Temperature-Dependent Structural, Dielectric, and Raman Spectroscopy Studies on Magnetoelectric Co4Nb2O9

Co4Nb2O9 (CNO) is one of the interesting magnetoelectric materials. Here, we report comprehensive temperature-dependent structural, phonon, dielectric, and magnetic properties of CNO. In contrast to most of the earlier reports, a clear transition (upturn feature) is observed in temperature-dependent dielectric measurements at the antiferromagnetic ordering (T-N) similar to 27.2 K, even in the absence of magnetic field. The upturn-like feature at T-N (in zero magnetic field) becomes a sharp peak above the critical magnetic field. The isothermal magnetodielectric (MD) results infer magnetic field-induced transition below T-N with nonlinear behavior. The maximum MD effect (similar to 4%) is observed at T-N at 80 kOe. The terahertz measurements exhibit spin excitation mode below T-N in the absence of magnetic field. The temperature-dependent synchrotron X-ray diffraction results demonstrate that the crystal structure of CNO remained P (3) over bar c1 down to 12 K; however, small distortion in the lattice is observed at T-N. The temperature-dependent Raman measurements infer an unusual phonon shift and line width of optical phonons below T-N. These results illustrate that CNO exhibits dielectric transition even in the absence of magnetic field and exhibit magnetoelastic coupling. The correlation in these order parameters is also discussed.