The challenges of measuring spin Seebeck noise

TL;DR

This paper investigates the feasibility of measuring magnon shot noise via the spin Seebeck effect and inverse spin Hall effect, highlighting experimental challenges and theoretical ambiguities that hinder detection.

Contribution

It demonstrates the technical difficulties in detecting magnon shot noise using existing spin Seebeck and inverse spin Hall setups, and discusses theoretical uncertainties and geometric limitations.

Findings

Small voltage noise increase at low temperatures with magnetic field

Dependence on field orientation suggests noise not due to magnon shot noise

Geometric factors likely prevent detection of magnon shot noise

Abstract

Just as electronic shot noise in driven conductors results from the granularity of charge and the statistical variation in the arrival times of charge carriers, there are predictions for fundamental noise in magnon currents due to angular momentum being carried by discrete excitations. The inverse spin Hall effect as a transduction mechanism to convert spin current into charge current raises the prospect of experimental investigations of such magnon shot noise. Spin Seebeck effect measurements have demonstrated the electrical detection of thermally driven magnon currents and have been suggested as an avenue for accessing spin current fluctuations. Using spin Seebeck structures made from yttrium iron garnet on gadolinium gallium garnet, we demonstrate the technical challenges inherent in such noise measurements. While there is a small increase in voltage noise in the inverse spin Hall detector at low temperatures associated with adding a magnetic field, the dependence on field orientation implies that this is not due to magnon shot noise. We describe theoretical predictions for the expected magnitude of magnon shot noise, highlighting ambiguities that exist. Further, we show that magnon shot noise detection through the standard inverse spin Hall approach is likely impossible due to geometric factors. Implications for future attempts to measure magnon shot noise are discussed.