  At B3LYP level of theory, we predict that the half-metallicity in zigzag edge
graphene nanoribbon (ZGNR) can be realized when an external electric field is
applied across the ribbon. The critical electric field to induce the
half-metallicity decreases with the increase of the ribbon width. Both the spin
polarization and half-metallicity are removed when the edge state electrons
fully transferred from one side to the other under very strong electric field.
The electric field range under which ZGNR remain half-metallic increases with
the ribbon width. Our study demonstrates a rich field-induced spin polarization
behavior, which may leads to some important applications in spinstronics.
