1. Institute of Materials Physics, University of Münster 1 , 48149 Münster, Germany
2. Institut für Experimentelle und Angewandte Physik, Universität Regensburg 2 , 93040 Regensburg, Germany
3. Department of Physics and Astronomy, Aarhus University 3 , 8000 Aarhus, Denmark
4. Helmholtz-Zentrum Dresden-Rossendorf 4 , 01328 Dresden, Germany