1. Institute of Physics, Johannes Gutenberg-University Mainz 1 , 55128 Mainz, Germany
2. Peter Grünberg Institut and Institute for Advanced Simulation, Forschungszentrum Jülich and JARA 2 , D-52425 Jülich, Germany
3. Institute for Theoretical Physics and Astrophysics, University of Würzburg 3 , D-97074 Würzburg, Germany