1. Department of Applied Physics and Quantum-Phase Electronics Center (QPEC), University of Tokyo, Tokyo 113-8656, Japan
2. Cross-Correlated Materials Research Group (CMRG) and Correlated Electron Research Group (CERG), RIKEN ASI, Wako 351-0198, Japan
3. Multiferroics Project, ERATO, Japan Science and Technology Agency (JST), c/o RIKEN, Wako 351-0198, Japan