STATISTICAL THEORY OF I(1) MODEL ANTIFERROMAGNE-TISM ON A FACE CENTERED CUBIC LATTICE WITH NEAREST-NEIGHBOR INTERACTION——A SERIES EXPANSION METHOD

Abstract

The antiferromagnetism of Ising spin I(1) with the nearest neighbor interaction -J on fcc has been treated by the method of series expansion similar to [1]. The free energy function for the lower temperature ordered state is written in a series of exp(-J/kT) and that for the higher temperature disorder state in a series of J/kT. Using the Padé approximants we have shown clearly that the free energy curves of the two states cross at Tc= 1.33 J/k. The transition is one of the first order, the same conclusion as we obtained for the I(1/2) model, but Tc of I(1) is lower than that of I(1/2). The thermodynamic quantities such as the long-range and short-range order parameter, the internal energy, the entropy, the specific heat as well as the magnetic susceptibility were calculated in variation with temperature. They all change abruptly at Tc.We concluded that (1) on fcc the Ising spin AM-PM transtion being one of the first order should be attributed to the close-packing characteristics of this lattice; and (2) Tc(s) decreases as s increases.