Origin of dielectric response and magneto-electric properties of CoTiO₃

Magnetic materials with kagome and honeycomb lattices harbor topological properties. One example is CoTiO₃ that consists of layers of Co²⁺ ions on a honeycomb lattice and exhibits Dirac magnons, magnetoelectric coupling, and spin-orbit excitons. The system orders into an A-type antiferromagnetic structure below T_N = 38 K, with the spins aligned ferromagnetically within the plane and antiferromagnetically out of plane. Moreover, CoTiO₃ has a large dielectric response at high temperature and low frequencies. The analysis of the local structure from neutron diffraction data indicate that the Ti atoms are displaced in the TiO₆ octahedra that coincide with the dielectric response at high temperature. The Ti displacements are enhanced with increasing temperature, indicating that the Ti distortions lead to dipolar interactions leading to the dielectric behavior. From inelastic neutron scattering data, low energy (<15 meV) magnon peaks below T_N and higher-energy “spin-orbit excitons” (SEOs) near 29 and 58 meV are clearly observed. The SEOs represent crystal field excitations and remain present up to at least 300 K, in contrast to the spin waves, though the SEOs soften across T_N. Our results could provide insight into the relation between the local structure and magnetic or electric properties.