Affiliation: | a Laboratory for Nanoelectronics and Spintronics, Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan b Hitachi Kokusai Electric Inc., 2-1 Yasuuchi, Yatsuo-machi, Toyama 939-2393, Japan |