Tunable Kondo effect in double quantum dots coupled to ferromagnetic contacts

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.