1. Advanced Science Research Center, Japan Atomic Energy Agency (JAEA), Tokai 319-1195, Japan
2. Department of Advanced Functional Materials Research, National Institutes for Quantum and Radiological Science and Technology (QST), Takasaki 370-1292, Japan
3. Materials Sciences Research Center, Japan Atomic Energy Agency, Hyogo 679-5148, Japan
4. System Platform Research Laboratories, NEC Corporation, Kawasaki 211-8666, Japan
5. Department of Applied Physics, The University of Tokyo, Tokyo 113-8656, Japan
6. WPI Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
7. Center for Spintronics Research Network, Tohoku University, Sendai 980-8577, Japan