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

Spin waves represent the collective excitations of the magnetization field
within a magnetic material, providing dispersion curves that can be manipulated by
material design and external stimuli. Bulk and surface spin waves can be excited in a
thin film with positive or negative group velocities and, by incorporating a symmetrybreaking mechanism, magnetochiral features arise. Here we study the band diagram of
a chiral magnonic crystal consisting of a ferromagnetic film incorporating a periodic
Dzyaloshinskii−Moriya coupling via interfacial contact with an array of heavy-metal
nanowires. We provide experimental evidence for a strong asymmetry of the spin wave
amplitude induced by the modulated interfacial Dzyaloshinskii−Moriya interaction, which generates a nonreciprocal propagation.
Moreover, we observe the formation of flat spin-wave bands at low frequencies in the band diagram. Calculations reveal that
depending on the perpendicular anisotropy, the spin-wave localization associated with the flat modes occurs in the zones with or
without Dzyaloshinskii−Moriya interaction.