  We present a theoretical study based on the Anderson model of the transport
properties of a Kondo impurity (atom or quantum dot) connected to ferromagnetic
leads, which can sustain a non-equilibrium spin current. We analyze the case
where the spin current is injected by an external source and when it is
generated by the voltage bias. Due to the presence of ferromagnetic contacts, a
static exchange field is produced that eventually destroys the Kondo
correlations. We find that such a field can be compensated by an appropriated
combination of the spin-dependent chemical potentials leading to the
restoration of the Kondo resonance. In this respect, a Kondo impurity may be
regarded as a very sensitive sensor for non-equilibrium spin phenomena.
