  We study the spin-dependent transport properties of a spin valve based on a
double quantum dot. Each quantum dot is assumed to be strongly coupled to its
own ferromagnetic lead, while the coupling between the dots is relatively weak.
The current flowing through the system is determined within the perturbation
theory in the hopping between the dots, whereas the spectrum of a quantum
dot-ferromagnetic lead subsystem is determined by means of the numerical
renormalization group method. The spin-dependent charge fluctuations between
ferromagnets and quantum dots generate an effective exchange field, which
splits the double dot levels. Such field can be controlled, separately for each
quantum dot, by the gate voltages or by changing the magnetic configuration of
external leads. We demonstrate that the considered double quantum dot spin
valve setup exhibits enhanced magnetoresistive properties, including both
normal and inverse tunnel magnetoresistance. We also show that this system
allows for the generation of highly spin-polarized currents, which can be
controlled by purely electrical means. The considered double quantum dot with
ferromagnetic contacts can thus serve as an efficient voltage-tunable spin
valve characterized by high output parameters.
