1. Department of Microtechnology and Nanoscience (MC2), Chalmers University of Technology; SE-41296 Göteborg Sweden
2. Department of Physics, Universiteit Antwerpen; B-2020 Antwerpen Belgium
3. Institute for Theory of Statistical Physics, RWTH Aachen; 52074 Aachen Germany
4. Peter Grünberg Institut and JARA-FIT, Forschungszentrum Jülich; 52425 Jülich Germany