High-energy spin waves in the spin-1 square-lattice antiferromagnet La2NiO4
Inelastic neutron scattering is used to study the magnetic excitations of the S=1 square-lattice antiferromagnet La2NiO4. 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 La2NiO4. 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.