1. Key Laboratory of Magnetic Materials and Devices, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo, Zhejiang 315201, P. R. China
2. Nano Science and Technology Institute, University of Science and Technology of China, Suzhou, Jiangsu 215123, P. R. China
3. Hunan Key Laboratory for Super Microstructure and Ultrafast Process, School of Physics and Electronics, Central South University, Changsha, Hunan 410083, P. R. China
4. Laboratory for Nanoelectronics and NanoDevices, Department of Electronics Science and Technology, Hangzhou Dianzi University Hangzhou, Zhejiang 310018, P. R. China