1. Neutron Scattering Laboratory, Institute for Solid State Physics, The University of Tokyo, 106-1 Shirakata, Tokai, Ibaraki 319-11
2. Institute for Solid State Physics, The Univeristy of Tokyo, 7-22-1 Roppongi, Minato-ku, Tokyo 106
3. Department of Physics, Faculty of Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263