Hysteresis and Phase Transition Kinetics in Magnetocaloric Materials

Abstract

In the present paper, we review the recent research on the physics of magnetocaloric materials aiming to define a coherent theoretical framework in which hysteresis and kinetic effects can be appropriately discussed and interpreted in relation to intrinsic and extrinsic factors. We dedicate our efforts to introduce a thermodynamic description of the material, including the out-of-equilibrium aspects which are necessary to understand hysteresis, heat flux avalanches and thermal relaxation effects. In particular we show how intrinsic and extrinsic factors, contributing to define the energy landscape of the system, influence the resulting hysteresis and how different kinetic effects are expected depending on the phase transformation mechanisms, here described either as an out-of-equilibrium domain boundary motion or as a thermally activated process associated to energy barriers. Several applications of the theoretical models are discussed in relation with experiments on La(Fe-Si) 13 -based compounds and Mn-Bi.

Determining the vibrational entropy change in the giant magnetocaloric material LaFe11.6Si1.4 by nuclear resonant inelastic x-ray scattering

“…Our observations in the preceding paragraph indicate that the discrepancy between our work and previous phenomenological descriptions 42,43,50 regarding the counteracting role of ∆S vib might be traced back to the modeling of the coupling between magnetism and lattice degrees of freedom. In the spirit of the Bean-Rodbell model it is often assumed that the molecular field, which derives from the magnetic exchange interactions, depends mainly on the interatomic spacing, as for instance predicted by the Bethe-Slater curve.…”

“…In the spirit of the Bean-Rodbell model it is often assumed that the molecular field, which derives from the magnetic exchange interactions, depends mainly on the interatomic spacing, as for instance predicted by the Bethe-Slater curve. 43 In turn, lattice degrees of freedom depend on the atomic volume as well, which is often modeled in terms of a conventional Grüneisen behavior. Our work shows, however, that in La(Fe,Si) 13 -based compounds a different mechanism based on adiabatic electron-phonon-coupling dominates the magnetoelastic interactions, which derives solely from local changes in the electronic structure, 46 which are by definition not considered in a Bean-Rodbell-type model.…”

“…Mixed interactions, i.e., cross terms in the relation above, are often neglected, although they can be expected for 3d metals 2 , in particular, since generally applicable approximations for the respective terms are missing. Model calculations [41][42][43] , based on the phenomenological description of the magnetoelastic coupling between spin and lattice degrees of freedom by Bean and Rodbell 44 , show that magnetoelastic coupling appears to be responsible for the enhanced MCE in magnetocaloric materials with a first-order phase transition. A similar conclusion was reached by the authors of Ref.…”

Hysteresis Design of Magnetocaloric Materials—From Basic Mechanisms to Applications

“…In general, is a positive, material‐specific constant, and we thus expect a blue‐shift (increase) of the VDOS for Δ V< 0, which corresponds to a smaller entropy according to Equation (5). Empirical calculations of the MCE of La–Fe–Si combine the conventional Grüneisen law with localized spin models to yield a good agreement with experiments …”

“…Empirical calculations of the MCE of La-Fe-Si combine the conventionalG r üneisen law with localized spin models to yield agood agreementwith experiments. [74,76,77] To obtain more detailed information on the subtle interplay of itinerant magnetism and lattice entropyw ee xamined the VDOS at different temperatures above and below T t . Using temperature-dependent nuclear resonant inelastic Xray scattering (NRIXS) we measured the vibrational part of the entropy S lat .A tp resent,N RIXS measurements of the 57 Fe projected VDOS, g(e), have been performed at the Sector3 beam line at the Advanced Photon Sourcea tA rgonne National Laboratory.B yt uning the incident X-ray around the nuclear resonance of 57 Fe at 14.41 keV with a bandwidth as narrowa s1meV, [78] the 57 Fe-projected VDOS can be extracted from the measured NRIXS spectra.…”