1. Spintronics Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan
2. Center for Science and Innovation in Spintronics (Core Research Cluster), Tohoku University, Sendai, Miyagi 980-8577, Japan
3. Center for Spintronics Research Network, Tohoku University, Sendai, Miyagi 980-8577, Japan
4. Department of Electronic Engineering, Tohoku University, Sendai, Miyagi 980-8579, Japan