Electrical detectability of magnon-mediated spin current shot noise

TL;DR

This paper analyzes the magnitude of spin current shot noise in magnonic spin pumping experiments, finding it is much smaller than Johnson-Nyquist noise and discussing implications for detecting squeezed magnons.

Contribution

It provides a detailed analysis of spin current shot noise in charge detection, highlighting limitations for experimental detection of squeezed magnons.

Findings

Voltage noise from spin shot noise is much smaller than Johnson-Nyquist noise.

The ratio of spin shot noise to Johnson-Nyquist noise cannot be enhanced by sample geometry.

The ratio depends on material-specific transport properties.

Abstract

A magnonic spin current crossing a ferromagnet-metal interface is accompanied by spin current shot noise arising from the discrete quanta of spin carried by magnons. In thin films, e.g., the spin of so-called squeezed magnons have been shown to deviate from the common value $\hbar$, with corresponding changes in the spin noise. In experiments, spin currents are typically converted to charge currents via the inverse spin Hall effect. We here analyze the magnitude of the spin current shot noise in the charge channel for a typical electrically detected spin pumping experiment, and find that the voltage noise originating from the spin current shot noise is much smaller than the inevitable Johnson-Nyquist noise. Furthermore, we find that due to the local nature of the spin-charge conversion, the ratio of spin current shot noise and Johnson-Nyquist noise cannot be systematically enhanced by tuning the sample geometry, in contrast to the linear increase in dc spin pumping voltage with sample length. Instead, the ratio depends sensitively on material-specific transport properties. Our analysis thus provides guidance for the experimental detection of squeezed magnons through spin pumping shot noise.