Spin-flop-like transition as quantum critical point in Cs$_2$RuO$_4$

TL;DR

This study investigates Cs$_2$RuO$_4$, revealing a unique spin-flop-like transition linked to a quantum critical point, driven by high-level magnetic frustration in a 4d transition metal system.

Contribution

It provides the first detailed magnetic phase diagram and Hamiltonian model for Cs$_2$RuO$_4$, highlighting unprecedented magnetic frustration effects.

Findings

Identified a continuous spin-flop-like transition at the quantum critical point.

Derived a magnetic Hamiltonian explaining the high frustration level.

Mapped the H-T phase diagram revealing novel magnetic behavior.

Abstract

We report thermodynamic, neutron diffraction, and inelastic neutron scattering measurements on Cs$_2$RuO$_4$, a member of the celebrated family of frustrated magnets Cs$_2$MX$_4$ (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.