Phase transitions in a frustrated Ising antiferromagnet on a square lattice.

The phase transitions occurring in the frustrated Ising square antiferromagnet with first- (J1 < 0) and second-nearest-neighbor (J2 < 0) interactions are studied within the framework of the effective-field theory with correlations based on the different cluster sizes and for a wide range of R = J2/\J1 |. Despite the simplicity of the model, it has proved difficult to precisely determine the order of the phase transitions. In contrast to the previous effective-field study, we have found a first-order transition line in the region close to R = --0.5 not only between the superantiferromagnetic and paramagnetic (R < - 0.5) but also between antiferromagnetic and paramagnetic (R > --0.5) phases. [ABSTRACT FROM AUTHOR]