Bulk-mediated reflection of chirality-protected surface spin waves

TL;DR

This study investigates how chirality affects surface spin wave reflection in thick magnetic films, revealing bulk mode excitations as the key factor enabling wave reversal and defining limits of backscattering immunity.

Contribution

It demonstrates that bulk-mode excitations enable the reversal of chiral surface waves in thick films, extending understanding of nonreciprocal wave transport in magnetic media.

Findings

Reflection of chiral surface waves excites localized bulk modes.

Reciprocal backward-volume waves reflect nearly elastically.

Bulk excitations lead to energy accumulation and dissipation.

Abstract

Surface spin waves of the Damon-Eshbach type exhibit intrinsically nonreciprocal transport properties with a chiral dynamical field structure that localizes counterpropagating waves at opposite film surfaces. Such chirality has been predicted to suppress direct backscattering in thin films within frequency ranges free of bulk modes. However, how chirality influences reflection in thicker three-dimensional magnetic media, where a dense spectrum of bulk excitations overlaps with surface waves, remains unclear. Here we demonstrate that, in micrometer-thick yttrium iron garnet films, reflection of the chiral Damon-Eshbach wave from the boundary of the magnetic medium is accompanied by excitation of spatially localized thickness-quantized bulk modes, whereas reciprocal backward-volume waves reflect nearly elastically. Brillouin light scattering spectroscopy, infrared thermography, and micromagnetic simulations reveal standing bulk excitations at the reflecting boundary and quantify the associated magnon energy accumulation and dissipation. These results identify bulk-mode excitations as the physical pathway enabling reversal of chirally localized surface waves in thick films, thereby defining the limits of chirality-based backscattering immunity and providing a general framework for wave transport in nonreciprocal magnetic media.