Magnetic and Structural Responses of FeRh and Ni2MnSn-Heusler Compounds to External Stimuli

This thesis concentrates on the response of the magnetic and structural subsystems of FeRh and Ni2MnSn-Heusler compounds to various external stimuli investigated by utilizing a variety of X-ray spectroscopy techniques to reveal the changes of the magnetic and structural subsystem for each compound. 

The first part of this thesis concentrates on the equiatomic alloy FeRh. Besides the study of the well-known metamagnetic phase transition of B2-FeRh, investigating the short-range order at low temperature utilizing extended X-ray absorption spectroscopy (EXAFS) points to the presence of a lattice instability. This lattice instability indicates that the assumed B2 crystal structure (CsCl structure) is not the thermodynamic ground state of FeRh.
Utilizing 57Fe nuclear resonant inelastic X-ray scattering (NRIXS), it is possible to investigate the Fe-partial lattice dynamics and the Fe-partial vibrational density of states along the metamagnetic phase transition. The Lamb-Mössbauer factor fLM indicates that with respect to phonons, the lattice is softer in the antiferromagnetic phase compared to the ferromagnetic phase. At the same time, the vibrational entropy Svib suggests that the change of the isotropic entropy change ΔSiso can be contributed to the vibrational modes of Rh. Based on these results obtained in a thermal steady-state, the lattice dynamics were investigated in a thermal non-equilibrium on a picosecond timescale. For these measurements, a novel pump-probe experiment has been designed, where an optical fs-laser pulse thermally excites the system and the lattice dynamics are probed via nuclear forward scattering (NFS) and nuclear inelastic X-ray scattering. Looking at the NFS time pattern, it is possible to suppress the observed beating structure due to a change of the coherence of the system, for example, by inducing a coexistence of the antiferromagnetic and ferromagnetic domains within the coherence length of the incident gamma pulse. The NRIXS measurements reveal different relaxation dynamics for optical and acoustic vibrational modes hinting towards differences in the electron-phonon or magnon-phonon coupling. Besides the temperature-dependent investigations, it was possible to study the effect of structural disorder in FeRh, which was introduced through ion irradiation. EXAFS measurements at low temperatures reveal that the introduction of open-volumes in the form of small lattice dislocations are sufficient to introduce ferromagnetism in an antiferromagnetically ordered sample.

In the second part, the formation of core-shell precipitates in Ni2MnSn-Heusler alloys is discussed utilizing 119Sn Mössbauer spectroscopy. It is demonstrated that the Sn moment is susceptible to the local environment and can, therefore, be used as a detector for the decomposition process in NiMn based Heusler alloys. 
The insights obtained in these thesis can be adopted to different materials and lead to new experimental techniques.


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