1. Department of Physics, Faculty of Science and Technology, Tokyo University of Science, Noda, Chiba 278-8510, Japan
2. Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
3. Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan
4. Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6393, U.S.A.