1. Graduate School of Electrical and Electronic Engineering, Chiba University, Chiba 263-8522, Japan
2. Department of Materials Science, Tohoku University, Sendai 980-8579, Japan
3. Spintronics Research Center, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8568, Japan
4. Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8573, Japan
5. Research Institute of Electrical Communication, Tohoku University, Sendai 980-8577, Japan