1. Department of Physics, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
2. Department of Applied Physics, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan
3. Center for Spintronics Research Network, Tohoku University, Sendai 980-8577, Japan
4. Center for Spintronics Integrated Systems, Tohoku University, Sendai 980-8577, Japan