Critical aging of Ising ferromagnets relaxing from an ordered state

We investigate the nonequilibrium behavior of the d-dimensional Ising model with purely dissipative dynamics during its critical relaxation from a magnetized initial configuration. The universal scaling forms of the two-time response and correlation functions of the magnetization are derived within the field-theoretical approach and the associated scaling functions are computed up to first order in the ǫ-expansion (ǫ = 4 − d). Aging behavior is clearly displayed and the associated universal fluctuation-dissipation ratio tends to X∞ = 4 5 [1− ( 73 480 − π 2 80 )ǫ+O(ǫ 2)] for long times. These results are confirmed by Monte Carlo simulations of the two-dimensional Ising model with Glauber dynamics, from which we findX∞ MC = 0.73(1). The crossover to the case of relaxation from a disordered state is discussed and the crossover function for the fluctuation-dissipation ratio is computed within the Gaussian approximation.