1. Peter Grünberg Institute, Research Center Jülich 1 , D-52425 Jülich, Germany
2. Physics Department, University Duisburg-Essen 2 , D-47057 Duisburg, Germany
3. Institute for Molecular Science 3 , Okazaki Aichi 444-0867, Japan
4. Physics Department, University of California, Davis 4 , Davis, California 95616