1. Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science, Tsukuba, Ibaraki 305-0003, Japan
2. Center for Spintronics Research Network, Osaka University, Toyonaka, Osaka 560-8531, Japan
3. Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan
4. Department of Applied Physics, Nagoya University, Nagoya 464-8063, Japan
5. Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan