Condensed Matter Seminars

Despite identical R3 crystal structures, honeycomb layered MTiO₃ ilmenites exhibit diverse magnetic orders and transition temperatures (T_N): G-type antiferromagnetic for MnTiO₃ (T_N=68 K) and A-type antiferromagnetic for CoTiO₃(T_N=38 K) and NiTiO₃ (T_N=22 K). This work focuses on this intriguing interplay between local structure, electronic properties, and magnetic configurations. CoTiO₃ has two magnon peaks around 5-14 meV with a distinct gapless Dirac node nestled between them are observed and the magnon modes are renormalized to lower energies. For CoTiO₃, magnetic excitations attributed to spin-orbit exciton multiplet transitions show the same temperature dependance as magnon with the intensity dissipating quickly above T_N.  The energy levels arising from crystal field and spin-orbit coupling are gradually thermally populated through T and reaching maximum at 100 K. However, the NiTiO₃ system shows a single low energy magnon peak around 2-4 meV which is renormalized into lower energies, but it does not show Dirac magnon properties. The calculated exchange interactions using SpinW confirm the weaker inter-plane interaction in CoTiO3 than NiTiO3. Across three system, both transition metal M⁺² ion and Ti⁺⁴ ions are in distorted octahedra environment, and the first four nearest neighbors are Ti-O < M-O < Ti-O < M-O with the given bond length order. Across three systems Ti-O and short M-O bond length variation is minimum. However, M-O bond length (MnTiO₃=2.28 Å, CoTiO₃=2.17 Å and NiTiO₃=2.12 Å) variation is significant which follows the same variation as reported dielectric constants (MnTiO₃=20.4, CoTiO₃=19.5 and NiTiO₃=17.8 ) and T_N and confirms the interplay between these parameters. Within the measured 100 K to 500 K, temperature dependance of local structure is insignificant and for the reported relative dielectric values, the variation is almost constant. This suggests that the interplay between local geometry and magnetic interactions governs the diverse behaviors observed in these honeycomb materials.