1. Department of Applied Physics, Tohoku University, Sendai 980-8579, Japan
2. Cybermedia Center (CMC), Osaka University, Toyonaka, Osaka 560-0043, Japan
3. Department of Physics, Ochanomizu University, Bunkyo, Tokyo 112-8610, Japan
4. Spintronics Research Center, AIST, Tsukuba, Ibaraki 305-8568, Japan