  Keldysh nonequilibrium Green's function method is utilized to study
theoretically the spin polarized transport through a graphene spin valve
irradiated by a monochromatic laser field. It is found that the bias dependence
of the differential conductance exhibits successive peaks corresponding to the
resonant tunneling through the photon-assisted sidebands. The multi photon
processes originate from the combined effects of the radiation field and the
graphene tunneling properties, and are shown to be substantially suppressed in
a graphene spin valve which results in a decrease of the differential
conductance for a high bias voltage. We also discussed the appearance of a
dynamical gap around zero bias due to the radiation field. The gap width can be
tuned by changing the radiation electric field strength and the frequency. This
leads to a shift of the resonant peaks in the differential conductance. We also
demonstrate numerically the dependencies of the radiation and spin valve
effects on the parameters of the external fields and those of the electrodes.
We find that the combined effects of the radiation field, the graphene, and the
spin valve properties bring about an oscillatory behavior in the tunnel
magnetoresistance (TMR), and this oscillatory amplitude can be changed by
scanning the radiation field strength and/or the frequency.
