The Monte Carlo study of structural disorder and anisotropy influence on nonequilibrium critical behavior of 3d Heisenberg model has performed. Superaging effects and three universality classes were identified for easy-axis anisotropy. Easy-plane anisotropy leads to XY-type behavior, which is insensitive to defects.
The results of a Monte Carlo study of features of structural defects influence on nonequi-librium critical behavior of three-dimensional isotropic and anisotropic Heisenberg models are presented with their evolution from different initial states. It is shown that presence of defects changes characteristics of nonequilibrium critical behavior of anisotropic model with easy axis anisotropy type and evolution from both high-temperature and low-temperature initial states. Presence defects is relevant only for characteristics of nonequi-librium critical behavior of isotropic model with evolution from low-temperature initial state leading to superaging effects.
Abstract Monte Carlo study of non-equilibrium critical behavior of three-dimensional Heisenberg model in isotropic case and with anisotropy of easy axis type is carried out. Relaxational Glauber-like dynamics of these models with evolution from high-temperature initial state is investigated. Realization of aging is demonstrated for two-time dependence of the autocorrelation function and dynamical susceptibility. Asymptotic fluctuation-dissipation ratios are determined for isotropic X ∞ = 0.383(6) and anisotropic X ∞ = 0.392(7) Heisenberg models. Significant influence of easy-axis anisotropy on non-equilibrium critical behavior of the 3D Heisenberg model leading to characteristics typical for the 3D Ising model is revealed.
Abstract A numerical Monte Carlo study has been performed on the influence of various initial states, easy-axis-type magnetic anisotropy, and structural defects on a nonequilibrium critical behavior of the classical three-dimensional Heisenberg model. An analysis of the time dependence of the magnetization and the autocorrelation function for the isotropic Heisenberg model shows substantial influence of the initial states on the relaxation of the magnetization and the aging effects in the behavior of the autocorrelation function, which are characterized by anomalous slowing down of the relaxation and correlation in the system as the waiting time increases. The study of the anisotropic Heisenberg model shows that the behavior of the magnetization and the autocorrelation function in a long-time regime is characterized by the critical exponents of the three-dimensional Ising model with a faster time decay of the autocorrelation function than that for the isotropic model. It has been revealed that the existence of structural defects in the case of the evolution of the system from the low-temperature initial state leads to an anomalously significant slowing down of the autocorrelation function. These features in the behavior of the autocorrelation function are characterized by the “superaging” effect with “superaging” exponent μ = 2.6(1) and related to the pinning of domain walls on structural defects during a nonequilibrium change in the domain structure of the system. In the case of evolution from the high-temperature initial state, the structural defects enhance the aging effect in the aging regime but their influence is irrelevant in the long-time regime.
The results of a Monte Carlo study of strongly structural disorder influence on nonequilibrium critical behavior of three-dimensional anisotropic Heisenberg models with spin concentration p = 0.5 are presented with its evolution from high and low temperature initial state. It is shown that presence of strongly disorder changes characteristics of nonequilibrium critical behavior of anisotropic model with easy axis anisotropy type in comparison with both pure anisotropic and isotropic Heisenberg models.