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

Abstract

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 symmetry-breaking 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.

Spin Waves in Ferromagnetic Nanorings with Interfacial Dzyaloshinskii–Moriya Interactions: II. Directional Effects

“…For example, DMIs are known to produce chiral and topological features in nanostructures [4][5][6], opening up new perspectives for device applications. Also, regarding the spin dynamics in bulk-like materials with no interfaces taken into account, it has been pointed out and verified experimentally that contributions linear in the wave-vector components occur in the dispersion relations at small wave vectors [3,[7][8][9][10][11][12][13], in addition to the quadratic contributions associated with the Heisenberg exchange. Several other studies have been conducted that elaborate on the wave-vector dependence of DMIs and their role in unidirectional spin-wave propagation and/or for Brillouin light scattering (BLS) (see [8][9][10][11][12][14][15][16]).…”

The 2024 magnonics roadmap

“…Interest in magnonic metamaterials [4] is freshly renewed by the possibility of inducing flat bands [42] and controlling the non-reciprocal SW frequency dispersion and hybrid systems composed of magnonic crystals coupled to superconducting materials, which have been experimentally demonstrated by BLS and FMR, respectively [43]. Of course, there are challenges associated with the low temperature to achieve a functioning system but, at the same time, it may appear to be adequate for application in cryogenic temperatures.…”