1. Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
2. Department of Applied Physics, Tohoku University, Sendai 980-8579, Japan
3. Spin Quantum Rectification Project, ERATO, Japan Science and Technology Agency, Sendai 980-8577, Japan
4. The Advanced Science Research Center, Japan Atomic Energy Agency, Tokai 319-1195, Japan
5. WPI Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan