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+ t(w),t(w)) and the integrated response (susceptibility) functions we obtain the FDT violation factor X(t+t(w),t(w)) for different values of the temperature, the waiting time t(w) and the quotient delta=J(0)/J(d), J(0) and J(d) being the strength of exchange and dipolar interactions, respectively. For positive values of delta this system develops a striped phase at low temperatures, in which the nonequilibrium dynamics presents two different regimes according to the value of delta. In each regime C(t+t(w),t(w)) 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 t(w) in the aging regime, a result that appears in domain growth processes in nonfrustrated ordered systems. [S0163-1829(99)03026-X].