1. National Institute of Advanced Industrial Science and Technology (AIST), Spintronics Research Center, Tsukuba, Ibaraki 305-8568, Japan
2. Graduate School of Information Science and Technology, The University of Tokyo, Bunkyo, Tokyo 113-8656, Japan
3. JST, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
4. The Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan