1. Advanced Science Institute, RIKEN, Wako,
Saitama 351-0198, Japan
2. Okinawa Institute of Science and Technology, Onna-son, Okinawa 904-0412, Japan
3. Central Research Laboratory, Hitachi, Ltd., Hatoyama, Saitama 350-0395, Japan
4. Japan Science and Technology Agency, Kawaguchi-shi,
Saitama 332-0012, Japan
5. Multiferroics Project, ERATO, Japan Science and Technology Agency, Tokyo 113-8656, Japan
6. Department of Applied Physics and Quantum-Phase Electronics
Center (QPEC), University of Tokyo, Tokyo
113-8656, Japan