1. Department of Physics, Jiangsu Key Laboratory
for Advanced Metallic Materials, Southeast University, Nanjing 211189, China
2. Spintronics and
Nanodevice Laboratory, Department of Electronics, University of York, York, U.K.
3. School of Electronic Science
and Engineering, Southeast University, Nanjing 210096, China
4. Department of Electronic Engineering, Royal
Holloway, University of London, Surrey, U.K.
5. National
Laboratory of Solid Microstructures and Center of Modern Analysis, Nanjing University, Nanjing, China