Spin-transfer torque in magnetic tunnel junctions: Scattering theory

TL;DR

This paper analyzes how bias voltage and barrier width influence spin-transfer torque in magnetic tunnel junctions using scattering theory, revealing conditions for torque reversal and quantum size effects.

Contribution

It introduces a scattering theory approach to model bias-dependent spin-transfer torque, predicting torque reversal and quantum size effects in magnetic tunnel junctions.

Findings

In-plane torque vanishes and reverses at certain bias and barrier widths.

The phase diagram predicts conditions for torque reversal based on bias and barrier parameters.

Quantum size effects influence spin-transfer torque as a function of normal metal layer thickness.

Abstract

We study the bias-dependent spin-transfer torque in magnetic tunnel junctions in the Stoner model by scattering theory. We show that the in-plane (Slonczewski type) torque vanishes and subsequently reverses its direction when the bias voltage becomes larger or the barrier wider than material and device-dependent critical values. We are able to reproduce the magnitude and the bias dependence of measured in-plane and out-of-plane torques using realistic parameters. The condition for the vanishing torque is summarized by a phase diagram depending on the applied bias and barrier width, which is explained in terms of an interface spin polarization and the electron focusing by the barrier. Quantum size effects in the spin-transfer torque are predicted as a function of the thickness of a normal metal layer inserted between the ferromagnet and tunnel barrier.