  The spin dependent charge transport in zigzag graphene nanoribbons (ZGNRs)
has been investigated by the nonequilibrium Green's function method combined
with the density functional theory at the local spin density approximation. The
current versus voltage curve shows distinguished behaviors for symmetric and
asymmetric ZGNRs and the doping on the ZGNR edges can manipulate the spin
transport. In special cases that a Be atom is substitutionally doped on one
edge of the symmetric ZGNRs, one spin current shows negative differential
resistance while the other increases monotonically with the bias. This property
might be used to design spin oscillators or other devices for spintronics.
