High-Pressure Synthesis and Magnetic States of Magnetoplumbite Cobaltates CaCo₁₂O₁₉ and BaCo₁₂O₁₉

Novel cobalt oxides, CaCo₁₂O₁₉ and BaCo₁₂O₁₉, have been synthesized under high-pressure and high-temperature conditions of 7 GPa and 1373 K, respectively. Rietveld refinement using synchrotron X-ray diffraction data indicates that the CaCo₁₂O₁₉ and BaCo₁₂O₁₉ crystallize in a magnetoplumbite structure with a hexagonal space group of P6₃/mmc (No. 194) as well as SrCo₁₂O₁₉. The magnetic study demonstrates that itinerant and localized 3d electrons coexist in all ACo₁₂O₁₉ (A = Ca, Sr, Ba) and the magnetic ground state transforms from antiferromagnetic (A = Ca) to ferrimagnetic (A = Sr) to antiferromagnetic (A = Ba), which is in stark contrast to the systematic change in the magnetoplumbite-related cobalt oxides of ACo₆O₁₁ from antiferromagnet (A = Ca) to ferrimagnet (A = Sr) to ferromagnet (A = Ba). The nonmonotonic magnetic evolution with isoelectronic A-site substitution in ACo₁₂O₁₉ is probably attributed to changes in the interactions between two magnetic sublattices of localized 3d electrons at trigonal-bipyramidal and tetrahedral sites for ACo₁₂O₁₉. This finding proposes the complex magnetic properties in the layered cobalt oxides with multiple magnetic sublattices in the coexistence system of itinerant and localized electrons.