Investigation of possible phase transition of the frustrated spin-1/2 J -J -J model on the square lattice.

The frustrated spin-1/2 J -J -J antiferromagnet with exchange anisotropy on the two-dimensional square lattice is investigated. The exchange anisotropy is presented by η with 0 ≤ η < 1. The effects of the J , J , J and anisotropy on the possible phase transition of the Néel state and collinear state are studied comprehensively. Our results indicate that for J  > 0 there are upper limits [Formula: see text] and η values. When 0 < J  ≤ [Formula: see text] and 0 ≤ η ≤ η , the Néel and collinear states have the same order-disorder transition point at J  = J /2. Nevertheless, when the J and η values beyond the upper limits, it is a paramagnetic phase at J  = J /2. For J  < 0, in the case of 0 ≤ η < 1, the two states always have the same critical temperature as long as J  = J /2. Therefore, for J  = J /2, under such parameters, a first-order phase transition between the two states for these two cases below the critical temperatures may occur. When J  ≠ J /2, the Néel and collinear states may also exist, while they have different critical temperatures. When J  > J /2, a first-order phase transition between the two states may also occur. However, for J  < J /2, the Néel state is always more stable than the collinear state.