1. Center for Spintronics Research Network, Tohoku University, Sendai 980-8577, Japan
2. WPI Research Center, Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
3. Department of Physics, Tohoku University, Sendai 980-8578, Japan
4. Research Institute of Electrical Communication, Tohoku University, 2 1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan