Ding K.-H., Zhu Z.-G., Berakdar J.

Based on the time-dependent nonequilibrium Green?s function method we investigate theoretically the timeand 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 an 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.

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