1. School of Physics, State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China
2. Peter Grünberg Institute, Forschungszentrum Jülich, 52425 Jülich, Germany
3. Department of Physics, RWTH Aachen University, 52056 Aachen, Germany
4. Institute of Physics, Johannes Gutenberg University Mainz, 55099 Mainz, Germany