1. II. Institute of Physics B, RWTH Aachen University and JARA-FIT, 52074 Aachen, Germany
2. Department of Applied Physics, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands
3. Peter Grünberg Institut (PGI-9) and JARA-FIT, Forschungszentrum Jülich, 52425 Jülich, Germany