1. York-Nanjing Joint Center (YNJC) for Spintronics and Nanoengineering, School of Electronics Science and Engineering, Nanjing University, Nanjing 210093, China.
2. Department of Electronic Engineering, Royal Holloway University of London, Egham TW20 0EX, UK.
3. Department of Electronic Engineering, The University of York, York YO10 5DD, UK.
4. Department of Electrical and Computer Engineering, Department of Materials Science and Engineering, and Department of Physics, University of California, Los Angeles, Los Angeles, CA 90095, USA.
5. Magnetic Spectroscopy Group, Diamond Light Source, Didcot OX11 0DE, UK.