1. Institute of Materials, Ningbo University of Technology, Ningbo 315211, P. R. China
2. School of Materials Science and Engineering, Shandong University, Jinan 250061, P. R. China
3. Department of Physics, Beijing Normal University, Beijing 100875, P. R. China
4. School of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450001, P. R. China