  We investigate the effect of edge defects (vacancies) and impurities
(substitutional dopants) on the robustness of spin-polarization in graphene
nanoribbons (GNRs) with zigzag edges, using density-functional-theory
calculations. We found that the stability of the spin state and its magnetic
moments decrease continuously with increasing concentration of defects or
impurities. The system generally becomes non-magnetic at the concentration of
one edge defect (impurity) per 10 angstrom. The spin suppression is shown to be
caused by reduction and removal of edge states at the Fermi energy. Our
analysis implies an important criterion on the GNR samples for spintronics
applications.
