Effect of the initial states, the anisotropy, and structural defects on a nonequilibrium critical behavior of the three-dimensional Heisenberg model

A numerical Monte Carlo study of different initial states, magnetic anisotropy of “easy axis” type, and structural defects influence on non-equilibrium critical behavior of the classical three-dimensional Heisenberg model has been carried out. Analysis of time dependence of the magnetization and the autocorrelation function for isotropic Heisenberg model has shown strong influence of initial states on relaxation of the magnetization and aging effects in behavior of the autocorrelation function, which are characterized by anomalous slowing down of relaxation and correlation in system with increasing waiting time. Study of the anisotropic Heisenberg model with magnetic anisotropy of “easy axis” type has revealed that behavior of the magnetization and autocorrelation function in long-time regime is characterized by critical exponents of the three-dimensional Ising model and more fast decay of the autocorrelation function than for isotropic Heisenberg model. It has been revealed that in structurally disordered isotropic Heisenberg model relaxing from the low-temperature initial state, the significant changes of non-equilibrium aging phenomena in comparison with pure model are observed due to the pinning of domain walls on defects. It has been shown that during this relaxation process a strong slowing-down of time dependence of the autocorrelation function is realized. As a result, superaging effects occur in long-term behavior of the autocorrelation function with values of the superaging exponent $\mu$=2.6(1). During evolution of system from high-temperature initial state, the presence of structural defects leads to increase of aging effects in aging regime, but their influence is found irrelevant in long-time regime.