Spin-orbit torque in a three-fold-symmetric bilayer and its effect on magnetization dynamics

TL;DR

This paper investigates the spin-orbit torque in a three-fold-symmetric bilayer, calculating its angular dependence and studying its effect on magnetization dynamics, revealing conditions for efficient field-free switching in magnetic films.

Contribution

It provides a first-principles calculation of the unique 3m spin-orbit torque and analyzes its impact on magnetization switching considering Dzyaloshinskii-Moriya interaction.

Findings

3m SOT is about 20% of conventional dampinglike SOT.

Large current densities are needed for switching with strong DMI and Néel domain walls.

Thinner films with weaker DMI allow switching at lower currents, matching experiments.

Abstract

Field-free switching of perpendicular magnetization has been observed in an epitaxial L1$_1$-ordered CoPt/CuPt bilayer and attributed to spin-orbit torque (SOT) arising from the crystallographic $3m$ point group of the interface. Using a first-principles nonequilibrium Green's function formalism combined with the Anderson disorder model, we calculate the angular dependence of the SOT in a CoPt/CuPt bilayer and find that the magnitude of the $3m$ SOT is about 20% of the conventional dampinglike SOT. We further study the magnetization dynamics in perpendicularly magnetized films in the presence of $3m$ SOT and Dzyaloshinskii-Moriya interaction, using the equations of motion for domain wall dynamics and micromagnetic simulations. We find that for systems with strong interfacial DMI characterized by the N\'eel character of domain walls, a very large current density is required to achieve deterministic switching because reorientation of the magnetization inside the domain wall is necessary to induce the switching asymmetry. For thicker films with relatively weak interfacial DMI and the Bloch character of domain walls the deterministic switching with much smaller currents is possible, which agrees with recent experimental findings.