1. Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan
2. Center for Advanced High Magnetic Field Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
3. SPring-8/JASRI, Sayo, Hyogo 679-5198, Japan
4. Condensed Matter Science Division, Japan Atomic Energy Agency, Sayo, Hyogo 679-5148, Japan