1. Center for Magnetic Materia ls and Devices, College of Physics and Electronic Engineering, Qujing Normal University, Qujing 655011, China
2. Key Laboratory for Advanced Functional and Low Dimensional Materials of Yunnan Higher Education Institute, Qujing Normal University, Qujing 655011, China
3. Institute of Plasma Physics, Chinese Academy of Sciences, P.O. Box 1129, Hefei 230031, People’s Republic of China