Thermodynamic properties of dilute antiferromagnets with mixed ferro- and antiferromagnetic interactions

A cumulant expansion is used to examine some thermodynamic properties of dilute, antiferromagnetic spin-12 system with two distinct exchange interactions. The averaged magnetic moment, transition temperature, susceptibility, specific heat and entropy are discussed for various crystallographical structures and various forms of exchange anisotropy in the whole range of temperatures and concentrations of magnetic components. It is shown that para- and antiferromagnetic phases are separated by a critical temperature surface, which at T = 0 K is reduced to a critical concentration curve. Below a certain concentration of magnetic atoms the paramagnetic susceptibility and the specific heat show an anomaly in a form of the maximum. The results obtained for the two-sublattice dilute model were found to be qualitatively similar with those obtained for a complementary total disordered system without sublattice structure.