1. School of Materials and Chemical Technology, Tokyo Institute of Technology, Kanagawa 226-8503, Japan
2. Department of Materials Science, Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan
3. Magnesium Research Center, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan