Ferroelectricity in an Ising Chain Magnet

We report discovery of collinear-magnetism-driven ferroelectricity in the Ising chain magnet ${\mathrm{Ca}}_3{\mathrm{Co}}_{2-x}{\mathrm{Mn}}_x{\mathrm{O}}_6$ ($x\approx 0.96$). Neutron diffraction shows that ${\mathrm{Co}}^{2+}$ and ${\mathrm{Mn}}^{4+}$ ions alternating along the chains exhibit an up-up-down-down ($\uparrow \uparrow \downarrow \downarrow$) magnetic order. The ferroelectricity results from the inversion symmetry breaking in the $\uparrow \uparrow \downarrow \downarrow$ spin chain with an alternating charge order. Unlike in spiral magnetoelectrics where antisymmetric exchange coupling is active, the symmetry breaking in ${\mathrm{Ca}}_3(\mathrm{Co},\mathrm{Mn}{)}_2{\mathrm{O}}_6$ occurs through exchange striction associated with symmetric superexchange.