Long spin coherence length and bulk-like spin-orbit torque in ferrimagnetic multilayers

TL;DR

This paper demonstrates that ferrimagnetic multilayers exhibit long spin coherence lengths and bulk-like spin-orbit torques, overcoming limitations of ferromagnets and enabling more energy-efficient spintronic devices.

Contribution

It reveals the existence of long spin coherence lengths and bulk-like torque behavior in ferrimagnetic multilayers, a phenomenon previously unexplored in such systems.

Findings

Transverse spin current passes through >10 nm-thick ferrimagnetic multilayers.

Switching efficiency increases with thickness up to 8 nm, then decreases.

Contrasts with 1/thickness dependence in ferromagnetic multilayers.

Abstract

Ferromagnetic spintronics has been a main focus as it offers non-volatile memory and logic applications through current-induced spin-transfer torques. Enabling wider applications of such magnetic devices requires a lower switching current for a smaller cell while keeping the thermal stability of magnetic cells for non-volatility. As the cell size reduces, however, it becomes extremely difficult to meet this requirement with ferromagnets because spin-transfer torque for ferromagnets is a surface torque due to rapid spin dephasing, leading to the 1/ferromagnet-thickness dependence of the spin-torque efficiency. Requirement of a larger switching current for a thicker and thus more thermally stable ferromagnetic cell is the fundamental obstacle for high-density non-volatile applications with ferromagnets. Theories predicted that antiferromagnets have a long spin coherence length due to the staggered spin order on an atomic scale, thereby resolving the above fundamental limitation. Despite several spin-torque experiments on antiferromagnets and ferrimagnetic alloys, this prediction has remained unexplored. Here we report a long spin coherence length and associated bulk-like-torque characteristic in an antiferromagnetically coupled ferrimagnetic multilayer. We find that a transverse spin current can pass through > 10 nm-thick ferrimagnetic Co/Tb multilayers whereas it is entirely absorbed by 1 nm-thick ferromagnetic Co/Ni multilayer. We also find that the switching efficiency of Co/Tb multilayers partially reflects a bulk-like-torque characteristic as it increases with the ferrimagnet-thickness up to 8 nm and then decreases, in clear contrast to 1/thickness-dependence of Co/Ni multilayers. Our results on antiferromagnetically coupled systems will invigorate researches towards energy-efficient spintronic technologies.