1. Spin Superstructure Project, Exploratory Research for Advanced Technology (ERATO), Japan Science and Technology Corporation, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 4, Tsukuba 305-8562, Japan.
2. Institute of Physics, Chinese Academy of Science, Beijing 100080, China.
3. Correlated Electron Research Center, AIST Tsukuba Central 4, Tsukuba 305-8562, Japan.
4. Department of Applied Physics, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.
5. Department of Condensed Matter Physics, University of Geneva, 24, quai Ernest-Ansermet, 1211 Geneva 4, Switzerland.