  Equilibrium transport properties of a single-level quantum dot tunnel-coupled
to ferromagnetic leads and exchange-coupled to a side nonmagnetic reservoir are
analyzed theoretically in the Kondo regime. The equilibrium spectral functions
and conductance through the dot are calculated using the numerical
renormalization group (NRG) method. It is shown that in the antiparallel
magnetic configuration, the system undergoes a quantum phase transition with
increasing exchange coupling $J$, where the conductance drops from its maximum
value to zero. In the parallel configuration, on the other hand, the
conductance is generally suppressed due to an effective spin splitting of the
dot level caused by the presence of ferromagnetic leads, irrespective of the
strength of exchange constant. However, for $J$ ranging from J=0 up to the
corresponding critical value, the Kondo effect and quantum critical behavior
can be restored by applying properly tuned compensating magnetic field.
