Measurement of Gilbert damping parameters in nanoscale CPP-GMR spin-valves

TL;DR

This study measures the intrinsic Gilbert damping in nanoscale CPP-GMR spin-valves using in-situ thermal noise spectral analysis, revealing significant damping differences between reference and free layers and validating the method against theoretical models.

Contribution

It introduces a novel in-situ measurement technique at the device level that accurately determines Gilbert damping parameters in nanoscale spin-valves, accounting for spin-torque effects.

Findings

Reference layer damping is ten times larger than free layer damping.

Magic-angle alignment enables direct measurement of intrinsic thermal spectra.

Experimental results agree with macrospin and micromagnetic models.

Abstract

In-situ, device level measurement of thermal mag-noise spectral linewidths in 60nm diameter CPP-GMR spin-valve stacks of IrMn/ref/Cu/free, with reference and free layer of similar CoFe/CoFeGe alloy, are used to simultaneously determine the intrinsic Gilbert damping for both magnetic layers. It is shown that careful alignment at a "magic-angle" between free and reference layer static equilibrium magnetization can allow direct measurement of the broadband intrinsic thermal spectra in the virtual absence of spin-torque effects which otherwise grossly distort the spectral line shapes and require linewidth extrapolations to zero current (which are nonetheless also shown to agree well with the direct method). The experimental magic-angle spectra are shown to be in good qualitative and quantitative agreement with both macrospin calculations and micromagnetic eigenmode analysis. Despite similar composition and thickness, it is repeatedly found that the IrMn exchange pinned reference layer has ten times larger intrinsic Gilbert damping (alpha ~ 0.1) than that of the free-layer (alpha ~ 0.01). It is argued that the large reference layer damping results from strong, off -resonant coupling to to lossy modes of an IrMn/ref couple, rather than commonly invoked two-magnon processes.