Critical ageing of Ising ferromagnets relaxing from an ordered state

We investigate the nonequilibrium behaviour 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 epsilon-expansion (epsilon = 4 - d). Ageing behaviour is clearly displayed and the associated universal fluctuation-dissipation ratio tends to X-infinity = 4/5 [1-(73/480 - pi(2)/80)epsilon + O(epsilon(2))] for long times. These results are confirmed by Monte Carlo simulations of the two-dimensional Ising model with Glauber dynamics, from which we find X-MC(infinity) = 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.