LAUSR

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.