  Electronic transport in a graphene-based ferromagnetic/normal/ferromagnetic
junction is investigated by means of Landauer-B\"{u}ttiker formulism and the
nonequilibrium Green's function technique. For the zigzag edge case, the
results show that the conductance is always larger than $e^{2}/h$ for the
parallel configuration of lead magnetizations, but for the antiparallel
configuration the conductance becomes zero because of the band-selective rule.
So a magnetoresistance (MR) plateau emerges with the value 100% when the Fermi
energy is located around the Dirac point. Besides, choosing narrower graphene
ribbons can obtain the wider 100% MR plateaus and the length change of the
central graphene region does not affect the 100% MR plateaus. Although the
disorder will reduce the MR plateau, the plateau value can be still kept about
50% even in a large disorder strength case. In addition, when the
magnetizations of the left and right leads have a relative angle, the
conductance changes as a cosine function of the angle. What is more, for the
armchair edge case, the MR is usually small. So, it is more favorable to
fabricate the graphene-based spin valve device by using the zigzag edge
graphene ribbon.
