Topological Magnon Bands in a Kagome Lattice Ferromagnet

TLDR

Materials with topological quasiparticles are sought after because they can host boundary excitations protected against disorder. Spin‑wave calculations using a Heisenberg Hamiltonian with Dzyaloshinskii–Moriya interaction explain the measured band structure. Neutron scattering shows that magnons in an insulating kagome ferromagnet form a topological band structure with a flat band, marking the first two‑dimensional topological magnon insulator that should exhibit a magnon Hall effect and protected chiral edge modes.

Abstract

There is great interest in finding materials possessing quasiparticles with topological properties. Such materials may have novel excitations that exist on their boundaries which are protected against disorder. We report experimental evidence that magnons in an insulating kagome ferromagnet can have a topological band structure. Our neutron scattering measurements further reveal that one of the bands is flat due to the unique geometry of the kagome lattice. Spin wave calculations show that the measured band structure follows from a simple Heisenberg Hamiltonian with a Dzyaloshinkii-Moriya interaction. This serves as the first realization of an effectively two-dimensional topological magnon insulator--a new class of magnetic material that should display both a magnon Hall effect and protected chiral edge modes.