1. Semiconductor Spintronics Project, Exploratory Research for Advanced Technology, Japan Science and Technology Agency, 1-18 Kitamemachi, Aoba-ku, Sendai 980-0023, Japan.
2. Laboratory for Nanoelectronics and Spintronics, Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan.
3. Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan.
4. Core Research for Evolutional Science and Technology, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi 332-0012, Japan.
5. Physics Department, University of Miami, Coral Gables, FL 33124, USA.