We investigate the transport properties of a serial double quantum dot when it is coupled to ferromagnetic contacts in the Kondo regime. The contact polarization generates effective magnetic fields and suppresses the Kondo effect in each dot. The super-exchange interaction (J), tuned by the inter-dot tunelling 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) leads to an effective spin-1 exchange-anisotropic Kondo model which exhibits a quantum phase transition in the presence of partially polarized contacts.
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