Spin-orbit exciton in a honeycomb lattice magnet CoTiO₃: Revealing a link between magnetism in d- and f-electron systems

We present inelastic neutron scattering study of the spin-orbit (SO) exciton in a single crystal sample of CoTiO₃ as a function of temperature. CoTiO₃ is a honeycomb magnet with dominant XY-type magnetic interaction and an A-type antiferromagnetic order below T_N ≈38 K. We observed strong temperature dependence of the SO exciton going from the ordered to paramagnetic phase: a significant softening and an increase in its bandwidth at T>T_N, as well as appearance of a second mode at intermediate temperatures below T_N. Such an unusual temperature dependence observed in this material suggests that its ground states (an S_eff = 1/2 doublet) and excited states multiplets are strongly coupled and therefore cannot be treated independently, as often done in a pseudospin model. Our observations can be explained by a multilevel theory within random phase approximation that explicitly takes into account both the ground and excited multiplets. The success of our theory, originally developed for the rare-earth systems, highlights the similarity between magnetic excitations in f- and d-electron systems with strong spin-orbit coupling.

Reference: Phys. Rev. B 102, 134404 (2020)