1. JARA-FIT and 2nd Institute of Physics, RWTH Aachen University, 52074 Aachen, Germany
2. Peter Grünberg Institute (PGI-8/9), Forschungszentrum Jülich, 52425 Jülich, Germany
3. Advanced Microelectronic Center Aachen (AMICA), AMO GmbH, 52074 Aachen, Germany
4. National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan