1. Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan
2. National Institute for Materials Science, Tsukuba, Ibaraki 305-0003, Japan
3. Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047, Japan
4. Center for Spintronics Research Network, Osaka University, Toyonaka, Osaka 560-8531, Japan