1. Storage Laboratory Samsung Advanced Institute of Technology, P.O. Box 111, Suwon 440-600, Korea
2. Department of Mechanical Engineering, Yonsei University, 134 Shinchon-dong, Sudaemun-ku, Seoul 120-749, Korea
3. Center for Information Storage Device, Yonsei University, 134 Shinchon-dong, Sudaemun-ku, Seoul 120-749, Korea