1. Future Technology Research Laboratory, ULVAC, Inc., Tsukuba 300-2635, Japan
2. Department of Applied Physics, Tohoku University, Sendai 980-8579, Japan
3. Department of Electronic Engineering, Tohoku University, Sendai 980-8579, Japan
4. Center for Spintronics Research Network, Tohoku University, Sendai 980-8577, Japan
5. Center for Science and Innovation in Spintronics (Core Research Cluster), Organization for Advanced Studies, Tohoku University, Sendai 980-8577, Japan