  We study the spin-resolved transport through single-level quantum dots
strongly coupled to ferromagnetic leads in the Kondo regime, with a focus on
contact and material asymmetry-related effects. By using the numerical
renormalization group method, we analyze the dependence of relevant spectral
functions, linear conductance and tunnel magnetoresistance on the system
asymmetry parameters. In the parallel magnetic configuration of the device the
Kondo effect is generally suppressed due to the presence of exchange field,
irrespective of system's asymmetry. In the antiparallel configuration, on the
other hand, the Kondo effect can develop if the system is symmetric. We show
that even relatively weak asymmetry may lead to the suppression of the Kondo
resonance in the antiparallel configuration and thus give rise to nontrivial
behavior of the tunnel magnetoresistance. In addition, by using the
second-order perturbation theory we derive general formulas for the exchange
field in both magnetic configurations of the system.
