  Neutron powder diffraction experiments reveal that Co4Nb2O9 forms a
noncollinear in-plane magnetic structure with Co2+ 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 neighbour exchange interactions, in-plane anisotropy
and the Dzyaloshinskii-Moriya interaction. The in-plane magnetic structure of
Co4Nb2O9 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
Co4Nb2O9 in fields can be explained by its special in-plane magnetic structure.
