1. Institute of Physics, Academy of Sciences of the Czech Republic, 182 21 Prague 8, Czech Republic
2. University of Chemistry and Technology Prague, 166 28 Prague 6, Czech Republic
3. Peter Grünberg Institute (PGI-6) and JARA-FIT, Research Center Jülich, 52425 Jülich, Germany
4. Fakultät für Physik and Center for Nanointegration Duisburg-Essen (CENIDE), Universität Duisburg-Essen, D-47048 Duisburg, Germany