1. Department of Physics, Aoyama Gakuin University, Setagaya-ku, Tokyo 157-8572
2. Neutron Scattering Laboratory, ISSP, University of Tokyo, Tokai, Ibaraki 319-1106
3. Advanced Science Research Center, Japan Atomic Energy Research Institute (JAERI), Tokai, Ibaraki 319-1195
4. Dapartment of Applied Physics, Nagoya University, Nagoya 464-8603
5. Department of Physics, Tohoku University, Sendai 980-8578