gutfleisch] Can you say something more about the search for metastable permanent magnet phases?
- [to V. Franco] What is an eddy current?
- [to V. Franco] Is energy loss in ferromagnetic insulators also common, especially in thin films ∼200nm-thick?
- If you take the example of the Epstein frame of an SST (Single-Shot Tester), how is the concept of demagnetization applied there?
- [to K. Sandeman] Can I get equation for δT (value of MCE) directly from the 1st energy conservation law? (for ferromagnets)
- [to S. Fähler] What about fatigue in magneto-elastic materials?
- [to G. Herzer] During annealing, do you change the shape of the nanocrystals? (cause of magnetostriction effect and diffusion)
- [to M. Viret] Discuss the Landau-Lifshitz-Gilbert equation versus the Landau-Lifshitz equation in the presence of spin torques. What is the physical interpretation of the two spin torque terms in each case?
- [to S. Valenzuela] Please discuss the inverse Hall effect, in particular the role of phonons in extrinsic inverse spin hall effect. Could you also provide some bibliography about this?
- [to S. Valenzuela] What is the (anomalous) Nernst effect? Is it different from the (spin) Seebeck effect?