1. WPI-Advanced Institute for Materials Research (WPI-AIMR), Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
2. Laboratory for Nanoelectronics and Spintronics, Research Institute of Electrical Communication, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
3. Center for Spintronics Integrated Systems, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
4. Center for Spintronics Research Network, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan