1. Institute of Physics, Johannes Gutenberg University Mainz, Staudinger Weg 7, 55128 Mainz, Germany
2. Max Planck Institute for Solid State Research, Heisenbergstraße 1, 70569 Stuttgart, Germany
3. State Key Laboratory for Artificial Microstructure and Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, China
4. Max Planck Institute for Polymer Research Mainz, Ackermannweg 10, 55128 Mainz, Germany
5. Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons, Peter Grünberg Institute, Forschungszentrum Jülich, 52425 Jülich, Germany
6. Department of Physics, University of Konstanz, Universitaetsstrasse 10, 78464 Konstanz, Germany
7. Centre for Quantum Spintronics, Department of Physics, Norwegian University of Science and Technology, 7491 Trondheim, Norway