Electron-phonon vs. moment-volume coupling in hydrogenated and Mn-doped La-Fe-Si compounds

Markus Ernst Gruner, University of Duisburg-Essen, Duisburg, Germany
Alexandra Terwey, University of Duisburg-Essen, Duisburg, Germany
Werner Keune, University of Duisburg-Essen, Duisburg, Germany
Joachim Landers, University of Duisburg-Essen, Duisburg, Germany
Soma Salamon, University of Duisburg-Essen, Duisburg, Germany
Katharina Ollefs, University of Duisburg-Essen, Duisburg, Germany
Iliya Radulov, Technical University Darmstadt, Darmstadt, Germany
Konstatin Skokov, Technical University Darmstadt, Darmstadt, Germany
Jiyong Zhao, Argonne National Laboratory, Argonne, USA
Michael Y. Hu, Argonne National Laboratory, Argonne, USA
Esen Ercan Alp, Argonne National Laboratory, Argonne, USA
Oliver Gutfleisch, Technical University Darmstadt, Darmstadt, Germany
Heiko Wende, University of Duisburg-Essen, Duisburg, Germany

Fully hydrogenated La-Fe-Si is one of the most interesting candidates for room temperature magnetic refrigeration. The first order nature of the magnetic transition is connected to its itinerant electron metamagnetism, which gives rise to a peculiar coupling between all microscopic degrees of freedom. Previously, we could show that for the hydrogen-free system itinerant nature of the Fe moments, which is responsible for the large volume change, gives also rise to the adiabatic electron-phonon coupling [1,2]. This leads to a cooperative contribution of magnetic, electronic and vibrational degrees of freedom to the entropy change, which results in the excellent caloric properties [1,3]. Hydrogenation shifts the operating range to ambient conditions, which is beneficial for mass market application.

By combining first principles calculations in the framework of density functional theory (DFT) and nuclear resonant inelastic X-ray scattering (NRIXS) we investigate the interplay of electronic structure, magnetism and vibrational degrees of freedom in the fully hydrogenated compound. We demonstrate that the same mechanism acting in the hydrogen-free compund is also responsible for the superior magnetocaloric properties of the hydrogenated system. We find that Hydrogen dominates the vibrational density of states at low energies, which one rather expects for heavy elements. Still, its contribution to the change in vibrational entropy remains rather small. Since full loading with hydrogen involves the occupation of only a part of the available (24d) lattice sites, we also discuss the site-occupation of hydrogen based on total energy calculations and by comparing vibrational density of states from DFT involving different distributions of hydrogen. Finally, we will give an outlook on the impact of a partial substitution of Fe with Mn, which is required fopr the fine-tuning of the transition, on the vibrational properties and the coupling mechanism.

References
[1] M. E. Gruner, W. Keune, B. Roldán Cuenya et al., Phys. Rev. Lett. 114, 057202 (2015).
[2] M. E. Gruner, W. Keune, J. Landers et al., Phys. Status Solidi B 255, 1700465 (2018).
[3] J. Landers, S. Salamon, W. Keune et al., Phys. Rev. B 98, 024417 (2018).

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