  We investigate the influence of periodically time-modulated cavity frequency
on the generation of atomic squeezed states for a collection of N two-level
atoms confined in a non-stationary cavity with a moving mirror. We show that
the two-photon character of the field generated from the vacuum state of field
plays a significant role in producing the atomic or spin squeezed states. We
further show that the maximum amount of persistent atomic squeezing is obtained
for the initial cavity field prepared in the vacuum state.
