1. Key Laboratory for Magnetism and Magnetic Materials of MOE; Key Laboratory of Special Function Materials and Structure Design of MOE; Lanzhou University; Lanzhou 730000 P. R. China
2. Department of Material Science and Engineering; National University of Singapore; Engineering Drive 3 117575 Singapore
3. Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province; The Research Center of Biomedical Nanotechnology; Lanzhou University; Lanzhou 730000 P. R. China