1. Department of Physics, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan
2. National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
3. Department of Physics, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan
4. Center for Spintronics Research Network, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-8531, Japan