Magnetic structure and spin dynamics of multiferroic system Co₄Nb₂O₉

Full text: Co₄Nb₂O₉, was recently reported to have large magneto-dielectric coupling effect under a certain magnetic field. This compound has a corundum-type crystal structure of space group P-3c1 of ref. [3] and undergoes antiferromagnetic phase transition around 27 K. It was previously believed that the magnetic moments of Co²⁺ order into a collinear antiferromagnetic structure in which Co²⁺ spins order parallel to the c-direction and form ferromagnetic chains with antiparallel inter-chain coupling. However, the recent study has shown that this magnetic structure model is incorrect. In this study, we found that the Co²⁺ magnetic moments align in the ab plane with a non-collinear configuration. Using inelastic neutron scattering, we measured the spin wave excitation from its magnetic phase along (h00) and (00l). A magnetic model was proposed to explain the observed spin dynamical behavior. There are two inequivalent Co sites, which form spin chains in an alternative way along c axis. Each Co²⁺ moment couples with its two inequivalent neighbors on the same chain with ferromagnetic interactions. Co²⁺ moments from each site form a zig-zag hexagonal ring perpendicular to the c axis, where antiferromagnetic interactions dominate. On the basis of this model, the observed spin wave spectra can be well simulated by SpinW. (author)