1. College of Mechanical and Electrical Engineering, Suzhou University, Suzhou 234000, Anhui, P. R. China
2. Key Laboratory of Spin Electron and Nanomaterials of Anhui Higher Education Institutes, Suzhou University, Suzhou 234000, Anhui, P. R. China
3. School of Physics and Materials Science, Anhui University, Hefei 230601, P. R. China