1. National Laboratory of Solid State Microstructures & Jiangsu Key Laboratory for Nano Technology; Department of Physics; Nanjing University; Nanjing 210093 P.R. China
2. Collaborative Innovation Center of Advanced Microstructures; Nanjing University; Nanjing 210093 P.R. China
3. State Key Laboratory for Magnetism; Beijing National Laboratory for Condensed Matter Physics; Institute of Physics; Chinese Academy of Sciences; Beijing 100190 P.R. China
4. Key Laboratory of Magnetic Materials and Devices; Ningbo Institute of MaterialTechnology and Engineering; Chinese Academy of Sciences; Ningbo 315201 P.R. China