Modelling & Simulation
Element-resolved thermodynamics of the conventional magnetocaloric system La-Fe-Si
Markus Gruner, Faculty of Physics and Center for Nanointegration, CENIDE, University of Duisburg-Essen, 47048 Duisburg, GermanyWerner Keune, Faculty of Physics and Center for NanoinUniversity of Duisburg-Essen, Duisburg, GermanyBeatriz Roldan Cuenya, Ruhr-University Bochum, Bochum, GermanyClaudia Weis, Faculty of Physics and CenUniversity of Duisburg-Essen, Duisburg, GermanyJoachim Landers, University of Duisburg-Essen, Duisburg, GermanySergei Makarov, University of Duisburg-Essen, Duisburg, GermanyDavid Klar, University of Duisburg-Essen, Duisburg, GermanySoma Salamon, University of Duisburg-Essen, Duisburg, GermanyMichael Hu, Argonne National Laboratory, Argonne, USAErsen Alp, Argonne National Laboratory, Argonne, USAJiyong Zhao, Argonne National Laboratory, Argonne, USAMaria Krautz, IFW Dresden, Dresden, GermanyOliver Gutfleisch, TU Darmstadt, Darmstadt, GermanyHeiko Wende, University of Duisburg-Essen, Duisburg, Germany
La-Fe-Si in the 1:13 phase is in its hydrogenated form one of the most promising systems for magnetic refrigeration applications. It undergoes a first-order magnetic transition with a narrow hysteresis providing large adiabatic temperature and isothermal entropy changes in an external magnetic field. The isothermal entropy change across the Curie temperature is usually divided up assuming independent contributions from the magnetic, lattice (vibrational) and electronic degrees of freedom, respectively. We combine two independent approaches, first-principles calculations in the framework of density functional theory and nuclear resonant inelastic X-ray scattering, in order to compare lattice dynamics in both the ferromagnetic and the paramagnetic phase and thus the vibrational entropy, resolving the contributions of the elemental constituents.
Our results demonstrate an unusual and significant effect of the magnetic phase transition on the element-resolved vibrational density of states of La-Fe-Si. This involves the disappearance of a high-energy peak in connection with an overall softening of phonons in the paramagnetic phase. The consequence is a significant increase in vibrational entropy near the transition, which contributes cooperatively with magnetic and electronic entropy to the good magneto- and barocaloric properties. This is unexpected, since conventional Grüneisen theory predicts a negative vibrational entropy according to the large volume decrease at the transition. Instead, we observe an anomalous magneto-elastic softening which is solely associated with the Fe-subsystem. It originates from adiabatic electron-phonon coupling, caused by specific changes in the electronic density of states at the Fermi level arising from the itinerant electron metamagnetism of Fe, and implies a substantial interdependence of all relevant degrees of freedom .
 M. E. Gruner, W. Keune, B. Roldan Cuenya, et al., Phys. Rev. Lett. 114, 057202 (2015).