  The Kondo effect in quantum dots (QDs) - artificial magnetic impurities -
attached to ferromagnetic leads is studied with the numerical renormalization
group (NRG) method. It is shown that the QD level is spin-split due to presence
of ferromagnetic electrodes, leading to a suppression of the Kondo effect. We
find that the Kondo effect can be restored by compensating this splitting with
a magnetic field. Although the resulting Kondo resonance then has an unusual
spin asymmetry with a reduced Kondo temperature, the ground state is still a
locally-screened state, describable by Fermi liquid theory and a generalized
Friedel sum rule, and transport in the unitary limit is not spin dependent.
