1. Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, 730000 Lanzhou, P. R. China
2. Key Laboratory of Special Function Materials and Structure Design, Ministry of Education, Lanzhou University, 730000 Lanzhou, P. R. China
3. Beijing Institute of Aerospace Micro Electro-Mechanical Technology, 100094 Beijing, P. R. China