1. University of Coimbra, CFisUC, Physics Department, P-3004-516 Coimbra, Portugal
2. Laboratoire de Physique Appliquée, Faculté des Sciences, Université de Sfax, B. P. 1171, 3000 Sfax, Tunisia
3. I3N and Physics Department, University of Aveiro, Aveiro, 3810-193, Portugal
4. INEGI, Department of Metallurgical and Materials Engineering, University of Porto, Rua Dr Roberto Frias, P-4200-465 Porto, Portugal