# Magnon dispersion in Ca2RuO4: impact of spin-orbit coupling and oxygen moments

## TL;DR

This study investigates magnon dispersion in Ca2RuO4, revealing the significant role of spin-orbit coupling and oxygen moments, and demonstrating the effectiveness of a spin-wave model aligned with density functional theory.

## Contribution

It provides a detailed analysis of magnetic excitations in Ca2RuO4, highlighting the influence of spin-orbit coupling and oxygen moments, and introduces a model explaining additional magnetic modes.

## Key findings

- Magnon dispersion well described by a spin-wave model.
- Spin-orbit coupling impacts magnetic excitation energies.
- Evidence for a magnetic moment on apical oxygens with weak ferromagnetic coupling.

## Abstract

The magnon dispersion of Ca$_2$RuO$_4$ has been studied by polarized and unpolarized neutron scattering experiments on crystals containing 0, 1 and 10 % of Ti. The entire dispersion of transverse magnons can be well described by a conventional spin-wave model with interaction and anisotropy parameters that agree with density functional theory calculations. Spin-orbit coupling strongly influences the magnetic excitations, which is most visible in large energies of the magnetic zone-center modes arising from magnetic anisotropy. We find evidence for a low-lying additional mode that exhibits strongest scattering intensity near the antiferromagnetic zone center. This extra signal can be explained by a sizable magnetic moment of 0.11 Bohr magnetons on the apical oxygens parallel to the Ru moment, which is found in the density functional theory calculations. The energy and the signal strength of the additional branch are well described by taking into account this oxygen moment with weak ferromagnetic coupling between Ru and O moments.