Electronic, magnetic, and structural properties of magnetocaloric materials La(Fe,Si)13, (Ho,Dy)Co2, Ni-Co-Mn-(Sn,Ti), and Fe-Rh as prototype systems

This thesis concentrates on studying magnetocaloric (MC) materials. Such materials utilize the magnetocaloric effect and are used to build magnetocaloric refrigerators that are promising candidates to replace contemporary vapor compression refrigerators. This leads to a more energy efficient and less greenhouse gas (GHG) emitting refrigeration technology. In addition, a second approach to reduce GHG concentration has been explored, aiming at the extraction of the GHG CO2 from gaseous media. These two different, but complementing pathways to reduce the global GHG load are studied in detail within this work. With respect to MC refrigerators, cooling power, but also the life time, mechanical stability, prices, and (geographical) availability, need to be investigated. A fundamental understanding of MC materials and factors that influence their cooling power, life time or mechanical stabiliy are essential. For that reason, the focus of this thesis is on the intrinsic properties, e.g. electronic, magnetic and structural contributions. Therefore, element- and isotope-specific measurement techniques like X-ray absorption spectroscopy (XAS), X-ray magnetic circular dichroism (XMCD), and absorption- and scattering-methods based on the Mössbauer effect, will be used to investigate MC materials. In the beginning, La(Fe,Mn,Si)13 is studied by extended X-ray absorption fine structure spectroscopy (EXAFS) to unravel the preferred occupation of substituted Mn atoms. Complemented by density functional theory (DFT) calculations, the conclusion can be drawn that Mn mainly occupies the 96i sites. Furthermore, La shows a local increased disorder when Mn is introduced into the system. The overall saturation magnetisation of (La,Ce,Nd)(Fe,Si)13 is investigated. Determined changes due to different stoichiometries are followed at an isotope- and element-specific level by Mössbauer spectroscopy and XMCD. Moreover, the influence of pressure is studied by nuclear resonant inelastic X-ray scattering (NRIXS) in (La,Ce)(Fe,Si)13. Here, the aim is to combine magneto- and elastocaloric effects. For this, temperature and pressure responses are studied separately and the isotope-specific sublattice contribution to the total entropy change is investigated. These are the first pressure-dependent investigations of a MC material. The results indicate a different response of Fe, situated on the symmetric 8b sites, in comparison to Fe, situated on 96i sites. In addition, the response of Fe, situated on 8b sites, differs for the external stimuli pressure and temperature. Secondly, Laves phases are investigated. These are mainly studied for MC applications at low temperatures, as their MC effect becomes maximal in this regime. Here, HoCo2 is investigated by XMCD to resolve the magnetic properties. The magnetic phase diagram of HoCo2 is mirrored by the complex interactions between the rare-earth (RE) 4f- / 5d magnetic moments and the Co d-orbital contribution. Also, magnetic properties of DyCo2 are studied by XMCD. NRIXS measurements unravel the isotope-specific sublattice contribution of 161Dy to the total entropy change - a first time investigation of a RE sublattice contribution to the total entropy change at a MC phase transition. The third material class are Ni-Co-Mn-(Ti,Sn) Heusler alloys. By elemental substitutions, the mechanical material properties are optimised for MC applications. Different stoichiometries cause changes in the elemental electronic hybridisations which is discussed as possible cause for the improved mechanical properties. However, this assumption is not based on any experimental evidence, yet. Within this thesis, electronic changes due to different stoichiometric changes will be examined by XAS. The XAS data are discussed in the context of complementary DFT calculations. The last investigated MC material is Fe-Rh. By high-resolution X-ray diffraction (HR-XRD), structural changes of Fe-Rh along the MC phase transition are studied. Arising changes are discussed within the context of a changing magnetic order. With respect to the second approach, CO2 molecule adsorption in zeolite is investigated. Here, pressure-dependent HR-XRD experiments unravel preferred sites of CO2 molecules within the crystal lattice. Furthermore, the occupations of these sites are determined during the adsorption process.