1. Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan.
2. Multiferroics Project, Exploratory Research for Advanced Technology, Japan Science and Technology Agency, care of Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan.
3. Kavli Institute for Theoretical Physics, University of California, Santa Barbara, CA 93106, USA.
4. Cross-Correlated Materials Research Group and Correlated Electron Research Group, Advanced Science Institute, RIKEN, Wako 351-0198, Japan.