Flat Bands, Indirect Gaps, and Unconventional Spin-Wave Behavior Induced by a Periodic Dzyaloshinskii-Moriya Interaction

TL;DR

This paper explores how periodic Dzyaloshinskii-Moriya interactions in magnetic metamaterials can create unique spin-wave band structures, including flat bands and indirect gaps, enabling advanced spin-wave device engineering.

Contribution

It introduces a theoretical and simulation-based approach to engineer magnonic crystals with periodic Dzyaloshinskii-Moriya interactions, revealing novel band phenomena.

Findings

Emergence of flat bands and indirect gaps in spin-wave spectra.

Unusual evolution of standing spin waves around band gaps.

Effects depend on the Dzyaloshinskii-Moriya interaction strength.

Abstract

Periodically patterned metamaterials are known for exhibiting wave properties similar to the ones observed in electronic band structures in crystal lattices. In particular, periodic ferromagnetic materials are characterized by the presence of bands and bandgaps in their spin-wave spectrum at tunable GHz frequencies. Recently, the fabrication of magnets hosting Dzyaloshinskii-Moriya interactions has been pursued with high interest since properties such as the stabilization of chiral spin textures and nonreciprocal spin-wave propagation emerge from this antisymmetric exchange coupling. In this context, to further engineer the magnon band structure, we propose the implementation of magnonic crystals with periodic Dzyaloshinskii-Moriya interactions, which can be obtained, for instance, via patterning of periodic arrays of heavy-metals wires on top of an ultrathin magnetic film. We demonstrate through theoretical calculations and micromagnetic simulations that such systems show an unusual evolution of the standing spin waves around the gaps in areas of the film that are in contact with the heavy-metal wires. We also predict the emergence of indirect gaps and flat bands and, effects that depend on the strength of the Dzyaloshinskii-Moriya interaction. This study opens new routes towards engineered metamaterials for spin-wave-based devices.