  We present a theoretical study on the spin-dependent transport through a spin
valve consisting of graphene sandwiched between two magnetic leads with an
arbitrary orientation of the lead magnetization. No gate voltage is applied.
Using Keldysh's nonequilibrium Green's function method we show that, in absence
of external magnetic fields, the current-voltage curves are nonlinear. Around
zero bias the differential conductance versus bias voltage possesses a strong
dip. The zero-bias anomaly in the tunnel magnetoresistance (TMR) is affected
strongly by the leads spin polarization. Depending on the value of the bias
voltage TMR exhibits a behavior ranging from an insulating to a metallic-type.
In presence of a static external magnetic field the differential conductance
and TMR as a function of the bias voltage and the strength of the magnetic
field show periodic oscillations due to Landau-level crossings. We also inspect
the effects of the temperature and the polarization degrees on the differential
conductance and TMR.
