1. Graduate School of Biomedical Engineering, Tohoku University, Sendai 980-8579, Japan
2. Laboratory for Nanoelectronics and Spintronics, Research Institute of Electrical Communication, Tohoku University, Sendai 980-8577, Japan
3. CREST, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan
4. Tokyo University of Technology, Hachioji, Tokyo 192-0982, Japan
5. Advanced Photon Source, Argonne National Laboratory, Lemont, Illinois 60439, USA