1. State Key Laboratory of Power Systems, Department of Electrical Engineering, Tsinghua University, Beijing 100084, China
2. Physics of Functional Materials, University of Vienna, 1090 Vienna, Austria
3. Christian Doppler Laboratory for Advanced Magnetic Sensing and Materials, 1090 Vienna, Austria
4. Center for Magnetic Nanotechnology, Stanford University, 450 Serra Mall, Stanford, California 94305, USA