Order by Static Disorder in the Ising Chain Magnet Ca3Co2-xMnxO6

Ising chain compound Ca3Co2-xMnxO6 exhibits up-up-down-down long-range magnetic order (LRO) in a broad range of 0.75 < x < 1. The LRO is abruptly lost in the narrow vicinity of x=1, and the magnetic state becomes incommensurate. The commensurate state (but not the LRO) is recovered for larger x. This is surprising because the stoichiometric x=1 state exhibits the best Co/Mn ionic order, and the magnetic LRO appears only in the samples with reduced ionic order. We argue that this order-by-static-disorder phenomenon may be related to the disruption of the long-range magnetic interactions by the magnetic-site disorder, reducing magnetic frustration.