Ferroelectricity of structural origin in spin-chain compounds Ca$_3$Co$_{2-x}$Mn$_x$O$_6$

TL;DR

This study reveals that Ca$_3$Co$_{2-x}$Mn$_x$O$_6$ exhibits ferroelectricity originating from structural distortions rather than magnetic order, with significant polarization observed along and perpendicular to the spin chains.

Contribution

It demonstrates ferroelectricity in Ca$_3$Co$_{2-x}$Mn$_x$O$_6$ due to structural effects, independent of magnetic transitions, highlighting a structural origin of ferroelectricity in spin-chain compounds.

Findings

Ferroelectric transition occurs at 35-40 K with large polarization values.

Electric polarization is observed both along and perpendicular to the spin chains.

No direct correlation between magnetic anomalies and ferroelectricity.

Abstract

We report a systematic study of the structure, electric and magnetic properties of Ca$_3$Co$_{2-x}$Mn$_x$O$_6$ single crystals with $x =$ 0.72 and 0.26. The DC and AC magnetic susceptibilities display anomalies with characteristic of the spin freezing. The crystals show ferroelectric transition at 40 K and 35 K ($T_{FE}$) for $x =$ 0.72 and 0.26, respectively, with a large value of 1400 $\mu$C/m$^2$ at 8 K for electric polarization ($P_c$) along the spin-chain ($c$-axis) direction. Interestingly, the electric polarization perpendicular to the chain direction ($P_{ab}$) can also be detected and has value of 450 and 500 $\mu$C/m$^2$ at 8 K for the $x =$ 0.72 and 0.26 samples, respectively. The specific heat and magnetic susceptibility show no anomaly around $T_{FE}$, which means that the electric polarization of these samples has no direct relationship with the magnetism. The X-ray diffraction and the Raman spectroscopy indicate that these samples may undergo Jahn-Teller distortions that could be the reason of electric polarization.