1. RIKEN Center for Emergent Matter Science (CEMS) Wako 351‐0198 Japan
2. Institute of Industrial Science The University of Tokyo Tokyo 153‐8505 Japan
3. Department of Applied Physics Waseda University Tokyo 169‐8555 Japan
4. Research and Development Group Hitachi, Ltd. Hatoyama 350‐0395 Japan
5. Department of Applied Physics and Tokyo College The University of Tokyo Tokyo 113‐8656 Japan