1. National Institute for Materials Science, Tsukuba 305-0047, Japan
2. Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
3. Center for Spintronics Research Network, Tohoku University, Sendai 980-8577, Japan
4. Center for Spintronics Research Network, Osaka University, Osaka 560-8531, Japan
5. Department of Applied Physics, Tohoku University, Sendai 980-8579, Japan
6. Center for Science and Innovation in Spintronics, Core Research Cluster, Tohoku University, Sendai 980-8577, Japan