1. Correlated Electron Research Center (CERC), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, 305-8562, Japan, Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan, Tokura Multiferroics Project, ERATO, JST, c/o AIST, Tsukuba, 305-8562, Japan, and Department of Applied Physics, The University of Tokyo, Tokyo 113-8656, Japan