1. Department of Applied Physics, The University of Tokyo, Bunkyo, Tokyo 113-8656, Japan
2. Central Research Institute of Electric Power Industry, Yokosuka, Kanagawa 240-0196, Japan
3. WPI-Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
4. Center for Spintronics Integrated Systems, Tohoku University, Sendai 980-8577, Japan
5. Laboratory for Nanoelectronics and Spintronics, Research Institute of Electrical Communication, Tohoku University, Sendai 980-8577, Japan
6. Center for Spintronics Research Network, Tohoku University, Sendai 980-8577, Japan