Aging and memory effects in the nonequilibrium critical behavior of structurally disordered magnetic materials in the course of their evolution from the low-temperature initial state

Abstract

The Monte Carlo study of the nonequilibrium critical evolution of structurally disordered anisotropic magnetic materials from the low-temperature initial state with the reduced magnetization m 0 = 1 is performed within the broad range of spin densities, p = 1.0, 0.95, 0.8, 0.6, and 0.5. It is shown that, in such systems, the pinning of domain walls by structural defects occurring when the evolution starts from the low-temperature state leads to significant changes in the nonequilibrium “aging” and “memory” effects in comparison to those characteristic of the “pure” system. As a result, in the long-term regime at times t–t w » t w » 1, an anomalously strong slowing down in the correlation effects is revealed. It is shown that a decrease in the autocorrelation function with time occurs according to a power law typical of the critical relaxation of the magnetization in contrast to a usual scaling dependence. Eventually, the limiting value of the fluctuation–dissipation ratio X∞ for the structurally disordered systems with p < 1 vanishes, whereas for the pure system, we have X∞ = 0.784(5). The nonequilibrium critical “superaging” stage is found. This stage is characterized by the critical exponent µ = 2.30(6) for weakly disordered systems and by µ = 2.80(7) for the systems with strong disorder.