High-energy spin waves in the spin-1 square-lattice antiferromagnet La$_2$NiO$_4$

TL;DR

This study uses inelastic neutron scattering to explore magnetic excitations in La$_2$NiO$_4$, revealing complex spin-wave behavior influenced by quantum effects, longer-range interactions, and covalency, challenging simple classical models.

Contribution

It demonstrates that La$_2$NiO$_4$ exhibits quantum dispersion and interactions similar to $S=1/2$ systems, highlighting the importance of longer-range exchanges and covalency in its magnetic excitations.

Findings

Spin waves deviate from simple Heisenberg model predictions.

Presence of quantum dispersion renormalization effects.

Suppressed spin-wave intensity indicating covalency effects.

Abstract

Inelastic neutron scattering is used to study the magnetic excitations of the $S=1$ square-lattice antiferromagnet La$_2$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 La$_2$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.