1. Ceramics Laboratory; EPFL-Swiss Federal Institute of Technology; CH-1015 Lausanne Switzerland
2. Peter Grünberg Institute and Ernst Ruska Center for Microscopy and Spectroscopy with Electrons; Research Center Jülich; D-52425 Jülich Germany
3. The School of Electronic and Information Engineering; Xi'an Jiaotong University; Xi'an 710049 China