1. Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
2. Department of Physics, University of Tokyo, Tokyo 113-0033, Japan
3. Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
4. Advanced Science Research Center, Japan Atomic Energy Agency, Tokai 319-1195, Japan
5. Center for Spintronics Research Network, Tohoku University, Sendai 980-8577, Japan