[1] Univ Parma, Dept Math Phys & Comp Sci, Parco Area Sci 7-A, I-43124 Parma, Italy
[2] IMEM CNR Inst, Parco Area Sci 37-A, I-43124 Parma, Italy
[3] Univ Los Andes, Lab Cristalog, Dept Quim, Fac Ciencias, Merida, Venezuela
[4] Univ Los Andes, Lab Magnetismo, Dept Fis, Fac Ciencias, Merida, Venezuela