1. Department of Applied Physics and Physico-Informatics, Keio University, Yokohama 223-8522, Japan
2. Department of Electrical and Computer Engineering, Rice University, Houston, Texas 77005, United States
3. Department of Physics and Astronomy, Rice University, Houston, Texas 77005, United States
4. Department of Materials Science and NanoEngineering, Rice University, Houston, Texas 77005, United States
5. Center for Spintronics Research Network, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan