We report thermodynamic, neutron diffraction, and inelastic neutron scattering measurements on ${\mathrm{Cs}}_2{\mathrm{RuO}}_4$, a member of the celebrated family of frustrated magnets ${\mathrm{Cs}}_{2}M{X}_4$ ($M$ = Cu, Co, $X$ = Br, Cl). Unlike the previously studied members, it is based on $4d$ transition metal ions with $S=1$. Mapping out the $H\text{−}T$ 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.

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• Frustrated magnetism
• Magnetic anisotropy
• Magnetic phase transitions
• Quantum criticality
• Inelastic neutron scattering
• Neutron diffraction
• Spin wave theory