1. School of Electronic and Information Engineering and; State Key Laboratory for Mechanical Behavior of Materials; Xi'an Jiaotong University; Xi'an 710049 P. R. China
2. State Key Laboratory for Mechanical Behavior of Materials and; School of Material Science and Engineering; Xi'an Jiaotong University; Xi'an 710049 P. R. China
3. Frontier Institute of Science and Technology; Xi'an Jiaotong University; Xi'an 710049 China
4. Peter Grünberg Institute and Ernst Ruska Centre for Microscopy and Spectroscopy with Electrons; Forschungszentrum Jülich; D-52425 Jülich Germany