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The crystallographic structure and the magnetic order of the distorted perovskite ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{3}$ $(0.10l~xl~0.30)$ 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 $0.15l~xl~0.25$ 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.

The critical exponents of the second-order ferromagnetic-paramagnetic phase transition of (Formula presented) (Formula presented) compounds are obtained. The results (Formula presented) (Formula presented) and (Formula presented) are qualitatively different than those reported in the literature for parent manganites, and are taken as the indication of the absence of long-range interactions below the Curie point. Electronic phase segregation in hole-rich ferromagnetic and hole-poor (Formula presented)-like regions is supposed to explain the observations. The hole-poor regions would dilute the magnetic lattice and prevent the occurrence of long-range order. Also, a spin transition of the (Formula presented) ions at (Formula presented) or the hole-poor regions (acting equivalently to missed spin clusters in amorphous ferromagnets) could be the reason for the high β value in the framework of a Heisenberg model. © 1999 The American Physical Society.

We present Lu${}_{2}$MnCoO${}_{6}$, a multiferroic member of the double perovskites that we have investigated using x-ray and neutron diffraction, specific heat, magnetization, electric polarization, and dielectric constant measurements. This material possesses a net electric polarization strongly coupled to a net magnetization below 35 K, despite the antiferromagnetic ordering of the $S=3/2$ Mn${}^{4+}$ and Co${}^{2+}$ spins in an $\ensuremath{\uparrow}\ensuremath{\uparrow}\ensuremath{\downarrow}\ensuremath{\downarrow}$ configuration along the $c$ direction. We discuss the magnetic order in terms of a condensation of domain boundaries between $\ensuremath{\uparrow}\ensuremath{\uparrow}$ and $\ensuremath{\downarrow}\ensuremath{\downarrow}$ ferromagnetic domains, with each domain boundary producing an electric polarization due to spatial inversion symmetry breaking. In an applied magnetic field the domain boundaries slide, controlling the size of the magnetization, electric polarization, and magnetoelectric coupling.