1. Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117576, Singapore
2. Key Laboratory of Magnetic Molecules and Magnetic Information, Materials of Ministry of Education, School of Chemistry and Materials Science, Shanxi Normal University, Taiyuan 030006, China
3. Institute of Materials Research & Engineering, Agency for Science, Technology & Research (A*STAR), Singapore 138634, Singapore
4. Department of Physics, National University of Singapore, Singapore 117551, Singapore