Spin-Orbit Exciton in a Honeycomb Lattice Magnet CoTiO$_3$: Revealing Link Between Rare Earth and Transition Metal Magnetism

TL;DR

This study uses inelastic neutron scattering to investigate the temperature-dependent spin-orbit exciton in CoTiO$_3$, revealing strong coupling between ground and excited states and drawing parallels between rare-earth and transition metal magnetism.

Contribution

It demonstrates the temperature evolution of spin-orbit excitons in CoTiO$_3$ and introduces a multi-level RPA theory that captures the coupling effects, bridging rare-earth and transition metal magnetism.

Findings

SO exciton softens and broadens above T_N

An additional mode appears at intermediate temperatures

Multi-level theory successfully explains temperature dependence

Abstract

We carried out inelastic neutron scattering to study the spin-orbital (SO) exciton in a single crystal sample of CoTiO$_3$ as a function of temperature. CoTiO$_3$ is a honeycomb magnet with dominant XY-type magnetic interaction and an A-type antiferromagnetic order below $\mathrm{T_N} \approx 38$~K. We found that the SO exciton becomes softer, but acquires a larger bandwidth in the paramagnetic phase, compared to that in the magnetically ordered phase. Moreover, an additional mode is only observed in the intermediate temperature range, as the sample is warmed up above the lowest accessible temperature below $\mathrm{T_N}$. Such an unusual temperature dependence observed in this material suggests that its ground states (an $S_{\mathrm{eff}}=\frac{1}{2}$ doublet) and excited states multiplets are strongly coupled, and therefore cannot be treated independently, as often done in a pseudo-spin model. Our observations can be explained by a multi-level theory within random phase approximation that explicitly takes into account both the ground and excited multiplets. The success of our theory, which is originally developed to explain temperature dependence of magnetic excitations in the rare-earth magnets, highlight the similarity between the magnetic excitations in rare-earth systems and those in transition metal systems with strong spin orbit coupling.