Today's research in diluted magnetic semiconductors (DMS) is concentrated on large band gap semi­conductors with the scope to obtain carrier mediated high temperature ferromagnetism by substitution of appropriate transition metal ions into the host matrix. This study investigates the possibility of alternative approach, such as the creation a of DMS starting from a ferromagnetic oxide. The control of stoichiometry during growth of thin oxide film will be used as a tool to modify the physical properties of the oxide such as magnetism, valence state and consequently transport properties. In particular, ferrimagnetic iron oxides like garnets or illmenites show already high intrinsic Curie temperatures (T_C > 400 K) and are suitable candidates for this case study. Among the different systems, the Yttrium Iron Garnet (YIG) will be presented in detail. Pulsed laser deposition of Yttrium Iron Garnet thin films allows to explore a new part of its phase diagram, e.g. controlled stabilization of iron and yttrium vacancies within the oxygen sub-lattice, and to tune magnetism by off-stoichiometry. Magnetization and Curie temperatures are measured by polar magnetic circular dichroism (MCD). A significant increase of the Curie temperature (+10%) indicates changes of the super-exchange coupling through the variation of the Fe-O-Fe distance. Simultaneously, an important increase of the magnetization (up to 120%) is observed. The temperature dependent MCD measurements demonstrate that the increase of magnetization is due to a preferential occupation of the iron vacancies on the octahedral sites. Iron valence presumably changes from Fe³⁺ to Fe⁴⁺ accompanied by the presence of iron and yttrium vacancies. Its consequences to the carrier doping in application to spintronics will be discussed.

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