The aim of the present work was to develop a detailed picture of the electronic and magnetic properties of a number of interesting transition metal compounds. A number of complementary experimental and theoretical techniques have been applied, special emphasis was given to x-ray spectroscopies. The studies led to a number of results, and the following conclusions can be drawn: The influence of the magnetic ground state (high-spin (FeO) vs. low-spin (FeS2)) with respect to the recorded x-ray spectra was investigated. In particular, by performing RXES on the Fe L edge of the two compounds, very different ratios of La / Lβ integrated intensity for excitation energies close to the L2 edge have been observed. This effect has been explained in terms of the magnetic structure of FeO (high spin), which inhibits Coster-Kronig processes. Special attention has been given to the direct investigation of orbital ordering in a three dimensional CMR manganite, namely La7/8Sr1/8MnO3, by means of x-ray linear dichroism (XLD). We obtained, for the first time, rather strong indications that the coherently distorted Jahn-Teller phase in La7/8Sr1/8MnO3 is accompanied by a predominantly cross type (x2-z2) / (y2-z2) orbital ordering. In addition to manganites the double perovskite Sr2FeMoO6 the combined study by means of x-ray spectroscopies, magnetic measurements and theoretical band structure calculations could resolve some points discussed controversially in the literature. Both, paramagnetic measurements as well as core level spectroscopy of the Fe 2p, Fe 3s and the Mo 3d states suggest a mixed iron valence state involving around 30% Fe3+- Mo5+ and 70% Fe2+ - Mo6+ states in highly ordered Sr2FeMoO6. XPS valence band studies reveal that the Fe 3d states are not extremely localized, and we find evidence that charge transfer between Fe 3d and O 2p states plays an essential role.

In this work the electronic and magnetic structure of the crystals Sr2FeMoO6, Fe0.5Cu0.5Cr2S4, LuFe2O4 and the molecules FeStar, Mo72Fe30, W72Fe30 are investigated by means of X-ray spectroscopic techniques. These advanced materials exhibit very interesting properties like magnetoresistance or multiferroic behaviour. In case of the molecules they also could be used as spin model systems. A long standing issue concerning the investigation of these materials are contradicting results found for the magnetic and electronic state of the iron (Fe) ions present in these compounds. Therefore this work focuses on the Fe state of these materials in order to elucidate reasons for these problems. Thereby the experimental results are compared to multiplet simulations.