A systematic cluster extension of the effective-interaction approximation for quenched-bond disorder in random Ising systems is proposed. For bond dilution, exact results for the critical concentrations are obtained in the single-bond approximation for the self-dual square lattice and in the three-bonds approximation for the dual triangular and honeycomb lattices. Qualitative changes in the critical line are observed in the frustration model when going from the single-bond to the three-bonds approximations.
The ferromagnetic-phase stability limit and magnetization of a quenched bond-mixed spin-1/2 transverse Ising model in an anisotropic simple square lattice are studied within the framework of an effective-field theory considering both competing and noncompeting interactions. In the bond-disordered case, where bonds
J1 and
J2 occur with probabilities 1-p and p, respectively, critical curves are obtained for various \ensuremath{\alpha}'s (\ensuremath{\alpha}\ensuremath{\equiv}
J1/
J2), and the critical bond concentrations
pc at which the ferromagnetic phase breaks down are determined. As \ensuremath{\alpha}\ensuremath{\rightarrow}0, the mixed model reduces to the diluted Ising model and the results obtained are compared to those available in the literature. The effect of the transverse field (for different \ensuremath{\alpha}'s) in the thermal behavior of the magnetization as a function of the concentration is also discussed.