We study the Kondo effect in a quantum dot which is coupled to ferromagnetic leads and analyse its properties as a function of the spin polarization of the leads. Based on a scaling approach we predict that for parallel alignment of the magnetizations in the leads the strong-coupling limit of the Kondo effect is reached at a finite value of the magnetic field. Using an equation-of-motion technique we study nonlinear transport through the dot. For parallel alignment the zero-bias anomaly may be split even in the absence of an external magnetic field. For antiparallel spin alignment and symmetric coupling, the peak is split only in the presence of a magnetic field, but shows a characteristic asymmetry in amplitude and position.PACS numbers: PACS numbers: 75.20.Hr, 72.15.Qm, 73.23.Hk The Kondo effect [1] in electron transport through a quantum dot (QD) with an odd number of electrons is experimentally well established [2, 3]. Screening of the dot spin due to the exchange coupling with lead electrons yields, at low temperatures, a Kondo resonance. The main goal of the present work is to investigate how ferromagnetic leads influence the Kondo effect. In the extreme case of half-metallic leads, minority-spin electrons are completely absent, i.e., the screening of the dot spin is not possible, and no Kondo-correlated state can form. What happens, however, for the generic case of partially spin polarized leads? How does the spin-asymmetry affect the Kondo effect? Is there still a strong coupling limit, and how are transport properties modified?Based on a poor man's scaling analysis we first show that the strong-coupling limit can still be reached in this case if an external magnetic field is applied. This is familiar from the Kondo effect in QDs with an even number of electrons [4, 5, 6, 7], which occurs at finite magnetic fields, although the physical mechanism is different in the present case. In the second part of the paper we analyze within an equation-of-motion (EOM) approach the nonlinear transport through the QD. We find that for parallel alignment of the lead magnetizations the zerobias anomaly is split. This splitting can be removed by appropriately tuning the strength of an external magnetic field B. In the antiparallel configuration of the lead magnetizations no splitting occurs at zero field.The Anderson Hamiltonian for a QD with a single level at energy ǫ 0 coupled to ferromagnetic leads iswhere c rkσ and d σ are the Fermi operators for electrons with wavevector k and spin σ in the leads, r = L, R, and in the QD, V rk is the tunneling amplitude,, and the last term is the Zeeman energy of the dot. (Stray fields from the leads are neglected.) We assume identical leads and symmetric coupling, V Lk = V Rk . The ferromagnetism of the leads is accounted for by different densities of states (DOS) ν r↑ (ω) and ν r↓ (ω) for up and down-spin electrons.In the following we study the two cases of parallel (P) and antiparallel (AP) alignment of the leads' magnetic moments. For the AP configuration and zero magnetic f...

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The Kondo effect in quantum dots (QDs)-artificial magnetic impurities-attached to ferromagnetic leads is studied with the numerical renormalization group method. It is shown that the QD level is spin split due to the presence of ferromagnetic electrodes, leading to a suppression of the Kondo effect. We find that the Kondo effect can be restored by compensating this splitting with a magnetic field. Although the resulting Kondo resonance then has an unusual spin asymmetry with a reduced Kondo temperature, the ground state is still a locally screened state, describable by Fermi liquid theory and a generalized Friedel sum rule, and transport at zero temperature is spin independent.