The XYZ model in the mean-field approximation in terms of Pauli spin matrices

Abstract

The mean-field approximation (MFA) of spin-1/2 XYZ model was studied by using the Pauli spin matrices and their exponentials which led to very nice hyperbolic tangent functions in nonlinear form. The magnetization components Mx, My and Mz were obtained for the ferromagnetic (FM) case and then it was modified for the antiferromagnetic (AFM) case by introducing the sublattices. The bilinear exchange interaction parameters Jx, Jy and Jz, and the external magnetic fields Hx, Hy and Hz were considered along the three-dimensions for various coordination numbers q = 3, 4 and 6. The thermal variations of the magnetizations and thus the phase diagrams were obtained to illustrate the behaviours of phase transition lines in the AFM case.