A nanoscopic junction consisting of a quantum dot attached to ferromagnetic electrodes can act as a spin valve. Tunnelling current flowing through the system strongly depends on the relative orientation of magnetic moments in the leads. Usually the conductance is maximal for collinear alignment of the magnetic moments and is partly suppressed in canted configurations.

Spin-polarized transport through a quantum dot strongly coupled to ferromagnetic electrodes with non-collinear magnetic moments is analysed theoretically in the frame of the non-equilibrium Green’s function formalism. Due to strong coupling of the dot to ferromagnetic electrodes the effective exchange field B_ex created by the electrodes leads to spin splitting of the dot level [1,2]. Magnitude of the field depends on the coupling strengths and on the intra-dot correlation parameter U. We have studied the interplay of spin-dependent transport and exchange field in a strongly correlated quantum dot spin valve. Both, linear and non-linear-response regimes have been analysed. We have found that the effective field significantly influences the linear conductance for large values of U, which is a non-monotonic function of the angle between magnetic moments in the leads [2]. A monotonic dependence is obtained for the system with weak correlations, in which the spin-splitting is reduced. I-V characteristics and TMR are strongly influenced by the presence of exchange field. The low-bias Coulomb blockade can be partly reduced in parallel or nearly parallel configuration of magnetic moments by the spin splitting of the dot level. As a result a significant increase in the low bias TMR can be observed. On the other hand, for gate voltages for which the dot level is very close to the Fermi level in the leads, the spin-splitting induced by exchange field lowers the current leading to reduction of low-bias TMR.

[1] M. Braun et al., Phys. Rev. B 70, 195345 (2004).

[2] J. Fransson, cond-mat/0502288v1 (2005)

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