1. Department of Materials Science and Engineering, National University of Singapore, 117575 Singapore, Singapore.
2. College of Electron and Information Engineering, Hebei University, Baoding 071002, China.
3. Department of Physics and Astronomy and Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, NE 68588-0299, USA.
4. Center of Electron Microscope, State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China.
5. Key Laboratory of Magnetic Molecules and Magnetic Information Materials of Ministry of Education, School of Chemistry and Materials Science, Shanxi Normal University, Linfen 041004, China.