Super-Poissonian shot noise of squeezed-magnon mediated spin transport

TL;DR

This paper investigates the shot noise in spin currents across ferromagnet-normal metal interfaces under microwave drive, revealing super-Poissonian noise due to squeezed magnon quasi-particles.

Contribution

It introduces the concept of super-Poissonian shot noise in spin transport caused by magnon squeezing, a novel insight into spin current fluctuations.

Findings

Shot noise is frequency independent up to the drive frequency.

Noise increases linearly with frequency beyond the drive frequency.

Super-Poissonian noise arises from effective spin $ abla ext{hbar}^* = abla ext{hbar} (1 + abla ext{delta})$ due to magnon squeezing.

Abstract

The magnetization of a ferromagnet (F) driven out of equilibrium injects pure spin current into an adjacent conductor (N). Such F$|$N bilayers have become basic building blocks in a wide variety of spin based devices. We evaluate the shot noise of the spin current traversing the F$|$N interface when F is subjected to a coherent microwave drive. We find that the noise spectrum is frequency independent up to the drive frequency, and increases linearly with frequency thereafter. The low frequency noise indicates super-Poissonian spin transfer, which results from quasi-particles with effective spin $\hbar^* = \hbar (1 + \delta)$. For typical ferromagnetic thin films, $\delta \sim 1$ is related to the dipolar interaction-mediated squeezing of F eigenmodes.