1. Department of Physics and Astronomy, Seoul National University, Seoul 151-747, Korea
2. School of Mathematics and Physics, China University of Geosciences, Wuhan 430074, China
3. Faculty of Mechanical and Electronic
Information, China University of Geosciences, Wuhan 430074, China
4. Peter-Grünberg-Institute,
PGI-8, Research Center Jülich and JARA Fundamentals of Future Information Technology, Jülich 52425,
Germany