  Spin-polarized transport through quantum dots is analyzed theoretically in
the cotunneling regime. It is shown that the zero-bias anomaly, found recently
in the antiparallel configuration, can also exist in the case when one
electrode is magnetic while the other one is nonmagnetic. Physical mechanism of
the anomaly is also discussed. It is demonstrated that intrinsic spin
relaxation in the dot has a significant influence on the zero-bias maximum in
the differential conductance -- the anomaly becomes enhanced by weak spin-flip
scattering in the dot and then disappears in the limit of fast spin relaxation.
Apart from this, inverse tunnel magnetoresistance has been found in the limit
of fast intrinsic spin relaxation in the dot. The diode-like behavior of
transport characteristics in the cotunneling regime has been found in the case
of quantum dots asymmetrically coupled to the leads. This behavior may be
enhanced by the spin-flip relaxation processes.
