1. WPI Advanced Institute for Materials Research, Tohoku University, Katahira 2-1-1, Sendai 980-8577, Japan
2. Center for Spintronics Research Network, Tohoku University, Sendai 980-8577, Japan
3. Electrical Engineering and Electronics, Kyoto Institute of Technology, Kyoto 606-8585, Japan
4. Department of Applied Physics, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan