1. Department of Quantum Nanoscience, Kavli Institute of Nanoscience, Delft University of Technology, 2628 CJ Delft, the Netherlands.
2. RWTH Aachen University, Institute of Physics, D-52074 Aachen, Germany.
3. Peter-Grünberg-Institute, Forschungszentrum Jülich, D-52425 Jülich, Germany.