We report thermodynamic, neutron diffraction, and inelastic neutron scattering measurements on Cs2RuO4, a member of the celebrated family of frustrated magnets Cs2MX4 (M = Cu, Co, X = Br, Cl). Unlike the previously studied members, it is based on 4d transition metal ions with S=1. Mapping out the H−T magnetic phase diagram reveals an unusual continuous spin-flop-like phase transition associated with a quantum critical point within the antiferromagnetically ordered phase. A quantitative analysis of the complex magnetic excitation spectrum measured in zero field allows us to derive a model magnetic Hamiltonian for this compound. Its main feature is a frustration of magnetic anisotropy on a level that is much higher than in any of the previously studied species. This frustration naturally explains the peculiar phase transition observed.
@article{arxiv.2507.19853,
title = {Spin-flop-like transition as quantum critical point in Cs$_2$RuO$_4$},
author = {S. D. Nabi and M. Zhu and K. Yu. Povarov and D. G. Mazzone and J. Lass and Y. Wu and Z. Yan and S. Gvasaliya and A. Zheludev},
journal= {arXiv preprint arXiv:2507.19853},
year = {2025}
}