1. Fachbereich Physik, Universität Osnabrück, Barbarastraße 7, 49069 Osnabrück, Germany
2. Center of Physics and Chemistry of New Materials, Universität Osnabrück, Barbarastraße 7, 49069 Osnabrück, Germany
3. DESY, Photon Science, Notkestr. 85, 22607 Hamburg, Germany
4. Physics Department, Center for Spinelectronic Materials and Devices, Bielefeld University, Universitätsstraße 25, 33501 Bielefeld, Germany