Violation of the fluctuation-dissipation theorem in a two-dimensional Ising model with dipolar interactions

The violation of the fluctuation-dissipation theorem (FDT) in a two-dimensional Ising model with both ferromagnetic exchange and antiferromagnetic dipolar interactions is established and investigated via Monte Carlo simulations. Through the computation of the autocorrelation C(t+tw,tw) and the integrated response (susceptibility) functions we obtain the FDT violation factor X(t+tw,tw) for different values of the temperature, the waiting time tw and the quotient δ=J₀/Jd, J₀ and Jd being the strength of exchange and dipolar interactions, respectively. For positive values of δ this system develops a striped phase at low temperatures, in which the nonequilibrium dynamics presents two different regimes according to the value of δ. In each regime C(t+tw,tw) displays different scaling laws. Our results show that such different regimes are not reflected in the FDT violation factor, where X goes always to zero for high values of tw in the aging regime, a result that appears in domain growth processes in nonfrustrated ordered systems.