  Using the non-equilibrium Green's functions formalism in a tight binding
model, the spin-dependent transport in armchair graphene nanoribbon (GNR)
structures controlled by a ferromagnetic gate is investigated. Beyond the
oscillatory behavior of conductance and spin polarization with respect to the
barrier height, which can be tuned by the gate voltage, we especially analyze
the effect of width-dependent band gap and the nature of contacts. The
oscillation of spin polarization in the GNRs with a large band gap is strong in
comparison with 2D-graphene structures. Very high spin polarization (close to
100%) is observed in normal-conductor/graphene/normal-conductor junctions.
Moreover, we find that the difference of electronic structure between normal
conductor and graphene generates confined states in the device which have a
strong influence on the transport quantities. It suggests that the device
should be carefully designed to obtain high controllability of spin current.
