  We investigate the Andreev transport through double-quantum-dot Cooper pair
splitters with ferromagnetic leads. The analysis is performed with the aid of
the real-time diagrammatic technique in the sequential tunneling regime. We
study the dependence of the Andreev current, the differential conductance, and
the tunnel magnetoresistance on various parameters of the model in both the
linear and nonlinear response regimes. In particular, we analyze the
spin-resolved transport in the crossed Andreev reflection regime, where a
blockade of the current occurs due to enhanced occupation of the triplet state.
We show that in the triplet blockade, finite intradot correlations can lead to
considerable leakage current due to direct Andreev reflection processes.
Furthermore, we find additional regimes of current suppression resulting from
enhanced occupation of singlet states, which decreases the rate of crossed
Andreev reflection.We also study how the splitting of Andreev bound states,
triggered by either dot level detuning, finite hopping between the dots, or
finite magnetic field, affects the Andreev current. While in the first two
cases the number of Andreev bound states is doubled, whereas transport
properties are qualitatively similar, in the case of finite magnetic field
further level splitting occurs, leading to a nontrivial behavior of
spin-resolved transport characteristics, and especially that of tunneling
magnetoresistance. Finally, we discuss the entanglement fidelity between split
Cooper pair electrons and show that by tuning the device parameters, fidelity
can reach unity.
