Wang-Landau study of the random bond square Ising model with nearest- and next-nearest-neighbor interactions

We report results of a Wang-Landau study of the random bond square Ising model with nearest- ($J_{nn}$) and next-nearest-neighbor ($J_{nnn}$) antiferromagnetic interactions. We consider the case $R=J_{nn}/J_{nnn}=1$ for which the competitive nature of interactions produces a sublattice ordering known as superantiferromagnetism and the pure system undergoes a second-order transition with a positive specific heat exponent $\alpha$. For a particular disorder strength we study the effects of bond randomness and we find that, while the critical exponents of the correlation length $\nu$, magnetization $\beta$, and magnetic susceptibility $\gamma$ increase when compared to the pure model, the ratios $\beta/\nu$ and $\gamma/\nu$ remain unchanged. Thus, the disordered system obeys weak universality and hyperscaling similarly to other two-dimensional disordered systems. However, the specific heat exhibits an unusually strong saturating behavior which distinguishes the present case of competing interactions from other two-dimensional random bond systems studied previously.