Influence Of Current Leads On Critical Current For Spin Precession In Magnetic Multilayers

TL;DR

This paper investigates how non-uniform current leads in magnetic multilayers reduce spin accumulation and alter the critical current for inducing spin precession, aligning theoretical predictions with experimental observations.

Contribution

It develops a spin-diffusion model for variable cross section leads and shows how these leads significantly decrease spin accumulation in the pillar.

Findings

Spin accumulation is reduced by at least a factor of 2 in non-uniform leads.

Critical current density becomes nearly independent of the magnetic layer thickness.

Results align better with experimental measurements by Albert et al. (2002).

Abstract

In magnetic multilayers, a dc current induces a spin precession above a certain critical current. Drive torques responsible for this can be calculated from the spin accumulation $\bar{\Delta\mu}$. Existing calculations of $\bar{\Delta\mu}$ assume a uniform cross section of conductors. But most multilayer samples are pillars with current leads flaring out immediately to a much wider cross-section area than that of the pillar itself. We write spin-diffusion equations of a form valid for variable cross section, and solve the case of flat electrodes with radial current distribution perpendicular to the axis of the pillar. Because of the increased volume available for conduction-electron spin relaxation in such leads, $\bar{\Delta\mu}$ is reduced in the pillar by at least a factor of 2 below its value for uniform cross section, for given current density in the pillar. Also, $\bar{\Delta\mu}$ and the critical current density for spin precession become nearly independent of the thickness of the pinned magnetic layer, and more dependent on the thickness of the spacer, in better agreement with measurements by Albert et al. (2002).