Investigation of spin waves and magnetic structure of NiTiO₃

Magnetoelectric multiferroics with coexisting magnetic and electric orders that can be manipulated by external electric and magnetic fields have wide-ranging applications. Investigating the origin of these magnetic and electric properties is crucial for understanding the intriguing underlying physics. NiTiO₃ is such a system with the honeycomb structure with R space group. It has an antiferromagnetic (AFM) stacking order along the c-axis and ferromagnetic (FM) in-plane. The Néel temperature is 22 K. The local structure, magnetism and exchange interactions were investigated by neutron scattering. The local structure obtained from the pair distribution function analysis of the diffraction data at high temperatures shows that the Ti-O bond length changes as a function of temperature and this could be linked to the ferroelectric properties above room temperature. At low temperatures, a negative thermal expansion is observed along a-axis that may be linked to magnetoelastic coupling and anisotropic exchange interactions in the system. From inelastic neutron scattering data, a single, broad magnon peak centered at ~ 2.5 meV is observed and calculations suggested considerable out of plane interactions.