1. Department of Applied Physics, The University of Tokyo, Bunkyo, Tokyo 113-8656, Japan
2. Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047, Japan
3. Murata Manufacturing Co., Ltd., Nagaokakyo, Kyoto 617-8555, Japan
4. Center for Spintronics Research Network, Osaka University, Toyonaka, Osaka 560-8531, Japan