Crystal field effect on the magnetic field dependence cooling exponent of 3D simple cubic spin-7/2 Ising model: A Monte Carlo study

Abstract

Based on the Monte Carlo simulation, the magneto caloric effect of a spin-7/2 3D-Ising system with pure ferromagnetic interaction was studied. The effective magnetic field h dependent entropy change ΔS was examined and result shows that ΔS follows a quadratic polynomial of hn where n is the cooling exponent. In the high field region the quadratic term h2n becomes comparable with the linear term hn. The quadratic term is strongly suppressed by the increment of the crystal field coefficient D. The ratio of the coefficient of the linear and quadratic term B/C is nearly 250 for D = 4 J (J is the exchange coefficient) and the quadratic term becomes negligibly small compared with the linear term. Furthermore in this ferromagnetic Ising system n continuously decreases from 0.71 to 0.62 for the rise of effective crystal field coefficient De from 0 to 4. This happens due to the enhancement of the + 7/2 spin from 70% to 87% for the increment of the same amount of De. It turns the system from spin-σ to spin-1/2 3D-Ising in nature.