1. Key Laboratory for Magnetism and Magnetic Materials of MOE; Lanzhou University; Lanzhou 730000 P. R. China
2. Key Laboratory of Special Function Materials and Structure Design; Ministry of Education; Lanzhou University; Lanzhou 730000 P. R. China
3. School of Microelectronics; Xi'an Jiaotong University; Xi'an 710049 P. R. China