MONTE-CARLO SIMULATION OF STRONGLY DISORDERED ISING-FERROMAGNETS

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 and as well as the universal amplitude A of the magnetization and the ratio C+/C- 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, which is a divergent thermal quantity. The corresponding critical exponent, which is related to the other exponents by =1-=(+)/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 =0.335 0.01 and =1.49 0.02. The exponent 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. © 1990 The American Physical Society.