1. Department of Electrical and Computer Engineering, Ajou University, Suwon 16499, South Korea
2. School of Electronic Engineering, Soongsil University, Seoul 06938, South Korea
3. Department of Electrical and Computer Engineering, University of Virginia, Charlottesville, Virginia 22904, United States
4. School of Science, Jimei University, Xiamen 361021, China
5. Center for Spintronics, Korea Institute of Science and Technology (KIST), Seoul 02792, South Korea
6. Division of Electrical Engineering, Hanyang University, Ansan 15588, South Korea