Unidirectional Chiral Magnonics in Cylindrical Synthetic Antiferromagnets

Asymmetric spin-wave propagation in magnetic nanostructures has received significant attention due to the potential applications of magnon-based devices. In curved nanostructures and planar multilayers, the classical dipole-dipole interaction induces a significant frequency nonreciprocity in which two coun-terpropagating waves excited at the same frequency exhibit different wavelengths. This work proposes a cylindrical synthetic antiferromagnet as a potential three-dimensional waveguide design to generate nonreciprocal spin waves. The magnetochiral properties emerge from two mechanisms: the asymmetric interlayer dipolar coupling and the asymmetric dipolar coupling of the curved membrane. It is demon-strated that the cylindrical bilayer presents a notable spin-wave asymmetry induced by the combined action of antiparallel magnetic vortices and the curvature of the inner and outer surfaces. A substantial frequency range with waves having only a negative phase velocity is predicted, where unidirectional wave propagation is allowed. It is also found that the nonreciprocity reaches a constant value as the curvature decreases, which is an essential advantage over isolated nanotubes, where the frequency shift vanishes at a large radius. Besides, analytical expressions are proposed to predict the frequency shift in the case of coupled cylindrical shells. These results are relevant from fundamental and practical points of view since magnetochirality is a crucial ingredient in visualizing future spin-wave-based logic devices.