1. National Laboratory of Solid State Microstructures and Key Laboratory of Nanomaterials for Jiang Su Province, Nanjing University, Nanjing 210093, People's Republic of China
2. Center for Integrated Spintronic Devices (CISD), Hangzhou Dianzi University, Hangzhou, Zhejiang 310018, China
3. Department of Physics, Southeast University, Nanjing 211189, China