Thesis presented September 26, 2018

Abstract:
The search for more efficient use of energy has been leading to a growing interest in the research field of magnetocaloric materials. The magnetocaloric effect (MCE) refers to a change of temperature or entropy of a magnetic material exposed to a change of magnetic field. The MCE requires the exchange of magnetic, lattice and/or electronic entropy during an adiabatic (de-)magnetization process. A large MCE at room temperature and low magnetic field for a material with abundant and environmentally friendly elements opens the way for magnetic cooling devices.
From the Mn_5−xFe_xSi₃ system, that exhibits a moderate MCE at low magnetic fields, two materials in single crystal form are under investigation: the ferromagnetic (FM) compound MnFe₄Si₃ and the parent compound Mn₅Si₃. The aim of this thesis is to investigate the spin and lattice dynamics and their couplings in these compounds that are up to nowadays unexplored with inelastic neutron scattering (INS) and inelastic X-ray scattering (IXS) measurements. Such studies might help to point out ingredients that may favour large MCE, such as phonon-magnon interaction, effect of spin fluctuations etc.
The FM compound MnFe₄Si₃ is a promising candidate for applications since it exhibits a moderate MCE near room temperature. Its magnetic excitation spectrum has been investigated by means of polarized and unpolarized INS. Spin-wave measurements at 1.5 K reveal a strong anisotropy of the magnetic exchange interactions along the (h00) and (00l) reciprocal directions of the hexagonal system, which also manifests itself in the q-dependent linewidths in the paramagnetic (PM) state. The correlation lengths indicate a short-range order, while the average linewidth is of the order of k_BT_C pointing to a behavior typical of many ferromagnets. In addition, the in- and out-of-plane spin-fluctuations are found to be isotropic around T_C and can be suppressed by a magnetic field of 2 T.
In order to study the spin and lattice dynamics and their interactions in MnFe₄Si₃, a combination of IXS and INS (polarized and unpolarized) measurements was performed. A remarkable feature evidenced by this combination of measurements is that along the (h00) direction the magnon branch close to the zone boundary falls exactly on the two transverse acoustic (TA) phonons. Furthermore, a large difference of intensities in the two non-spin-flip (NSF) channels was observed for one TA phonon mode. This difference of intensity between the two NSF channels can be attributed to the nuclear-magnetic interference term.
The parent compound Mn₅Si₃ has been extensively characterized as a model system by many groups in the past decades by magnetometry, X-ray and neutron diffraction on powder and single crystal samples. Previous studies indicate the existence of two stable antiferromagnetic (AF) phases at about 100 K (AF2) and 66 K (AF1), respectively. AF2 and AF1 transitions are of first-order and the inverse MCE (the sample heats up when an external magnetic field is applied adiabatically) is associated with the AF1-AF2 phase transition. INS experiments revealed that AF1 is characterized by sharp spin-waves, but AF2 is characterized by a mixed signal that resembles the one of the AF1 and PM state, indicating strong spin-fluctuations coexisting with spin-waves. Moreover, the application of a magnetic field in the AF1 phase induces spin-fluctuations, which points to their importance for the inverse MCE in Mn₅Si₃.

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Keywords

On-line thesis.