Ferroelectricity in an Ising Chain Magnet

We report discovery of collinear-magnetism-driven ferroelectricity in the Ising chain magnet ${mathrm{Ca}}_{3}{mathrm{Co}}_{2ensuremath{-}x}{mathrm{Mn}}_{x}{mathrm{O}}_{6}$ ($xensuremath{approx}0.96$). Neutron diffraction shows that ${mathrm{Co}}^{2+}$ and ${mathrm{Mn}}^{4+}$ ions alternating along the chains exhibit an up-up-down-down ($ensuremath{uparrow}ensuremath{uparrow}ensuremath{downarrow}ensuremath{downarrow}$) magnetic order. The ferroelectricity results from the inversion symmetry breaking in the $ensuremath{uparrow}ensuremath{uparrow}ensuremath{downarrow}ensuremath{downarrow}$ 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.