1. Department of Physics, Graduate School of Science, Tohoku University, Sendai 980-8578
2. Neutron Science Laboratory, Institute for Solid State Physics, University of Tokyo, Tokai, Ibaraki 319-1106
3. Department of Applied Physics, National Defense Academy, Yokosuka, Kanagawa 239-8686
4. Center for Low Temperature Science, Tohoku University, Sendai 980-8578