Cotunneling through quantum dots coupled to magnetic leads: Zero-bias anomaly for noncollinear magnetic configurations

Cotunneling transport through quantum dots weakly coupled to noncollinearly magnetized leads is analyzed theoretically by means of the real-time diagrammatic technique. The electric current, dot occupations, and dot spin are calculated in the Coulomb blockade regime and for the arbitrary magnetic configuration of the system. It is shown that an effective exchange field exerted on the dot by ferromagnetic leads can significantly modify the transport characteristics in noncollinear magnetic configurations, in particular the zero-bias anomaly found recently [I. Weymann , Phys. Rev. B 72, 113301 (2005)] for antiparallel configuration. For the asymmetric Anderson model, the exchange field gives rise to precession of the dot spin, which leads to a nonmonotonic dependence of the differential conductance and tunnel magnetoresistance on the angle between magnetic moments of the leads. An enhanced differential conductance and negative tunnel magnetoresistance were found for certain noncollinear configurations.