Stability of the collinear up-up-down-down magnetic ordering in Ca₃CoMnO₆

Ca₃CoMnO₆ −xMnxO6 is a quasi-one-dimensional Ising chain magnet with a unique collinear up-up-down-down(↑↑↓↓) magnetic ordering, that gives rise to ferroelectricity below the Néel temperature of 13 K. This uniquelong-range magnetic ordering, however, was argued to get stabilized only with Co excess (i.e., x < 1.0) nonstoichiometric Ca₃CoMnO₆ , and not so for the chemically stoichiometric compound x = 1.0. Combining severalexperiments along with first-principles density-functional-based calculations, we elucidate that this unique↑↑↓↓ magnetic ordering can be stabilized even in the stoichiometric Ca₃CoMnO₆ (i.e., x = 1.0) compound,albeit through the presence of optimum cationic positional disorder, where some Mn and Co ions occupy thetrigonal-prismatic (Co sites for the ordered case) and octahedral (Mn-sites for the ordered one) sites respectivelythrough relative tuning of the various magnetic exchange interactions. Specifically, the relative energy stabilityof the ↑↑↓↓ magnetic order in Ca3₃CoMnO₆ exhibits a nonmonotonic dependency on the extent of cationicdisorder with a maximum of around 16% disorder. Thus our study helps to elucidate the mechanism thatleads to stabilization of this unique functional ↑↑↓↓ magnetic ordering in this promising class of multiferroiccompounds.