  The problem of a magnetic impurity, atomic or molecular, absorbed on top of a
carbon atom in otherwise clean graphene is studied using the numerical
renormalization group. The spectral, thermodynamic, and scattering properties
of the impurity are described in detail. In the presence of a small magnetic
field, the low energy electronic features of graphene make possible to inject
spin polarized currents through the impurity using a scanning tunneling
microscope (STM). Furthermore, the impurity scattering becomes strongly spin
dependent and for a finite impurity concentration it leads to spin polarized
bulk currents and a large magnetoresistance. In gated graphene the impurity
spin is Kondo screened at low temperatures. However, at temperatures larger
than the Kondo temperature, the anomalous magnetotransport properties are
recovered.
