1. Department of Materials Science and Engineering, National University of Singapore, Singapore 117575, Singapore
2. MIIT Key Laboratory of Spintronics, School of Integrated Circuit Science and Engineering, Beihang University, Beijing 100191, China
3. Department of Mechanical Engineering, National University of Singapore, Singapore 117575, Singapore
4. Beijing Superstring Academy of Memory Technology, Beijing 100176, China
5. Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117575, Singapore
6. Chongqing Research Institute, National University of Singapore, Chongqing 401120, China