Spin-flip noise due to nonequilibrium spin accumulation

TL;DR

This paper demonstrates that nonequilibrium spin accumulation in magnetic tunnel junctions generates a distinct low frequency noise, which can be modeled similarly to shot noise but driven by spin-polarized current rather than voltage.

Contribution

It introduces a novel understanding of low frequency noise as a nonequilibrium effect caused by spin accumulation, with a new model relating noise power to spin-polarized current.

Findings

LFN is a nonequilibrium effect due to spin accumulation.

Noise power fits a model similar to shot noise with spin-polarized current.

The model links spin-flip probability to measurable noise characteristics.

Abstract

When current flows through a magnetic tunnel junction (MTJ), there is spin accumulation at the electrode-barrier interfaces if the magnetic moments of the two ferromagnetic electrodes are not aligned. Here we report that such nonequilibrium spin accumulation generates its own characteristic low frequency noise (LFN). Past work viewed the LFN in MTJs as an equilibrium effect arising from resistance fluctuations ($S_R$) which a passively applied current ($I$) converts to measurable voltage fluctuations ($S_{V}=I^{2}S_{R}$). We treat the LFN associated with spin accumulation as a nonequilibrium effect, and find that the noise power can be fitted in terms of the spin-polarized current by $S_{I}f=aI\coth(\frac{I}{b})-ab$, resembling the form of the shot noise for a tunnel junction, but with current now taking the role of the bias voltage, and spin-flip probability taking the role of tunneling probability.