EXTENDED MEAN-FIELD ANALYSIS OF THE STACKED-TRIANGULAR ISING ANTIFERROMAGNET

Phase transitions of the frustrated Ising model on a stacked-triangular lattice with the antiferromagnetic nearest-neighbor and the ferromagnetic next-nearest-neighbor interactions are studied. For this system, an earlier mean-field analysis yielded successive phase transitions, with a partial disordered phase stabilized in an intermediate temperature range and ferrimagnetic phases stabilized at lower temperatures. We have performed an extended mean-field analysis based on the variational principle in which correlation effects in the frustrated triangular layer are taken into account. It is found that the partial disordered phase is largely suppressed relative to the ferrimagnetic phase due to the correlation effects.