1. Materials Research and Technology Department, Luxembourg Institute of Science and Technology, 41 rue du Brill, L-4422 Belvaux, Luxembourg
2. Department of Physics and Materials Science, University of Luxembourg, Campus Limpertsberg, 162 Avenue de la Faïencerie, L-1511 Luxembourg, Luxembourg
3. Université de Lorraine, CNRS, Institut Jean Lamour, UMR 7198, F-54000 Nancy, France