1. Department of Physics ‘E.R. Caianiello’, University of Salerno, via Giovanni Paolo II, 132, Fisciano, I-84084 Salerno, Italy
2. CNR-SPIN Salerno, via Giovanni Paolo II, 132, Fisciano, I-84084 Salerno, Italy
3. Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee Blvd., 1784 Sofia, Bulgaria
4. Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Acad. G. Bonchev Street, bl. 10, 1113 Sofia, Bulgaria