[1] Department of Material Science, University of Hyogo, Kamigoori 672-1297, Japan
[2] Department of Physics, University of Tokyo, Tokyo 113-0033, Japan
[3] Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
[4] Department of Physics, Bilkent University, 06800 Ankara, Turkey
[5] Department of Physics, Emory University, Atlanta, GA 30322