  We investigate the evolution of a Zeeman coherence grating induced in a cold
atomic cesium sample in the presence of an external magnetic field. The
gratings are created in a three-beam light storage configuration using two
quasi-collinear writing laser pulses and reading with a counterpropagating
pulse after a variable time delay. The phase conjugated pulse arising from the
atomic sample is monitored. Collapses and revivals of the retrieved pulse are
observed for different polarizations of the laser beams and for different
directions of the applied magnetic field. While magnetic field inhomogeneities
are responsible for the decay of the coherent atomic response, a five-fold
increase in the coherence decay time, with respect to no applied magnetic
field, is obtained for an appropriate choice of the direction of the applied
magnetic field. A simplified theoretical model illustrates the role of the
magnetic field mean and its inhomogeneity on the collective atomic response.
