1. Solid State Physics Laboratory, ETH Zurich, 8093 Zurich, Switzerland
2. Tohoku Institute of Technology, Sendai 982-8577, Japan
3. Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
4. Center for Spintronics Research Network, Tohoku University, Sendai 980-8577, Japan
5. Center for Science and Innovation in Spintronics (Core Research Cluster), Tohoku University, Sendai 980-8577, Japan