  We investigate the effects induced by spin polarization in the contacts
attached to a serial double quantum dot. The polarization generates effective
magnetic fields and suppresses the Kondo effect in each dot. The super-exchange
interaction ($J_{\mathrm{AFM}}$), tuned by the inter-dot tunnelling rate $t$,
can be used to compensate the effective fields and restore the Kondo resonance
when the contact polarizations are aligned. As a consequence, the direction of
the spin conductance can be controlled and even reversed using electrostatic
gates alone. Furthermore, we study the associated two-impurity Kondo model and
show that a ferromagnetic exchange coupling ($J_{\mathrm{FM}}$) leads to an
effective spin-1 exchange-anisotropic Kondo model which exhibits a quantum
phase transition in the presence of partially polarized contacts.
