1. Department of Physics, Tokyo University of Science, Noda, Chiba 278-8510, Japan
2. Center for Spintronics Research Network, Osaka University, Toyonaka, Osaka 560-8531, Japan
3. Metro Inc., Shinagawa, Tokyo 141-0032, Japan
4. Department of Applied Physics, Tokyo University of Science, Katsushika, Tokyo 125-8585, Japan