Large spin-driven dielectric response and magnetoelectric coupling in the buckled honeycomb Fe 4 Nb 2 O 9

We present the significant spin-driven dielectric anomaly ($\ensuremath{\sim}40%$ drop) and magnetoelectric coupling near the magnetic ordering temperature in single crystal ${\mathrm{Fe}}_{4}{\mathrm{Nb}}_{2}{\mathrm{O}}_{9}$. By combining neutron and x-ray single crystal diffraction techniques, we unambiguously determined its magnetic symmetry and studied the structural phase transition at ${T}_{S}$ = 70 K. The temperature-dependent static dielectric constant is strongly anisotropic, rendering two dielectric anomalies along the $a$ axis in the hexagonal lattice with the first one coupled to the magnetic ordering around ${T}_{N}$ = 97 K and the second one accompanying with a first-order structural transition around ${T}_{S}$ = 70 K. Below ${T}_{N}$, we found that the anomalous dielectric constant is practically proportional to the square of the magnetic moment from neutron diffraction data, indicating that the exchange striction is likely responsible for the strong spin-lattice coupling. Magnetic-field-induced magnetoelectric coupling was observed and is compatible with the determined magnetic structure that is characteristic of antiferromagnetically arranged ferromagnetic chains in the honeycomb plane. We propose that such magnetic symmetry should be immune to external magnetic fields to some extent favored by the freedom of rotation of moments in the honeycomb plane, laying out a promising system to control the magnetoelectric properties by magnetic fields.