1. Center for Spintronics, Korea Institute of Science and Technology (KIST) 1 , Seoul 02792, Republic of Korea
2. Department of Physics and Astronomy, Seoul National University 2 , Seoul 08826, Republic of Korea
3. Department of Physics, Chonnam National University 3 , Gwangju 61186, Republic of Korea
4. Division of Scientific Instrumentation and Management, Korea Basic Science Institute (KBSI) 4 , Daejeon 34133, Republic of Korea