Questions raised in 2013 (processed):

Basic concepts

• 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 M_eff=sqrt(J(J+1)). Which one is measured experimentally?
• Why is M_eff=3.4µ_B measured for Eu³⁺ although one expects M_eff=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/T_c)^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 T_c 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 C_nm(J')=C_nm(θ,φ).C_n0(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 La_1-xCa_xMnO₃? [ 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 [001] and[110]. 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?

Magnetic characterization

• 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 m_ℓ/m_s 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)?

Functional materials

• 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: T_C, 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 ]

Spintronics

• What is the spin transfer torque?

Questions raised in 2013 (pending):

Basic concepts

• 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?

Magnetic characterization

• [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?

Functional materials

• [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 H_a (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)

Spintronic

• [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?