Element-specific Characterization of Magnetocaloric La(Fe, Si)13-based Compounds

Abstract

This thesis focuses on the element-specific contributions to the magnetocaloric effect in La(Fe,Si)13-based compounds in dependence on Mn doping and interstitial hydrogenation. By utilizing nuclear resonant inelastic X-ray scattering (NRIXS), the local Fe-partial vibrational (phonon) density of states and thermodynamics were evaluated across the meta-magnetic phase transition, which is the source of the magnetocaloric effect in this material class. The change in the vibrational entropy cooperatively contributes to the overall entropy change across the first order phase transition, which is rooted in strong electron-phonon coupling. An anomalous lattice softening by heating through the transition from the ferro- to paramagnetic state is present. The experimental data is then compared to first-principles calculations. The introduction of Mn results in a strong suppression of the entropy increase upon heating from the ferro- to paramagnetic phase. Combining X-ray magnetic circular dichroism (XMCD) measurements at the Fe K- and La L2,3-edges with 57Fe Mossbauer spectroscopy made it possible to identify the local element-specific magnetism of the Fe and La atoms in the compound depending on temperature and Mn content. An induced La magnetic moment, coupled antiparallel to the iron magnetic moment and a significant reduction in the Fe moment by increasing Mn doping is found due to hybridization between the Fe 3d and Mn 3d-states, altering the electronic structure. With rising Mn content, the spin frustration in the system drastically increases, partaking in the reduction of the Fe magnetic moment through increasing antiferromagnetic coupling between Fe and Mn. Due to strong hybridization between Fe and Mn, as well as through increase in temperature, the spectral shape of the X-ray absorption edges are strongly altered. Measuring the extended X-ray absorption fine structure (EXAFS) with rising Mn content indicates changes in the local geometry and electronic structure at the La L3- and the Fe K-edges. Due to an isostructural volume decrease upon the phase transition upon heating from the ferro- to paramagnetic state, EXAFS revealed contractions of the next neighbor distances. An increase in Mn content results in a rising static part of the relative mean square atomic displacement which contributes to the reduction of the Fe moment and an alteration of the long range order. Hydrogenation affects the short range order, expanding the next neighbor distances. EXAFS, modeled by ab-initio multiple scattering calculations are compared to the experiments. The combined approach of element-specific characterization methods reveals a strong interplay between the various microscopic degrees of freedom in the system, which contribute to the thermodynamic behavior of La(Fe,Si)13-based compounds.