M. Wilczyński

Spin effects in electron tunneling through a quantum dot coupled to noncollinearly polarized ferromagnetic leads

Spin-dependent transport through an interacting single-level quantum dot coupled to ferromagnetic leads with noncollinear magnetizations is analyzed theoretically. The transport properties and average spin of the dot are investigated within the nonequilibrium Green function technique based on the equation of motion in the Hartree-Fock approximation. Numerical results show that Coulomb correlations on the dot and strong spin polarization of the leads significantly enhance precession of the average dot spin around the effective molecular field created by the external electrodes. Moreover, they also show that spin precession may lead to negative differential conductance in the voltage range between the two relevant threshold voltages. Nonmonotonous angular variation of electric current and change in sign of the tunnel magnetoresistance are also found. It is also shown that the diodelike behavior in asymmetrical junctions with one electrode being half-metallic is significantly reduced in noncollinear configurations.

Tunnel magnetoresistance in ferromagnetic double-barrier planar junctions: coherent tunneling regime

Coherent tunneling in a double-barrier system consisting of two external ferromagnetic electrodes and a nonmagnetic central one is studied theoretically within the free-electron approximation. It is shown that the junction resistance depends on the relative orientation of magnetic moments of the ferromagnetic electrodes (so-called tunnel magnetoresistance). The magnetoresistance vs. thickness of the central electrode shows pronounced peaks related to the resonant tunneling through the whole system. Variation of the magnetoresistance with bias voltage is also studied. This variation is generally nonmonotonous.

Electron tunnelling in a double ferromagnetic junction with a magnetic dot as a spacer

Electron tunnelling in a double-barrier junction with ferromagnetic external electrodes and a magnetic quantum dot as a spacer is analysed theoretically in the framework of the nonequilibrium Green function technique. The considerations are restricted to spin-conserving tunnelling processes through a quantum dot with a single spin-split discrete level. The Coulomb correlations on the dot are taken into account in terms of the Hubbard Hamiltonian. Transport characteristics, including tunnelling magnetoresistance due to rotation of the magnetic moments of external electrodes, are calculated selfconsistently.

The Kondo effect in quantum dots coupled to ferromagnetic leads with noncollinear magnetizations: effects due to electron–phonon coupling

We study the Kondo effect in a quantum dot coupled to ferromagnetic leads and analyze 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.Spin-polarized transport through a quantum dot strongly coupled to ferromagnetic electrodes with noncollinear magnetic moments is analysed theoretically in terms of the nonequilibrium Green function formalism based on the equation of motion method. Electrons in the dot are assumed to be coupled to a phonon bath. The influence of electron–phonon coupling on tunnelling current, linear and nonlinear conductance, and tunnel magnetoresistance is studied in detail. Variation of the main Kondo peaks and phonon satellites with the angle between the magnetic moments of the leads is analysed.

Spin-polarized transport through a single-level quantum dot in the Kondo regime

Nonequilibrium electronic transport through a quantum dot coupled to ferromagnetic leads (electrodes) is studied theoretically by the nonequilibrium Green function technique. The system is described by the Anderson model with arbitrary correlation parameter U. Exchange interaction between the dot and ferromagnetic electrodes is taken into account via an effective molecular field. The following situations are analysed numerically: (i) the dot is symmetrically coupled to two ferromagnetic leads, (ii) one of the two ferromagnetic leads is half-metallic with almost total spin polarization of electron states at the Fermi level, and (iii) one of the two electrodes is nonmagnetic whereas the other one is ferromagnetic. Generally, the Kondo peak in the density of states (DOS) becomes spin-split when the total exchange field acting on the dot is nonzero. The spin-splitting of the Kondo peak in DOS leads to splitting and suppression of the corresponding zero-bias anomaly in the differential conductance.

Thermopower, figure of merit and spin-transfer torque induced by the temperature gradient in planar tunnel junctions

The thermopower, charge and thermal conductance, and figure of merit as well as the spin-transfer torque generated by the temperature gradient in the planar tunnel junction consisting of ferromagnetic layers and the nonmagnetic tunnel barrier are investigated in the free-electron-like spin-polarized one-band model. In particular, the influence of the parameters of the junction as well as the influence of the relative orientation of magnetic moments on the studied phenomena are investigated. The thermopower can be related to the voltage drop generated by the temperature difference between electrodes under the condition that the charge current vanishes. It depends on the magnetic configuration of the junction. In junctions with high barriers the thermopower is maximal in the antiparallel configuration and it can be enhanced in junctions with strong spin-splitting of the electron bands. The component of the torque studied in the present paper is oriented in the plane formed by magnetic moments and it appears in the absence of the bias voltage. Its magnitude is insensitive to the sign of the temperature difference in contrast to the bias-induced in-plane torque which strongly depends on the polarization of the bias. The studied torque is usually smaller than the torque generated by the bias: however, it can be significant in junctions with low barriers.

Electron tunnelling in planar double junctions with ferromagnetic barriers

Electron tunnelling in double junctions with ferromagnetic barriers and nonmagnetic electrodes is analysed in the sequential and coherent limit of electron tunnelling. The free-electron-like one band model is used. The tunnelling current and its spin polarisation, as well as tunnel magnetoresistance (TMR) are determined. The spin accumulation in the central electrode, leading to a nonvanishing TMR effect, is taken into account and analysed in the sequential limit. In the coherent limit the influence of resonant states on the results obtained is analysed. The conditions leading to an enhancement of TMR and negative differential resistance are discussed. The influence of the parameters of the junction on the results obtained is also investigated.

Transport through a quantum dot subject to spin and charge bias

Abstract Spin and charge transport through a quantum dot coupled to external nonmagnetic leads is analyzed theoretically in terms of the non-equilibrium Green function formalism based on the equation of motion method. The dot is assumed to be subject to spin and charge bias, and the considerations are focused on the Kondo effect in spin and charge transport. It is shown that the differential spin conductance as a function of spin bias reveals a typical zero-bias Kondo anomaly which becomes split when either magnetic field or charge bias are applied. Significantly different behavior is found for mixed charge/spin conductance. The influence of electron–phonon coupling in the dot on tunneling current as well as on both spin and charge conductance is also analyzed.