1. National Institute for Materials Science, Tsukuba 305-0003
2. Center for Low Temperature Science, Tohoku University, Sendai 980-8578
3. National High Magnetic Field Laboratory, Tallahassee, FL 32310, U.S.A.
4. Japan Atomic Energy Research Institute, Tokai 319-1195
5. Department of Physics, Tohoku University, Sendai 980-8578