Topological Weyl magnons and thermal Hall effect in layered honeycomb ferromagnets

TL;DR

This paper investigates the topological properties of magnons in layered honeycomb ferromagnets, revealing how interlayer coupling influences magnon band topology and thermal Hall effects, with implications for experimental tuning in materials like CrI3.

Contribution

It demonstrates the impact of interlayer coupling on magnon topology and thermal Hall effect in layered honeycomb ferromagnets, highlighting tunability via pressure.

Findings

Magnon band structure and Chern numbers are affected by interlayer coupling Jc.

Presence of Weyl points separating different Chern insulating phases.

Thermal Hall conductivity depends on Jc and intra-layer couplings.

Abstract

In this work, we study the topological properties and magnon Hall effect of a three-dimensional ferromagnet in the ABC stacking honeycomb lattice, motivated by the recent inelastic neutron scattering study of CrI$_3$. We show that the magnon band structure and Chern numbers of the magnon branches are significantly affected by the interlayer coupling $J_c$, which moreover has a qualitatively different effect in the ABC stacking compared to the AA stacking adopted by other authors. The nontrivial Chern number of the lowest magnon band is stabilized by the next-nearest-neighbour Dzyaloshinsky-Moriya interaction in each honeycomb layer, resulting in the hopping term similar to that in the electronic Haldane model for graphene. However, we also find several gapless Weyl points, separating the non-equivalent Chern insulating phases, tuned by the ratio of the interlayer coupling $J_c$ and the third-neighbour Heisenberg interaction $J_3$. We further show that the topological character of magnon bands results in non-zero thermal Hall conductivity, whose sign and magnitude depend on $J_c$ and the intra-layer couplings. Since the interlayer coupling strength $J_c$ can be easily tuned by applying pressure to the quasi-2D material such as CrI$_3$, this provides a potential route to tuning the magnon thermal Hall effect in an experiment.