1. Peter Grünberg Research Center Nanjing University of Posts and Telecommunications Nanjing 210003 China
2. State Key Laboratory of Luminescence and Applications Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences Changchun 130033 China
3. National Center for International Joint Research of Electronic Materials and Systems School of Information Engineering Zhengzhou University Zhengzhou 450001 China
4. Institute of Materials and Systems for Sustainability Nagoya University Nagoya 464-8601 Japan