  We investigate spin effects in transport across fully interacting, finite
size graphene armchair nanoribbons (ACNs) contacted to collinearly
spin-polarized leads. In such systems, the presence of short ranged Coulomb
interaction between bulk states and states localized at the ribbon ends leads
to novel spin-dependent phenomena. Specifically, the total spin of the low
energy many-body states is conserved during tunneling but that of the bulk and
end states is not. As a consequence, in the single-electron regime, dominated
by Coulomb blockade phenomena, we find pronounced negative differential
conductance features for ACNs contacted to parallel polarized leads. These
features are however absent for an anti-parallel contact configuration, which
in turn leads within a certain gate and bias voltage region to a negative
tunneling magneto-resistance. Moreover, we analyze the changes in the transport
characteristics under the influence of an external magnetic field.
