Spin-transfer in bilayer magnetic nanopillars at high fields as a function of free layer thickness

TL;DR

This study investigates how free-layer thickness affects spin transfer and magnetic excitations in bilayer nanopillars under high magnetic fields, revealing a finite critical current at zero thickness and insights into damping mechanisms.

Contribution

It provides the first detailed phase diagram of current-induced magnetic excitations at high fields as a function of free-layer thickness in bilayer nanopillars.

Findings

Critical current decreases linearly with free-layer thickness.

Finite critical current persists at zero thickness.

Spin-pumping contributes to magnetization damping.

Abstract

Spin transfer in asymmetric Co/Cu/Co bilayer magnetic nanopillars junctions has been studied at low temperature as a function of free-layer thickness. The phase diagram for current-induced magnetic excitations has been determined for magnetic fields up to 7.5 T applied perpendicular to the junction surface and free-layers thicknesses from 2 to 5 nm. The junction magnetoresistance is independent of thickness. The critical current for magnetic excitations decreases linearly with decreasing free-layer thickness, but extrapolates to a finite critical current in the limit of zero thickness. The limiting current is in quantitative agreement with that expected due to a spin-pumping contribution to the magnetization damping. It may also be indicative of a decrease in the spin-transfer torque efficiency in ultrathin magnetic layers.