1. NTT Basic Research Laboratories, NTT Corporation, Atsugi, Kanagawa 243-0198, Japan
2. Department of Electrical Engineering and Information Systems, The University of Tokyo, Bunkyo, Tokyo 113-8656, Japan
3. Institute of Engineering Innovation, The University of Tokyo, Bunkyo, Tokyo 113-8656, Japan
4. Center for Spintronics Research Network (CSRN), The University of Tokyo, Bunkyo, Tokyo 113-8656, Japan