1. Department of Applied Physics and Physico-Informatics, Keio University, Yokohama 223-8522, Japan
2. IBM Research - Tokyo, Kawasaki 212-0032, Japan
3. Keio Institute of Pure and Applied Science, Keio University, Yokohama 223-8522, Japan
4. Center for Spintronics Research Network, Keio University, Yokohama 223-8522, Japan