1. Choetsu Kaken Co., Ltd., Suehiro, Tsurumi, Yokohama, Kanagawa 230-0045, Japan
2. Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan
3. Neutron Science and Technology Center, Comprehensive Research Organization for Science and Society, Tokai, Ibaraki 319-1106, Japan