Confined Magnons

TL;DR

This paper develops a theoretical framework to analyze confined magnon modes at magnetic surfaces and interfaces, revealing their spectral properties and how anisotropy influences magnon confinement, with implications for experimental detection.

Contribution

The authors present a new theory for calculating confined magnon spectra and scattering functions, including analytical expressions for confinement length scales in various dimensions.

Findings

Confined magnon resonances are predicted beyond usual magnons.

Surface anisotropy critically influences magnon confinement.

Differences between ferromagnetic and antiferromagnetic models are significant in 1D.

Abstract

Magnetic structures are known to possess magnon excitations confined to their surfaces and interfaces, but these spatially localized modes are often not resolved in spectroscopy experiments. We develop a theory to calculate the confined magnon spectra and its associated spin scattering function, which is the physical observable in neutron and electron scattering, and a proxy for photon spectroscopy based on X-ray, Raman and THz sources. We show that extra anisotropy at the surface or interface plays a key role in magnon confinement. We obtain analytical expressions for the confinement length scale, and show that it is qualitatively similar for ferromagnets and antiferromagnets in dimension d>=2. For d=1 we find remarkable differences between ferromagnetic and antiferromagnetic models. The theory indicates the presence of several confined magnon resonances in addition to the usual magnons thought to explain the excitations of magnetic nanostructures. Detecting these modes may elucidate the impact of the interface on spin anisotropy and magnetic order.