1. Neutron Scattering Laboratory, Institute for Solid State Physics, University of Tokyo, Shirakata 106-1, Tokai, Ibaraki 319-11
2. Institute for Solid State Physics, University of Tokyo, Roppongi, Minato-ku, Tokyo 106
3. Department of Physics, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo 162
4. Department of Physics, Tohoku University, Sendai 980-77