1. National Laboratory of Solid State Microstructures and Department of Physics Nanjing University, Nanjing 210093 (P. R. China)
2. Laboratorium voor Vaste‐Stoffysica en Magnetisme Kathelieke University of Leuven Leuven 3001 (Belgium)
3. Australian Key Centre for Microscopy and Microanalysis University of Sydney Sydney NSW 2006 (Australia)
4. Present address: Department of Physics, Soochow University 215006 Suzhou (P. R. China)
5. National Laboratory of Solid State Microstructures and Department of Material Science and Engineering Nanjing University Nanjing 210093 (P. R. China)