# Magnonic Band Structure Established by Chiral Spin-Density Waves in Thin Film Ferromagnets

## TL;DR

This paper demonstrates how chiral spin-density waves in thin ferromagnetic films create a magnonic band structure, which can be tuned via film thickness and SDW velocity, enabling reconfigurable magnonic crystals.

## Contribution

It introduces a hydrodynamic approach to analyze SDWs in thin films, revealing their role in forming tunable magnonic band gaps.

## Key findings

- Nonlocal dipole fields modulate SDW fluid velocity.
- Magnonic band gaps of GHz order depend on SDW velocity and film thickness.
- Micromagnetic simulations validate analytical predictions.

## Abstract

Recent theoretical studies have demonstrated the possibility to excite and sustain noncollinear magnetization states in ferromagnetic nanowires. The resulting state is referred to as a spin-density wave (SDW). SDWs can be interpreted as hydrodynamic states with a constant fluid density and fluid velocity in systems with easy-plane anisotropy. Here, we consider the effect of the nonlocal dipole field arising from the finite thickness of magnetic thin films on the spatial profile of the SDW and on the associated magnon dispersion. Utilizing a hydrodynamic formulation of the Larmor torque equation, it is found that the nonlocal dipole field modulates the fluid velocity. Such a modulation induces a magnonic band structure unlike the typical dispersion relation for magnons on uniform magnetization. The analytical results are validated by micromagnetic simulations. Band gaps on the order of GHz are numerically observed to depend on the SDW fluid velocity and film thickness for realistic material parameters. The presented results suggest that SDWs can find applications as reconfigurable magnonic crystals.