Spin Current and Current-Induced Spin Transfer Torque in Ferromagnet-Quantum Dot-Ferromagnet Coupled Systems

TL;DR

This paper investigates spin-dependent transport in ferromagnet-quantum dot-ferromagnet systems using nonequilibrium Green functions, revealing unique spin current behaviors and enhanced spin transfer torque at resonant conditions.

Contribution

It introduces a detailed analysis of spin currents and spin transfer torque in ferromagnet-QD-ferromagnet systems, highlighting their dependence on magnetization orientation and bias voltage.

Findings

Spin current differs significantly from electrical current.

Spin current can change magnitude and sign with magnetization orientation.

Resonant bias voltages greatly enhance spin transfer torque.

Abstract

Based on Keldysh's nonequilibrium Green function method, the spin-dependent transport properties in a ferromagnet-quantum dot (QD)-ferromagnet coupled system are investigated. It is shown the spin current shows quite different characteristics from its electrical counterpart, and by changing the relative orientation of both magnetizations, it can change its magnitude even sign. The current-induced spin transfer torque (CISTT) is uncovered to be greatly enhanced when the bias voltage meets with the discrete levels of the QD at resonant positions. The relationship between the CISTT, the electrical current and the spin current is also addressed.