1. Department of Advanced Materials Science, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5-603 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan, Advanced Research Laboratory, Hitachi, Ltd., Hatoyama 350-0395, Japan, Department of Physics, Tokyo Institute of Technology, Meguro 152-8550, Japan, and WPI Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan