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