1. Advanced Research Center for Science and Engineering, Waseda University, Shinjuku‐ku, Tokyo 169, Japan
2. Institute for Materials Research, Tohoku University, Aoba‐ku, Sendai 980‐77, Japan
3. Department of Applied Chemistry, School of Science and Engineering, Kagami Memorial Laboratory for Materials Science and Technology, Waseda University, Shinjuku‐ku, Tokyo 169, Japan
4. Department of Metallurgy, Chiba Institute of Technology, Narashino, Chiba 275, Japan