  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 and
fluctuation-dissipation ratio are computed up to first order in the
epsilon-expansion. Aging behavior is clearly displayed during the critical
relaxation. These results are confirmed by Monte Carlo simulations of the
two-dimensional Ising model with Glauber dynamics. 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.
