We report extensive Monte Carlo simulations of disordered Ising systems in the ferromagnetic region with concentrations of magnetic sites between p=1.0 and 0.5. The magnetization, the susceptibility, and the caloric properties have been studied in the critical region. The critical exponents β and γ as well as the universal amplitude A of the magnetization and the ratio ${\mathit{C}}_{+}$/${\mathit{C}}_{\mathrm{-}}$ of the susceptibility amplitudes have been determined with high precision. In addition to the cusplike specific heat, we have also measured the magnetization-energy correlation function Γ, which is a divergent thermal quantity. The corresponding critical exponent ζ, which is related to the other exponents by ζ=1-β=(α+γ)/2, has been determined. We have found that all quantities show power-law behavior within the temperature range of our simulation. All critical exponents change continuously with dilution. Even in the range of weak dilution (p≥0.8), the effective critical exponents are concentration dependent and are clearly different from their pure system values. In the strongly diluted regime the critical exponents gradually reach new asymptotic behavior at p=0.5–0.6 with values of β=0.335±0.01 and γ=1.49±0.02. The exponent α of the specific heat becomes -0.17±0.04, which corresponds to a cusplike singularity. We conclude that disorder profoundly changes the critical behavior for weakly as well as strongly disordered spin systems.

• Received 16 May 1990

DOI:https://doi.org/10.1103/PhysRevB.42.6476