1. Department of Earth and Planetary Sciences, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan
2. National Institute for Materials Science, Tsukuba 305-0044, Japan
3. JEOL Ltd., Akishima 196-8558, Japan
4. Department of Physics, Graduate School of Science, Osaka University, Toyonaka 560-0043, Japan
5. Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga 816-8580, Japan