  We theoretically design a graphene-based all-organic ferromagnetic
semiconductor by terminating zigzag graphene nanoribbons (ZGNRs) with organic
magnets. A large spin-split gap with 100% spin polarized density of states near
the Fermi energy is obtained, which is of potential application in spin
transistors. The interplays among electron, spin and lattice degrees of freedom
are studied using the first-principles calculations combined with fundamental
model analysis. All of the calculations consistently demonstrate that although
no d electrons existing, the antiferromagnetic \pi-\pi exchange together with
the strong spin-lattice interactions between organic magnets and ZGNRs make the
ground state ferromagnetic. The fundamental physics makes it possible to
optimally select the organic magnets towards practical applications.
