Spontaneous magnon decays in planar ferromagnet

TL;DR

This paper predicts that magnons in an easy-plane ferromagnet have finite lifetimes at zero temperature due to spontaneous decay processes, with decay mechanisms depending on magnetic field conditions, supported by theoretical calculations.

Contribution

It introduces the prediction of spontaneous magnon decays at zero temperature in planar ferromagnets and provides explicit theoretical calculations within the spin-wave framework.

Findings

Magnons have finite lifetime at zero temperature due to spontaneous decay.

Decay mechanisms depend on magnetic field: three-magnon in zero field, two-particle in transverse field.

Decay occurs for anisotropy parameter $\lambda$ between approximately 1/7 and 1.

Abstract

We predict that spin-waves in an easy-plane ferromagnet have a finite lifetime at zero temperature due to spontaneous decays. In zero field the damping is determined by three-magnon decay processes, whereas decays in the two-particle channel dominate in a transverse magnetic field. Explicit calculations of the magnon damping are performed in the framework of the spin-wave theory for the $XXZ$ square-lattice ferromagnet with an anisotropy parameter $\lambda<1$. In zero magnetic field the decays occur for $\lambda^*<\lambda<1$ with $\lambda^*\approx 1/7$. We also discuss possibility of experimental observation of the predicted effect in a number of ferromagnetic insulators.