UNIVERSIT .

The investigation of magnetic nanoparticles is of great interest for potential applications. Due to huge surface, the properties of nanoparticles deviate from those of bulk material. In order to understand and control these deviations, X-ray Absorption Spectroscopy has been used in this thesis. Specifically, transition metal nanoparticles, prepared by wet-chemical synthesis which allows control of the size of the particles, were investigated to determi- ne their electronic and geometric structure. For Co nanoparticles, prepared in presence of aluminumtrialkyl compounds, a detailed model of metallic core formation and further sta- bilization under different synthesis conditions can be established. The size of the particles is sensitive to the initial molar ratio of precursor to aluminium organics and the surfactant used for stabilization. Using a molar ratio of Co2(CO)8:Al(oct)3 = 10:1 and KorantinSH as a surfactant leads to the production of 10 nm H.C.P. nanoparticles, whereas the addition of a second surfactant results on formation of 10 nm F.C.C. nanoparticles. The thickness of the protection shell, formed around metallic core is dependent on the chain length of aluminumtrialkyl. Application of 'smooth oxidation' to this system leads to the formation of a CoCO3 shell. This metrical can be further modified by different surfactants to obtain air stable ferrofluids. Another sample system discussed in this thesis is CoTi doped BaFe12O19. In this system, X-ray Absorption Spectroscopy provides an explanation for the pronounced size-dependent changes in magnetic properties. Magnetic properties are mostly influenced by surface effects, such as chemical re-/decomposition towards the BaO phase, which are found in the nanoparticles below the supermagnetic size limit, i.e. > 10 nm.