  Based on the time-dependent nonequilibrium Green's function method we
investigate theoretically the time and spin-dependent transport through a
graphene layer upon the application of a static bias voltage to the electrodes
and a time-alternating gate voltage to graphene. The electrodes are magnetic
with arbitrary mutual orientations of their magnetizations. We find features in
the current that are governed by an interplay of the strength of the
alternating field and the Dirac point in graphene: The influence of a weak
alternating field on the zero bias conductance is strongly suppressed by the
zero density of state at the Dirac point. In contrast, for a strong amplitude
of the alternating field the current is dominated by several resonant peaks, in
particular a marked peak appears at zero bias. This subtle competition results
in a transition of the tunnel magnetoresistance from a broad peak to a sharp
dip at a zero bias voltage applied to the electrodes. The dip amplitude can be
manipulated by tuning the ac field frequency.
