Ising Magnetism and Ferroelectricity in Ca₃CoMnO₆

The origin of both the Ising chain magnetism and ferroelectricity in Ca₃CoMnO₆ is studied by ab initio electronic structure calculations and x-ray absorption spectroscopy. We find that Ca₃CoMnO₆ has alternate trigonal prismatic Co²⁺ and octahedral Mn⁴⁺ sites in the spin chain. Both the Co²⁺ and Mn⁴⁺ are in the high-spin state. In addition, the Co²⁺ has a huge orbital moment of 1.7μ_B which is responsible for the significant Ising magnetism. The centrosymmetric crystal structure known so far is calculated to be unstable with respect to exchange striction in the experimentally observed ↑↑↓↓ antiferromagnetic structure for the Ising chain. The calculated inequivalence of the Co-Mn distances accounts for the ferroelectricity.