Modification of medium phase memory under influence of pulsed inhomogeneous magnetic field

We consider retrieving of a light pulse stored in a medium of atoms with Λ-configuration of levels under conditions of electromagnetically induced transparency when a pulse of the external magnetic field inhomogeneous over the medium length is applied to the medium during the absence of the control field. It is shown that the coherence of lower levels of medium atoms acquires a phase depending on the coordinate along the medium and that the temporal shape of the retrieved pulse, if it is recorded interferometrically, reflects the coordinate dependence of the magnetic field.