Spin dynamics and magnetoelectric coupling mechanism of Co4Nb2O9

Neutron powder diffraction experiments reveal that $\mathrm{C}{\mathrm{o}}_{4}\mathrm{N}{\mathrm{b}}_{2}{\mathrm{O}}_{9}$ forms a noncollinear in-plane magnetic structure with $\mathrm{C}{\mathrm{o}}^{2+}$ moments lying in the $ab$ plane. The spin-wave excitations of this magnet were measured by using inelastic neutron scattering and soundly simulated by a dynamic model involving nearest- and next-nearest-neighbor exchange interactions, in-plane anisotropy, and the Dzyaloshinskii-Moriya interaction. The in-plane magnetic structure of $\mathrm{C}{\mathrm{o}}_{4}\mathrm{N}{\mathrm{b}}_{2}{\mathrm{O}}_{9}$ is attributed to the large in-plane anisotropy, while the noncollinearity of the spin configuration is attributed to the Dzyaloshinskii-Moriya interaction. The high magnetoelectric coupling effect of $\mathrm{C}{\mathrm{o}}_{4}\mathrm{N}{\mathrm{b}}_{2}{\mathrm{O}}_{9}$ in fields can be explained by its special in-plane magnetic structure.