  We investigate the spin-resolved transport properties, such as the linear
conductance and the tunnel magnetoresistance, of a double quantum dot device
attached to ferromagnetic leads and look for signatures of SU(4) symmetry in
the Kondo regime. We show that the transport behavior greatly depends on the
magnetic configuration of the device, and the spin-SU(2) as well as the orbital
and spin-SU(4) Kondo effects become generally suppressed when the magnetic
configuration of the leads varies from the antiparallel to the parallel one.
Furthermore, a finite spin polarization of the leads lifts the spin degeneracy
and drives the system from the SU(4) to an orbital-SU(2) Kondo state. We
analyze in detail the crossover and show that the Kondo temperature between the
two fixed points has a non-monotonic dependence on the degree of spin
polarization of the leads. In terms of methods used, we characterize transport
by using a combination of analytical and numerical renormalization group
approaches.
