Spin- und Ladungsordnung in dem neu bewerteten Antiferromagneten YbFe$_{2}$O$_{4}$

The main focus of this thesis was dedicated to uncovering the true intrinsic magnetic and charge order properties of YbFe2O4, with a large emphasis on the crystal structure determination at 200 and 90 K. Previous research on its isostructural neighbor LuFe2O4, uncovered what was thought to initially be; ferroelectricity through valence ordering". The high temperature RFe2O4 structure (space group R-3m) can be described as Fe2+ and Fe3+ valence mixed bilayers, separated by R3+-O mono-layers. The concept of ferroelectricity through valence ordering was later suspended after the refinement on a highly stoichiometric LuFe2O4 single crystal, which resulted ina lower symmetry C2/m space group, and charged rather than polar bilayers. This sparked new interest in this series of compounds and provided the inspiration for this thesis; to determine if highly stoichiometric YbFe2O4 exhibits ferroelectricity through charge ordering. The charge order investigated in this thesis exhibits the same incommensurate phase at room temperature, but in contrast to LuFe2O4, a second phase transition to a commensurate charge order was also observed. This commensurate charge order was refined in the P-1 space group using single crystal x-ray diffraction data measured at 200 and 90 K. This space group is representative of all temperatures until above the 3D charge order phase, at which point the charge order becomes diffuse and the structure can be refined in the R-3m space group. For the first time, the magnetic phase diagram for highly stoichiometric YbFe2O4 has been established, and shows a ferrimagnetic state even in zero field, in the temperature range 240-260 K, which was not observed in LuFe2O4. From the analysis of neutron diffraction data measured on YbFe2O4, the same ferri (fM) and antiferromagnetic (AFM) spin structures intrinsic to LuFe2O4 are present in YbFe2O4, and deduced from almost identical intensity ratios of reflections measured along ( 1/3 , 1/3 , ℓ), for the three magnetic domain populations. The two magnetic phases are newly described in the proper magnetic space groups; P-1 (fM phase) and P-1’ (AFM phase) in contrast to the previous refinement of LuFe2O4, where the observed reflections could not be refined in a proper monoclinic magnetic space group. The final charge order structure, determined through bond valence sum analysis, provides polar bilayers with an anti-polar stacking in the P-1 space group. This charge order is characterized by three modes; Y1, Y2 and T1+, and results from a combination of 2 irreducible representations, namely a Y1 and Y2 propagation vector. This was not considered before, as it is very unlikely for a first order charge order phase transition to be described by 2 irreducible representations. It was the successful refinement in the P-1 space group, which led to the understanding that a combination of 2 irreducible representations is required, as it is this that reduces the space group symmetry. The final correct spin-charge structure was determined through x-ray magnetic circular dichroism measurements, which yielded the same spin arrangement as that observed for LuFe2O4, except it is described in the lower symmetry P-1 space group, in contrast to the original refinement in the C2/m symmetry