1. Institute of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305
2. Department of Physics, Tokyo Metropolitan University, Minami-Osawa 1-1, Hachioji, Tokyo 192-03
3. Physics Department, Sapporo Campus, Hokkaido University of Education, Sapporo, Hokkaido 002
4. The Oarai Branch, Institute for Materials Research, Tohoku University, Oarai, Ibaraki 311-13