A generalized model of magnon kinetics and subgap magnetic noise

TL;DR

This paper develops a comprehensive theory of magnon transport in two-dimensional magnetic insulators, bridging diffusive and ballistic regimes, and predicts magnetic noise signals detectable by NV centers across various length scales.

Contribution

It introduces a generalized model of magnon kinetics that unifies diffusive and ballistic transport regimes and predicts NV center magnetic noise signals in these regimes.

Findings

The model aligns with diffusive and ballistic limits for large and small length scales.

Provides a modified Lindhard form for the dynamic spin susceptibility.

Predicts NV transition rates across different magnon transport regimes.

Abstract

Magnetic noise spectroscopy provides a noninvasive probe of spin dynamics in magnetic materials. We consider two-dimensional magnetically ordered insulators with magnon excitations, especially those supporting long-distance magnon transport, where nitrogen-vacancy (NV) centers enable the access to (nearly) ballistic transport regime of magnons. We develop a generalized theory to describe the magnon transport across a wide range of length scales. The longitudinal dynamic spin susceptibility is derived from the Boltzmann equation and extended to a Lindhard form, which is modified by both the spin-conserving magnon collisions and spin relaxation. Our result is consistent with the diffusive (ballistic) model for the length scale much larger (smaller) than the magnon mean free path, and provides a description for the intermediate regime. We also give a prediction for the NV transition rate in different magnon transport regimes.