Inelastic neutron scattering is used to study the magnetic excitations of the $S=1$ square-lattice antiferromagnet ${\mathrm{La}}_2{\mathrm{NiO}}_4$. We find that the spin waves cannot be described by a simple classical (harmonic) Heisenberg model with only nearest-neighbor interactions. The spin-wave dispersion measured along the antiferromagnetic Brillouin-zone boundary shows a minimum energy at the $(1/2,0)$ position as is observed in some $S=1/2$ square-lattice antiferromagnets. Thus, our results suggest that the quantum dispersion renormalization effects or longer-range exchange interactions observed in cuprates and other $S=1/2$ square-lattice antiferromagnets are also present in ${\mathrm{La}}_2{\mathrm{NiO}}_4$. We also find that the overall intensity of the spin-wave excitations is suppressed relative to linear spin-wave theory, indicating that covalency is important. Two-magnon scattering is also observed.

• Received 5 April 2023
• Revised 11 July 2023
• Accepted 13 July 2023

DOI:https://doi.org/10.1103/PhysRevResearch.5.033113

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Published by the American Physical Society