1. Faculty of Science and Technology, Keio University, Yokohama, Kanagawa 223-8522, Japan
2. Research Institute for Electronic Science, Hokkaido University, Sapporo, Hokkaido 001-0020, Japan
3. Graduate School of Science and Engineering, Ibaraki University, Hitachi, Ibaraki 316-8511, Japan
4. Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, Miyagi 980-8577, Japan
5. Center for Spintronics Research Network, Keio University, Yokohama, Kanagawa 223-8522, Japan