| Riassunto: | <p>Inelastic neutron scattering is used to study the magnetic excitations of the <em>S</em>=1 square-lattice antiferromagnet <span tabindex="0">La<sub>2</sub>NiO<sub>4</sub></span>. 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 <em>S</em><span tabindex="0">=1/2</span> square-lattice antiferromagnets. Thus, our results suggest that the quantum dispersion renormalization effects or longer-range exchange interactions observed in cuprates and other <em>S</em> =1/2 square-lattice antiferromagnets are also present in La<sub>2</sub>NiO<sub>4</sub>. 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.</p>
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