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Electronic transport in a ferromagnetic single-electron transistor with non-collinear magnetizations in the co-tunnelling regime

Wiśniewska J., Barnaś J.

Materials Science Poland

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2007

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Vol. 25, No. 2

467--472

EN

Spin-dependent electronic transport in a ferromagnetic single-electron transistor (FM SET) is studied theoretically in the Coulomb blockade regime [1]. Two external electrodes and the central part (island) of the device are assumed to be ferromagnetic, with the corresponding magnetizations being non-collinear in a general case. First order (sequential) transport is suppressed in the Coulomb blockade regime, so the second order (co-tunnelling) processes give the dominant contribution to the current. The co-tunnelling processes take place via four intermediate (virtual) states of the island: two of them are with one extra electron on the central electrode of the device (in the spin-majority or spin-minority subbands), whereas the other two virtual states are with a hole (in the spin-majority or spin-minority subbands) in the central electrode. The co-tunnelling processes create electron-hole excitations of the central electrode, and in a general case they also can create spin excitations. However, we assume relatively fast spin relaxation in the island, hence the spin accumulation is neglected. Basic transport characteristics, like tunnelling current and tunnel magnetoresistance are calculated for an arbitrary magnetic configuration of the system.

2

Transport characteristics of ferromagnetic single-electron transistors with non-collinear magnetizations

Wiśniewska J., Kowalik M., Barnaś J.

Materials Science Poland

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2006

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Vol. 24, No. 3

761--767

EN

Theoretical analysis of spin-polarized transport through a ferromagnetic single-electron transistor (FM SET) has been carried out in the sequential tunneling regime. Two external electrodes and the central part (island) of the device are assumed to be ferromagnetic, with the corresponding magnetizations being generally non-collinear. Transport properties of the FM SET are analyzed within the master equation approach, with the respective transition rates determined from the Fermi golden rule. It is assumed that spin relaxation processes on the island are sufficiently fast to neglect spin accumulation. It is shown that electric current and tunnel magnetoresistance (TMR) strongly depend on magnetic configuration of the device. Transport characteristics of symmetrical and asymmetrical structures have been calculated as a function of bias and the gate voltages.

3

Torque due to spin-polarized current in ferromagnetic single-electron transistors

Kowalik M., Wiśniewska J., Barnaś J.

Materials Science Poland

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2006

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Vol. 24, No. 3

815--820

EN

Theoretical analysis of the current-induced torque acting on magnetic moment of the central part (island) of a ferromagnetic single-electron transistor has been carried out in the regime of sequential tunneling. The island is assumed to be ferromagnetic and attached to two leads (electrodes). One of the leads is ferromagnetic, and the corresponding magnetic moment is oriented arbitrarily. The torque is calculated from the spin current absorbed by the magnetic moment of the island, and the calculations are carried out in the limit of fast spin relaxation on the island (no spin accumulation).

4

Spin and orbital Kondo effect in electrostatically coupled quantum dots

Lipiński S.

Materials Science Poland

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2006

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Vol. 24, No. 3

689--693

EN

The spin polarization of conductance of the system of two capacitively coupled quantum dots is studied in the Kondo regime by the equation of motion method. For the case of orbital degeneracy in the one spin channel the system can operate as a spin filter.

5

Spin-dependent transport in ferromagnetic single-electron transistors with non-collinear magnetizations

Wiśniewska J., Weymann I., Barnaś J.

Materials Science Poland

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2004

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Vol. 22, No. 4

461--467

EN

Electronic transport in a ferromagnetic single-electron transistor is analysed theoretically in the sequential tunnelling regime. One of the external electrodes and the central part (island) of the device are assumed to be ferromagnetic, with the corresponding magnetizations being non-collinear. The analysis is based on the master equation method, and the respective transition rates are determined from the Fermi golden rule. It is shown that the electric current and corresponding tunnel magnetoresistance (TMR) strongly depend on the angle between the magnetizations. For an arbitrary magnetic configuration, TMR is modulated by charging effects, which give rise to characteristic dips (cusps) at the bias voltages corresponding to the Coulomb steps in the current-voltage characteristics.

6

Effective exchange interaction in tunnelling junctions based on a quantum dot with non-collinear magnetic moments of the leads

Rudziński W.

Materials Science Poland

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2004

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Vol. 22, No. 4

523--528

EN

Electron tunnelling through a spin-split discrete level of an interacting quantum dot coupled to two ferromagnetic electrodes (leads) is investigated theoretically in the sequential-tunnelling regime. Spinsplitting of the dot level is induced by an effective exchange interaction between the spin on the dot and spins in the leads. The calculations apply to arbitrary angles enclosed between the magnetizations of the external electrodes. It is shown that the interplay between effective exchange field and Coulomb correlations on the dot may enhance the tunnel magnetoresistance at certain bias voltages. It is also found that a large spin splitting appearing for strong Coulomb correlations gives rise to an enhanced diode-like effect. Finally, it is shown that by rotating the magnetization of one of the electrodes, one can modulate the amplitude of the spin-polarized current, from a blockade in the parallel or antiparallel configuration to its maximum value in the non-collinear case.