Monte Carlo simulations of the short-time dynamic behavior are reported for three-dimensional weakly site-diluted Ising model with spin concentrations p=0.95 and 0.8 at criticality. In contrast to studies of the critical behavior of the pure systems by the short-time dynamics method, our investigations of site-diluted Ising model have revealed three stages of the dynamic evolution characterizing a crossover phenomenon from the critical behavior typical for the pure systems to behavior determined by the influence of disorder. The static and dynamic critical exponents are determined with the use of the corrections to scaling for systems starting separately from ordered and disordered initial states. The obtained values of the exponents demonstrate a universal behavior of weakly site-diluted Ising model in the critical region. The values of the exponents are compared to results of numerical simulations which have been obtained in various works and, also, with results of the renormalization-group description of this model.

The critical behavior of the disordered ferromagnetic Ising model is studied numerically by the Monte Carlo method in a wide range of variation of concentration of nonmagnetic impurity atoms. The temperature dependences of correlation length and magnetic susceptibility are determined for samples with various spin concentra-*

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Theoretical and experimental studies of the critical behavior of disordered systems with quenched struc tural defects are of significant current interest. Most of the real solids contain quenched structural defects, which affect their characteristics and can appreciably modify the behavior characterizing phase transitions. This gives rise to novel complex phenomena in struc turally disordered systems. These phenomena are determined by the effects caused by anomalously strong interactions between fluctuations of some ther mal parameters. As a result, any perturbation intro duced by the structural defects even at their low den sity can lead to drastic changes in the state of the sys tem. The treatment of such systems demands developing special analytical and numerical tech niques.The analysis of the effect of the structural disorder on second order phase transitions leads to the follow ing two questions. First, do the critical exponents of a "pure" magnetic system change at the dilution of it by nonmagnetic impurities? Second, if they do change, are these new critical exponents universal, i.e., inde pendent of the density of structural effects up to the percolation threshold? The answer to the first question was given in [1]. It was shown that the critical expo nents for the systems with quenched structural defects differ from those characteristic of similar systems without defects if the critical exponent for the specific heat in the pure system is positive. This criterion is met only for three dimensional systems with the critical behavior described by the Ising model.The critical behavior of dilute Ising type magnetic systems was studied in [2-4] using the renormaliza tion group techniques, numerical Monte Carlo simu lations, and experimental methods. Currently, we have a positive answer to the question concerning the exist ence of the novel universality class for dilute Ising type magnetic systems. However, it is still not quite clear whether the asymptotic values of the critical expo nents are independent of the degree of dilution in the system, how the crossover effects can change these val ues, and whether two or more regimes of the critical behavior for weakly and strongly disordered systems can exist. These questions are still open and are actively discussed.In several studies of the critical dynamics in the dis ordered three dimensional Ising model with the use of both the renormalization group approach and numer ical simulations, it is shown that structural defects most clearly manifest themselves in the changes in the dynamic critical exponent z. This exponent character izes the anomalous growth (as compared to the changes in typical static critical exponents) of the relaxation rate in the system when it approaches the phase transition temperature. In this work, we use the damage spreading method to find out the effect of structural disorder on the dynamic critical behavior of the three dimensional Ising model with spin densities p = 0.6 and 0.8. This computer simulation technique is implemented as a to...

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