1. College of Physics, National Demonstration Center for Experimental Applied Physics Education, Qingdao University, Qingdao 266071, China
2. Peter Grünberg Institute, Forschungszentrum Jülich, 52425 Jülich, Germany
3. Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000, China
4. Key Laboratory for Special Function Materials and Structural Design of the Ministry of the Education, Lanzhou University, Lanzhou 730000, China