1. National Institute for Materials Science (NIMS), Tsukuba 305-0047, Japan
2. Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8577, Japan
3. Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
4. Center for Spintronics Research Network, Tohoku University, Sendai 980-8577, Japan