Magnons and Electromagnons in Dirac Antiferromagnet CoTiO₃

With an effective spin 1/2 ground state of Co2+ ions in a layered honeycomb lattice, CoTiO₃ has recently been in the spotlight due to its promise of topological magnetic behavior. Specifically, Dirac nodal lines have been measured around the K points in its magnon spectrum by inelastic neutron scattering. This spectrum also displays high energy spin-orbit excitons, coming from the higher effective spin multiplets. This makes this material a potential platform for the existence of topological magnons where spin-orbit coupling plays a crucial role.<br/>We report on our work combining time domain Terahertz (THz) and Raman spectroscopy on CoTiO₃. By controlling the polarization of the THz wave with respect to the crystal axes, we determine the nature of the magnetic excitations; at zero magnetic field, we find that the 1.3 THz (~5.4 meV) magnon is excited only by the electric field of the THz wave, making this magnon an electromagnon. In addition, we determine the zone-center gap via the measurement of the in-plane magnetic-dipole antiferromagnetic resonance at ~220 GHz (~0.9 meV). These two magnons are also detected by Raman spectroscopy. These experiments provide a high-resolution determination of these modes at k=0. Finally, by applying an external magnetic field along the c-axis and along a hexagonal axis, we revealed the highly anisotropic g-factor of these magnons. Surprisingly, we find that the frequencies of these two magnons approach each other with a non-linear dependence on the magnetic field. We will discuss these results and the possible mechanisms that explain them.