1. Research Center for Magnetic and Spintronic Materials, National Institute for Materials Science (NIMS), Tsukuba 305-0047, Japan
2. PRESTO, Japan Science and Technology Agency, Saitama 332-0012, Japan
3. Center for Spintronics Research Network, Osaka University, Osaka 560-8531, Japan
4. Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
5. Center for Spintronics Research Network, Tohoku University, Sendai 980-8577, Japan