1. Center for Spintronics, Korea Institute of Science and Technology, Seoul 136-791, South Korea
2. Department of Physics, Korea University, Seoul 136-713, 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 305-350, South Korea