Orbital ordering and superexchange in manganite oxides

Abstract

The magnetic couplings in insulating ${\mathrm{LaMnO}}_3$, ${\mathrm{CaMnO}}_3$, and the planar analogs ${\mathrm{La}}_{\mathrm{n}+1\mathrm{}}$ ${\mathrm{Mn}}_{\mathrm{n}}$ ${\mathrm{O}}_{3\mathrm{n}+1}$ are estimated using standard superexchange arguments. The orbital ordering observed in ${\mathrm{LaMnO}}_3$ is found to lead to the observed magnetic exchange constants if the effective Mn on-site interaction ${\mathrm{U}}_{\mathrm{Mn}}$ is larger than the charge transfer energy. Differences between the pseudocubic and the planar materials are accounted for. The effect of doping is discussed.