1. Department of Physics, Southeast University, Nanjing 211189, China
2. NanoElectronics Group, MESA + Institute for Nanotechnology, University of Twente, P. O. Box 217, 7500 AE Enschede, The Netherlands
3. Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543, Singapore
4. Department of Physics, University of York, York YO10 5DD, UK
5. Spintronics and Nanodevice Laboratory, Department of Electronics, University of York, York, YO10 5DD, UK