Twisting and tweezing the spin wave: on vortices, skyrmions, helical waves, and the magnonic spiral phase plate

TL;DR

This paper proposes a novel magnetic spiral phase plate to generate and control twisted spin waves with orbital angular momentum in magnetic nanowires, demonstrated through detailed micromagnetic simulations.

Contribution

It introduces the first design for a magnetic spiral phase plate that manipulates spin wave phase and orbital angular momentum using material synthesis.

Findings

Homogeneous spin waves can be twisted to carry orbital angular momentum.

The proposed design effectively imparts a twist to spin waves despite complex demagnetization fields.

Simulations confirm the feasibility of generating twisted spin waves with controlled phase.

Abstract

Spin waves are the low-energy excitations of magnetically ordered materials. They are key elements in the stability analysis of the ordered phase and have a wealth of technological applications. Recently, we showed that spin waves of a magnetic nanowire may carry a definite amount of orbital angular momentum components along the propagation direction. This helical, in addition to the chiral, character of the spin waves is related to the spatial modulations of the spin wave phase across the wire. It, however, remains a challenge to generate and control such modes with conventional magnetic fields. Here, we make the first proposal for a \textit{magnetic} spiral phase plate by appropriately synthesizing two magnetic materials that have different speeds of spin waves. It is demonstrated with full-numerical micromagnetic simulations that despite the complicated structure of demagnetization fields, a homogeneous spin wave passing through the spiral phase plate attains the required twist and propagates further with the desired orbital angular momentum. While excitations from the ordered phase may have a twist, the magnetization itself can be twisted due to internal fields and forms what is known as a magnetic vortex. We point out the differences between both types of magnetic phenomena and discuss their possible interaction.