抄録
An Ising model with nearest neighbor antiferromagnetic and next nearest neighbor ferromagnetic interactions is studied by the phenomenological renormalization method. The phase transition to the \sqrt3×\sqrt3 structure is of the second order for weak fields, and the critical exponents yT≅1.2 and η≅0.30 are consistent with the conjectured values of the 3-state Potts model. Beyond a tricritical point the transition becomes of the first order for strong fields. The tricritical exponent yT, t≅1.73 and ηt≅0.21 are also in agreement with the 3-state Potts tricritical values yT,t3P=12⁄7 and ηt3P=4⁄21. The XY-like phase transition at zero fields, which was obtained by the Monte Carlo simulations, is not found in the present calculation.
