Lattice dynamics and magnetic properties in selected functional iron compounds

The presented thesis reports about to the investigation of four different materials with respect to
their magnetic properties and their lattice dynamics. The carbodiimide FeNCN, which was recently synthesized for the first time and can be considered as an non-oxidic analogue to FeO, is discussed in chapter 2. The NCN 2− group bridges two iron and exhibits a double negative charge, similar to the oxygen in FeO. This material exhibits an increased magnetic order temperature as compared to FeO. FeNCN was investigated with magnetometry, Mössbauer spectroscopy and nuclear inelastic scattering, revealing an unusual behaviour of the hyperfine parameters below the ordering temperature. This observation is discussed with respect to the crystal field and the d-orbital level population with temperature.
The investigation of the magnetocaloric material Mn5−x FexSi3 is reported in chapter 3 with focus on the interaction of magnetism and lattice dynamics. Among the Mn5−xFe x Si3 compounds, MnFe4 Si3 is the most interesting one due to the possible application as a room temperature refrigeration material. The investigation reveals that a direct measurement of the influence of the magnetocaloric effect on the density of iron phonon states is not possible, however, the elasticity was observed to be strongly influenced by this effect.
The studies on the spin orientation in nanospheres and a NANOPERM alloy are discussed in chap
ter 4 and chapter 5. Two approaches were followed in order to investigated the spin orientation,
nuclear forward scattering and nuclear resonant small-angle X-ray scattering. In the first part,
maghemite nanospheres are analysed with in-field nuclear forward scattering, revealing that the
spins orient only partially along the direction of the applied magnetic field. A modeled magnetization calculated from the observed degree of spin orientation matches excellently the macroscopic magnetometry data and literature values. Though revealing the amount of oriented spins, nuclear forward scattering is not sensitive to their position in the nanoparticles. Thus, the nuclear forward scattering was combined with small-angle X-ray scattering, leading to small-angle patterns in dependence of the local spin orientation in the aforementioned samples.