The ilmenite ${\mathrm{NiTiO}}_3$ consists of a buckled honeycomb lattice, with the Ni spins aligned ferromagnetically in-plane and antiferromagnetically out-of-plane. Using neutron spectroscopy, the magnetic structure and the dynamics were investigated as a function of temperature. Dispersive acoustic bands and nearly dispersionless optical bands at $\approx 3.7$ meV are described by a highly anisotropy Heisenberg model with stronger antiferromagnetic (AFM) out-of-plane, weaker ferromagnetic (FM) in-plane interactions and an anisotropy gap of 0.95 meV. The order parameter yields a critical exponent between the Heisenberg and two-dimensional Ising models, consistent with highly anisotropic Heisenberg systems. The frustration parameter $\approx 2$ supports a weakly frustrated system.

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• Exchange interaction
• Magnetic anisotropy
• Magnetic interactions
• Magnetism
• Antiferromagnets
• Magnetic insulators