Phase Diagram of the ±JIsing Model in Two Dimensions

The ±JIsing model with asymmetric probability weight of ± bonds in two dimensions is studied in the p-^T plane by the numerical transfer-matrix method. We calculate finite-width susceptibilities and finite-width correlation lengths oflong strips of various sizes up to 14 x 10⁵. Finite-size scaling analysis suggests that the random antiphase state existsadjacent to the ferromagnetic phase. The boundary separating these phases is vertical (parallel to the temperature axis) in the phase diagram. The weak universality seems to hold along the ferromagnetic critical line.