We report comprehensive thermodynamic and neutron scattering measurements on the $S=3/2$ antiferromagnet ${\mathrm{Cs}}_2{\mathrm{CoI}}_4$, a member of the thoroughly studied family of frustrated magnets ${\mathrm{Cs}}_{2}M{X}_4$ ($M=\mathrm{Cu}$, Co, Ru, $X=\mathrm{Br}$, Cl, I, O). Unlike previously studied members, ${\mathrm{Cs}}_2{\mathrm{CoI}}_4$ undergoes a structural phase transition, for which we determine the low-temperature crystallographic structure. The resulting symmetry reduction strongly affects both the magnetic exchange interactions and single-ion anisotropy. Despite the large parameter space, we propose a minimal magnetic Hamiltonian that reasonably captures the observed excitation spectrum, analyzed using extended SU(4) linear spin-wave theory.

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• Frustrated magnetism
• Magnetostriction
• Calorimetry
• Inelastic neutron scattering
• Neutron diffraction
• Spin wave theory
• X-ray diffraction