1. Department of Physics, Tohoku University, Sendai 980-8578, Japan
2. Center for Spintronics Research Network, Tohoku University, Sendai 980-8577, Japan
3. WPI Research Center, Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
4. Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047, Japan