Catalog of Unconventional Magnons in Collinear Magnets

TL;DR

This paper develops a spin space group theory to classify and tabulate unconventional magnons in collinear magnets, revealing their topological features and potential for spintronic applications.

Contribution

It introduces a comprehensive SSG framework for understanding unconventional magnon band nodes, supported by database analysis and first-principles calculations.

Findings

Identified 498 collinear magnets with unconventional magnons

Over 200 magnon band structures analyzed using first-principles and linear spin wave theory

More than 80% of these magnets are dominated by Heisenberg interactions

Abstract

Topological magnons have garnered significant interest for their potential in both fundamental research and device applications, owing to their exotic, uncharged, yet topologically protected boundary modes. However, their comprehension has been hindered by the absence of fundamental symmetry descriptions of magnetic materials, which are primarily governed by isotropic Heisenberg interactions in spin Hamiltonians. The ensuing magnon dispersions enable gapless magnon band nodes that go beyond the scenario of representation theory of the magnetic space groups (MSGs), thus referred to as unconventional magnons. Here we developed spin space group (SSG) theory to elucidate collinear magnetic configurations, classifying the 1421 collinear SSGs into four types, constructing their band representations, and providing a comprehensive tabulation of unconventional magnons, such as duodecuple points, octuple nodal lines, and charge-4 octuple points. Based on the MAGNDATA database, we identified 498 collinear magnets with unconventional magnons, among which over 200 magnon band structures were obtained by using first-principles calculations and linear spin wave theory. Additionally, we evaluated the influence of the spin-orbit coupling-induced exchange interaction in these magnets and found that more than 80% are predominantly governed by the Heisenberg interactions, indicating that SSG serves as an ideal framework for describing magnon band nodes in most 3d, 4d and half-filled 4f collinear magnets. Our work offers new pathways for exploring uncharged transports in magnonic systems, holding promise for advancements in next-generation spintronic devices.