1. Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
2. Center for Spintronics Research Network, Tohoku University, Sendai 980-8577, Japan
3. Samsung R&D Institute Japan, Yokohama 230-0027, Japan
4. Center for Science and Innovation in Spintronics (Core Research Cluster), Tohoku University, Sendai 980-8577, Japan