Curvilinear spin-wave dynamics beyond the thin-shell approximation: Magnetic nanotubes as a case study

TL;DR

This paper investigates how increasing thickness in magnetic nanotubes affects spin-wave spectra, revealing curvature-induced non-reciprocity, hybridization, and unidirectional propagation, advancing understanding of curvilinear magnetic systems.

Contribution

It extends the analysis of spin-wave dynamics from thin shells to thick curvilinear shells, uncovering new effects like higher-order mode hybridization and non-reciprocal mode profiles.

Findings

Higher-order radial modes appear with increased thickness.

Curvature induces non-reciprocity in mode profiles.

Nonlinear three-wave splitting enables unidirectional spin-wave propagation.

Abstract

Surface curvature of magnetic systems can lead to many static and dynamic effects which are not present in flat systems of the same material. These emergent magnetochiral effects can lead to frequency nonreciprocity of spin waves, which has been shown to be a bulk effect of dipolar origin and is related to a curvature-induced symmetry breaking in the magnetic volume charges. So far, such effects have been investigated theoretically mostly for thin shells, where the spatial profiles of the spin waves can be assumed to be homogeneous along the thickness. Here, using a finite-element dynamic-matrix approach, we investigate the transition of the spin-wave spectrum from thin to thick curvilinear shells, at the example of magnetic nanotubes in the vortex state. With increasing thickness, we observe the appearance of higher-order radial modes which are strongly hybridized and resemble the perpendicular-standing-waves (PSSWs) in flat films. Along with an increasing dispersion asymmetry, we uncover the curvature-induced non-reciprocity of the mode profiles. This is explained in a very simple picture general for thick curvilinear shells, considering the inhomogeneity of the emergent geometric volume charges along the thickness of the shell. Such curvature-induced mode-profile asymmetry also leads to non-reciprocal hybridization which can facilitate unidirectional spin-wave propagation. With that, we also show how curvature allows for nonlinear three-wave splitting of a higher-order radial mode into secondary modes which can also propagate unidirectionally. We believe that our study provides a significant contribution to the understanding of the spin-wave dynamics in curvilinear magnetic systems, but also advertises these for novel magnonic applications.