Experimental Observation of Flat Bands in One-Dimensional Chiral Magnonic Crystals

Spin waves represent the collective excitations of themagnetizationfield within a magnetic material, providing dispersion curves thatcan be manipulated by material design and external stimuli. Bulk andsurface spin waves can be excited in a thin film with positive ornegative group velocities and, by incorporating a symmetry-breakingmechanism, magnetochiral features arise. Here we study the band diagramof a chiral magnonic crystal consisting of a ferromagnetic film incorporatinga periodic Dzyaloshinskii-Moriya coupling via interfacial contactwith an array of heavy-metal nanowires. We provide experimental evidencefor a strong asymmetry of the spin wave amplitude induced by the modulatedinterfacial Dzyaloshinskii-Moriya interaction, which generatesa nonreciprocal propagation. Moreover, we observe the formation offlat spin-wave bands at low frequencies in the band diagram. Calculationsreveal that depending on the perpendicular anisotropy, the spin-wavelocalization associated with the flat modes occurs in the zones withor without Dzyaloshinskii-Moriya interaction.