1. Department of Physics, Aristotle University of Thessaloniki, GR-54124 Thessaloniki, Greece
2. Institut Jean Lamour (UMR CNRS 7198), Université de Lorraine, Parc de Saurupt, 54011 Nancy cedex, France
3. Rensselaer Polytechnic Institute, Department of Materials Science and Engineering, Troy, New York 12180, United States