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

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 $\hat{I}¼$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 $\hat{I}¼$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.

11 pages, 16 figures, accepted for publication in Phys. Rev. B