1. WPI Advanced Institute for Materials Research (AIMR), Tohoku University, Sendai 980-8577, Japan
2. Department of Electronic Engineering, University of York, York YO10 5DD, England
3. Center for Science and Innovation in Spintronics (CSIS), Core Research Cluster (CRC), Tohoku University, Sendai 980-8577, Japan
4. Center for Spintronics Research Network (CSRN), Tohoku University, Sendai 980-8577, Japan