1. Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
2. WPI Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
3. Center for Spintronics Research Network, Tohoku University, Sendai 980-8577, Japan
4. Spin Quantum Rectification Project, ERATO, Japan Science and Technology Agency, Sendai 980-8577, Japan
5. Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan