Voltage Dependence of Spin Transfer Torque in Magnetic Tunnel Junctions

TL;DR

This paper theoretically examines how spin transfer torque in magnetic tunnel junctions varies with applied voltage, influenced by material parameters and electronic states, providing insights into the underlying mechanisms.

Contribution

It introduces a detailed theoretical model analyzing voltage-dependent spin transfer torque considering band parameters and electronic states in magnetic tunnel junctions.

Findings

Spin transfer torque behavior varies significantly with voltage and material parameters.

Both parallel and perpendicular torque components depend on spin and charge currents.

Exchange coupling at zero bias contributes to the perpendicular torque component.

Abstract

Theoretical investigations of spin transfer torque in magnetic tunnel junctions using the tight-binding model in the framework of non-equilibrium Green functions formalism are presented. We show that the behavior of the spin transfer torque as a function of applied voltage can vary over a wide range depending on the band parameters of the ferromagnetic electrodes and the insulator that comprise the magnetic tunnel junction. The behavior of both the parallel and perpendicular components of the spin torque is addressed. This behavior is explained in terms of the spin and charge current dependence and on the interplay between evanescent states in the insulator and the Fermi surfaces of ferromagnetic electrodes comprising the junction. The origin of the perpendicular (field-like) component of spin transfer torque at zero bias, i.e. exchange coupling through the barrier between ferromagnetic electrodes is discussed.