1. Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
2. Center for Spintronics Research Network, Tohoku University, Sendai 980-8577, Japan
3. National Institute of Technology, Fukushima College, Iwaki, Fukushima 970-8034, Japan
4. CREST, Japan Science and Technology Agency, Kawaguchi 332-0012, Japan