1. Department of Materials Science and Engineering; National University of Singapore; 9 Engineering Drive 1 Singapore 117575 Singapore
2. Data Storage Institute; Agency for Science, Technology and Research (A*STAR); 2 Fusionopolis Way, #08-01 Innovis Singapore 138634 Singapore
3. Key Laboratory for Magnetism and Magnetic Materials of MOE; Key Laboratory of Special Function Materials and Structure Design of MOE; Lanzhou University; Lanzhou 730000 P. R. China
4. Department of Physics; National University of Singapore; 2 Science Drive 3 Singapore 117542 Singapore