1. School of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju 61005, South Korea
2. College of Telecommunications and Information Engineering, Nanjing University of Posts and Telecommunications, Nanjing 210003, China
3. Grünberg Center for Magnetic Nanomaterials, Gwangju Institute of Science and Technology (GIST), Gwangju 61005, South Korea