1. Center for Spintronics Integrated Systems,Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
2. Laboratory for Nanoelectronics and Spintronics, Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
3. Central Research Laboratory, Hitachi, Ltd., Kokubunji, Tokyo 185-8601, Japan
4. WPI Advanced Institute for Materials Research, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan