1. National and Local Joint Engineering Laboratory of Optical-Conversion Materials and Technology, School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, P.R. China
2. School of Stomotology, Lanzhou University, Lanzhou 730000, P.R. China
3. Key Laboratory of Magnetism and Magnetic Materials of the Ministry of Education, School of Physical Science and Technology and Electron Microscope Center of Lanzhou University, Lanzhou 730000, P.R. China