1. Department of Advanced Materials Science, University of Tokyo, Kashiwa, Chiba 277-8561, Japan
2. Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan
3. Department of Materials Science, Osaka Metropolitan University, Osaka 599-8531, Japan
4. Materials Science Research Center, Japan Atomic Energy Agency, Tokai, Tokai, Ibaraki 319-1195, Japan