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Physics of the Solid State

Effect of the Initial States, the Anisotropy, and Structural Defects on a Nonequilibrium Critical Behavior of the Three-Dimensional Heisenberg Model

Vladimir V. Prudnikov, Pavel Vladimirovich Prudnikov, Лях Анастасия Сергеевна

Published May 1, 2020• 1 citation
Publisher

Abstract

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.

DOI
10.1134/s1063783420050261

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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.
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 μ=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.
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.
The effect of various initial magnetizations m0 and structural defects the nonequilibrium critical behavior of the three-dimensional Ising model is numerically studied. Based on an analysis of the time dependence of the magnetization and the two-time dependence of the autocorrelation function and dynamic susceptibility, the significant effect of initial states on relaxation magnetizations and aging effects characterized by anomalous relaxation inhibition and correlation in the system with increasing waiting time was revealed. The fluctuation–dissipation theorem violation was studied, and the values of the limit fluctuation–dissipation ratio (FDR) are calculated. It is shown that two universality subclasses can be distinguished in the nonequilibrium critical behavior of the three-dimensional Ising model with random initial magnetization m0 These subclasses correspond to the system evolution from the high-temperature (m0 = 0) and low-temperature (m0 = 1) initial states with limit FDRs characteristic of these states.
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.
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.
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.