1. National Institute for Materials Science, Tsukuba, Ibaraki 305-0003, Japan
2. Graduate School of Material Science, University of Hyogo, Kamigori, Hyogo 678-1297, Japan
3. Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8577, Japan
4. Department of Physics and National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32310, U.S.A.