1. Laboratory of Solid State Physics and Magnetism and INPACInstitute for Nanoscale Physics and Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200 D, 3001 Leuven, Belgium, and Department of Chemistry and INPACInstitute for Nanoscale Physics and Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200 F, 3001 Leuven, Belgium