1. Department of Electrical and Computer Engineering; National University of Singapore; 4 Engineering Drive 3 Singapore 117576 Singapore
2. Institute of Materials Research and Engineering; Agency for Science, Technology and Research; 3 Research Link Singapore 117602 Singapore
3. Singapore Institute of Manufacturing Technology; Agency for Science, Technology and Research; 71 Nanyang Drive Singapore 638075 Singapore
4. Department of Applied Physics; The Hong Kong Polytechnic University; Kowloon Hong Kong
5. The Blackett Laboratory, Department of Physics; Imperial College London; London SW7 2 AZ UK
6. Data Storage Institute, Agency for Science; Technology and Research; 5 Engineering Drive 1 Singapore 117608 Singapore
7. School of Physics & Astronomy; University of Birmingham; Birmingham B15 2TT UK