1. Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, National Center for Nanoscience and Technology (NCNST), Beijing 100083, People's Republic of China
2. Institute of Theoretical Physics, and Key Laboratory for Magnetism and Magnetic Materials of MOE, Lanzhou University, Lanzhou 730000, China
3. School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, USA