Magnon magic angles and tunable Hall conductivity in 2D twisted ferromagnetic bilayers

TL;DR

This paper introduces a magnonic analogue of twisted bilayer graphene, revealing how twist angles and Dzyaloshinskii-Moriya interactions create tunable topological magnon bands and Hall effects in ferromagnetic bilayers.

Contribution

It demonstrates the emergence of flat and topological magnon bands in twisted ferromagnetic bilayers with DMI, controlled by twist angles, expanding the scope of twistronics to magnonic systems.

Findings

Discrete magnon magic angles with flat minibands in absence of DMI

DMI induces topological magnon band structures at any twist angle

Control of magnon Hall and Nernst conductivities via twist angle

Abstract

Twistronics is currently one of the most active research fields in condensed matter physics, following the discovery of correlated insulating and superconducting phases in twisted bilayer graphene (tBLG). Here, we present a magnonic analogue of tBLG. We study magnons in twisted ferromagnetic bilayers (tFBL), including exchange and Dzyaloshinskii-Moriya interactions (DMI). For negligible DMI, tFBL presents discrete magnon magic angles and flat moir\'e minibands analogous to tBLG. The DMI, however, changes the picture and renders the system much more exotic. The DMI in tFBL induces a rich topological magnon band structure for any twist angle. The twist angle turns to a control knob for the magnon Hall and Nernst conductivities. Gapped flat bands appear in a continuum of magic angles in tFBL with DMI. In the lower limit of the continuum, the band structure reconstructs to form bundles of topological flat bands. The luxury of twist-angle control over band gaps, topological properties, number of flat bands, Hall and Nernst conductivities renders tFBL a novel device from fundamental and applied perspectives.