On the magnetic insulating states, spin frustration, and dominant spin exchange of the ordered double-perovskites Sr2CuOsO6 and Sr2NiOsO6: density functional analysis.

The ordered double-perovskites Sr(2)MOsO(6) (M = Cu, Ni) consisting of 3d and 5d transition-metal magnetic ions (M(2+) and Os(6+), respectively) are magnetic insulators; the magnetic susceptibilities of Sr(2)CuOsO(6) and Sr(2)NiOsO(6) obey the Curie-Weiss law with dominant antiferromagnetic and ferromagnetic interactions, respectively, and the zero-field-cooled and field-cooled susceptibility curves of both compounds diverge below ∼20 K. In contrast, the available density functional studies predicted both Sr(2)CuOsO(6) and Sr(2)NiOsO(6) to be metals. We resolved this discrepancy on the basis of systematic density functional calculations. The magnetic insulating states of Sr(2)MOsO(6) are found only when a substantially large on-site repulsion is employed for the Os atom, although it is a 5d element. The cause for the divergence between the zero-field-cooled and field-cooled susceptibility curves in both compounds and the reason for the difference in their dominant magnetic interactions were investigated by examining their spin exchange interactions.