  We theoretically investigate spin-resolved currents flowing in large-area
graphene, with and without defects, doped with single atoms of noble metals
(Cu, Ag and Au) and 3d-transition metals (Mn,Fe,Co and Ni). We show that the
presence of a local magnetic moment is a necessary but not sufficient condition
to have a non zero current polarization. An essential requirement is the
presence of spin-split localized levels near the Fermi energy that strongly
hybridize with the graphene pi-bands. We also show that a gate potential can be
used to tune the energy of these localized levels, leading to an external way
to control the degree of spin-polarized current without the application of a
magnetic field.
