Constant coupling approximation for antiferromagnets with mixed ferro- and antiferromagnetic interactions

Abstract

The constant-coupling approximation is extended to an antiferromagnetic spin- 1 2 system with two distinct anisotropic exchange interactions. The thermodynamic properties such as the transition temperature, magnetization, susceptibility and specific heat are discussed for three special cases: (i) ferro- and antiferromagnetic Ising interactions, (ii) isotropic ferro- and antiferromagnetic Heisenberg interactions; and (iii) isotropic ferromagnetic Heisenberg interactions and antiferromagnetic Ising interactions, allowing in each case for two different nearest-neighbour coupling constants. Numerical calculations have been performed for a layer structure with z = 6 intraplanar and z' = 6 interplanar nearest neighbours and the results are compared with those obtained in other approximations. Applying the theory to FeCl 2, the exchange constants are evaluated. It is shown that the calculated magnitudes of the interactions strongly depend upon the exchange-interaction model assumed.