Structural details and magnetic order ofLa1−xSrxCoO3(x<~0.3)

Abstract

The crystallographic structure and the magnetic order of the distorted perovskite ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{3}$ $(0.10<~x<~0.30)$ has been studied by neutron diffraction, high-resolution electron microscopy, and magnetic-susceptibility measurements. The results give direct evidence for an inhomogeneous distribution of the ${\mathrm{Sr}}^{2+}$ ions and the segregation of the material into hole-rich ferromagnetic regions and a hole-poor semiconducting matrix at lower values of x. The holes introduced by Sr doping are attracted to the ${\mathrm{Sr}}^{2+}$ ions where they stabilize to lowest temperatures an intermediate-spin state at neighboring trivalent cobalt. The antibonding e electrons so stabilized increase the mean unit-cell volume and are delocalized over the cobalt atoms of the cluster where they couple the localized ${t}^{5}$ configurations ferromagnetically. Long-range ferromagnetic order between clusters is realized even for Sr doping as low as $x=0.10.$ The transition to a spin glass state is observed only for Sr concentrations smaller than 0.10. The volume of a hole-rich cluster grows in a magnetic field, and the origin of the large negative magnetoresistance observed near ${T}_{C}$ for $0.15<~x<~0.25$ appears to be due to a growth of the clusters to a percolation threshold. For $x=0.30,$ the ${\ensuremath{\sigma}}^{*}$ band of the intermediate-spin state below ${T}_{C}$ is at the threshold of a transition from itinerant to polaronic conduction and, above ${T}_{C},$ the system transforms smoothly to a cluster state.