Ferroelectric order with a linear high-field magnetoelectric coupling in Na2Co2TeO6 : A proposed Kitaev compound

Abstract

We report ferroelectric (FE) order associated with a linear magnetoelectric consequence in ${\mathrm{Na}}_{2}{\mathrm{Co}}_{2}{\mathrm{TeO}}_{6}$. The ferroelectric order around $\ensuremath{\sim}$72 K (${T}_{FE}$), which is much above the antiferromagnetic ordering (${T}_{N}$) at 26 K, is confirmed by the pyroelectric as well as bias-electric field measurement. A reasonable value of the electrocaloric effect is observed around ${T}_{FE}$. The magnetic entropy change ($\mathrm{\ensuremath{\Delta}}{S}_{M}$) starts to emerge at the onset of FE order and exhibits a maximum around 35 K, which is also much higher than ${T}_{N}$, pointing towards a dominant short-range ordering above ${T}_{N}$. A structural transition is proposed to a polar $P{6}_{3}$ structure at ${T}_{FE}$ from a high-temperature $P{6}_{3}22$ space group, which accounts for the emergence of polar order. The distortion of the ${\mathrm{CoO}}_{6}$ octahedral as well as layered honeycomb structure around ${T}_{FE}$ and ${T}_{N}$ has been discussed to correlate significance of the magnetic frustration, the occurrence of FE order, multicaloric effect, and feasible Kitaev spin liquid state. The multifunctional properties of the compound have drawn special attention and are of fundamental interest.