1. Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577
2. Spin Superstructure Project, Exploratory Research for Advanced Technology (ERATO), Japan Science and Technology Agency, AIST Tsukuba Central 4, Tsukuba, Ibaraki 305-8562
3. Department of Applied Physics, University of Tokyo, Tokyo 113-8656
4. Multiferroics Project, Exploratory Research for Advanced Technology (ERATO), Japan Science and Technology Agency, AIST Tsukuba Central 4, Tsukuba, Ibaraki 305-8562