Sawtooth chain magnets have been a subject of historical interest in the field of frustrated magnetism, with classical olivine family M2TX4, (M - 3d, T - 4p, X - chalcogen elements) typically realizing simple k=(000) states. The magnetism of the Mn2GeO4 olivine is surprisingly complex, proceeding from commensurate states to a multiferroic commensurate + incommensurate phase. Here we report inelastic neutron scattering results from a Mn2GeO4 single crystal and develop an effective Hamiltonian including long-distance bilinear and dipolar interactions. The magnetic interactions are predominantly antiferromagnetic and span a three-dimensional exchange network consisting of coupled sawtooth chains. Based on the determined strength of the couplings, the dominant sawtooth chains appear at third- and fourth- rather than next-nearest-neighbor. However the next-nearest-neighbor interaction is, along with a modest Dzyaloshinskii-Moriya interaction, important for modeling the observed incommensurability. We use the best-fit Hamiltonian as the basis for Langevin dynamics simulations and Luttinger-Tisza calculations of the high-temperature commensurate transition.
@article{arxiv.2601.04908,
title = {Coupled sawtooth chain exchange network in olivine Mn$_2$GeO$_4$},
author = {Vincent C. Morano and Zeno Maesen and Stanislav Nikitin and Jonathan S. White and Takashi Honda and Tsuyoshi Kimura and Michel Kenzelmann and Daniel Pajerowski and Oksana Zaharko},
journal= {arXiv preprint arXiv:2601.04908},
year = {2026}
}