1. Department of Physics School of ScienceLanzhou University of Technology Lanzhou 730050 P. R. China
2. Laboratory of Clean Energy Chemistry and Materials, State Key Laboratory of Solid Lubrication Lanzhou Institute of Chemical PhysicsChinese Academy of Sciences Lanzhou 730000 P. R. China
3. Key Laboratory for Magnetism and Magnetic Materials of Ministry of EducationLanzhou University Lanzhou 730000 P. R. China
4. Dalian National Laboratory for Clean Energy Dalian Institute of Chemical PhysicsChinese Academy of Sciences Dalian 116000 P. R. China