1. Research Institute of Electrical Communication, Tohoku University, Sendai, Miyagi 980-8577, Japan
2. Center for Spintronics Research Network, Tohoku University, Sendai, Miyagi 980-8577, Japan
3. Center for Spintronics Research Network, Osaka University, Toyonaka, Osaka 560-8531, Japan
4. Department of Materials Science and Engineering, Tokyo Institute of Technology, Yokohama, Kanagawa 226-8502, Japan
5. The Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan
6. Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
7. Center for Spintronics Research Network, The University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan