Underscreened Kondo effect in quantum dots coupled to ferromagnetic leads

Abstract

We analyze the equilibrium transport properties of underscreened Kondo effect in the case of a two-level quantum dot coupled to ferromagnetic leads. Using the numerical renormalization-group (NRG) method, we have determined the gate voltage dependence of the dot's spin and level-resolved spectral functions. We have shown that the polarization of the dot is very susceptible to spin imbalance in the leads and changes sign in the middle of the $S=1$ Coulomb valley. Furthermore, we have also found that by fine tuning an external magnetic field one can compensate for the presence of ferromagnetic leads and restore the Kondo effect in the case of $S=\frac{1}{2}$ Coulomb valley. The underscreened Kondo effect is, however, extremely sensitive to the magnetic field which hinders the full restoration of the Kondo peak.