1. Solid state physics, Department of Physics, University of Siegen, 57068 Siegen, Germany
2. Deutsches Elektronen-Synchrotron, Notkestraße 85, 22607 Hamburg, Germany
3. Nano Lund, Department of Physics, Lund University, 221 00 Lund, Sweden
4. Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa 52242, USA
5. Peter Grünberg Institute and JARA-FIT, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany