1. Research Center for Magnetic and Spintronic Materials, National Institute for Materials Science 1 , Tsukuba 305-0047, Japan
2. Department of Mechanical Engineering, The University of Tokyo 2 , Tokyo 113-8656, Japan
3. Institute for Materials Research, Tohoku University 3 , Sendai 980-8577, Japan
4. Center for Spintronics Research Network, Osaka University 4 , Osaka 560-8531, Japan