1. Graduate School of Material Science, University of Hyogo, Kamigori, Hyogo 678-1297, Japan
2. Department of Material Science, Shimane University, Matsue 690-8504, Japan
3. Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan
4. Department of Physics and Earth Sciences, University of the Ryukyus, Nishihara, Okinawa 903-0213, Japan