1. Faculty of Engineering, Hokkai-Gakuen University, 1-1, Nishi 11, Minami 26, Chuo-ku, Sapporo 064-0926, Japan
2. Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan
3. Center for Spintronics Research Network, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan