1. Laboratory for Solid State Physics and Magnetism & INPAC-Institute for Nanoscale Physics and Chemistry, Katholieke Universiteit Leuven, B-3001 Leuven, Belgium
2. Department of Environmental Modeling, VITO-Flemish Institute for Technological Research, B-2400 Mol, Belgium
3. Department of Applied Mechanics, Chalmers University of Technology, SE-412 96 Göteborg, Sweden