1. Center for Spintronics, Post-Si Semiconductor Institute, Korea Institute of Science and Technology, Seoul 02792, South Korea
2. Department of Physics, Sookmyung Women's University, Seoul 04310, South Korea
3. KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul 136-701, South Korea
4. Department of Nanomaterials Science and Engineering, Korea University of Science and Technology, Daejeon 34113, South Korea