1. Institute of Inorganic and Analytical Chemistry, Justus Liebig University Giessen, Heinrich-Buff-Ring 58, D-35392 Giessen, Germany, Department of Physics and Material Sciences Center, Philipps University of Marburg, Renthof 5, D-35032 Marburg, Germany, and Experimental Physics V, Electronic Correlations and Magnetism, Institute of Physics, University of Augsburg, Universitätsstrasse 2, D-86135 Augsburg, Germany