1. WPI Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
2. Center for Spintronics Research Network, Tohoku University, Sendai 980-8577, Japan
3. Research Institute of Electrical Communication, Tohoku University, Sendai 980-8577, Japan
4. Department of Applied Physics, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan