1. Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
2. Center for Spintronics Research Network, Tohoku University, Sendai 980-8577, Japan
3. Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan
4. Research Institute of Electrical Communication, Tohoku University, Sendai 980-8577, Japan
5. Japan Synchrotron Radiation Research Institute (JASRI/SPring-8), Sayo, Hyogo 679-5198, Japan
6. WPI Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan