1. Department of Physics and Center
for Nanointegration Duisburg-Essen (CeNIDE), University of Duisburg-Essen, Lotharstrasse 1, 47057 Duisburg,
Germany
2. Department of Physics, University of Cologne, Zülpicher Strasse 77,
50937 Cologne, Germany
3. Institut NÉEL, CNRS & Université Joseph Fourier, BP166, F-38042 Grenoble Cedex 9, France
4. Physics of Interfaces and Nanomaterials,
MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, NL-7500AE Enschede, The Netherlands