High-Pressure Synthesis and Magnetic States of Magnetoplumbite Cobaltates CaCoO and BaCoO.

Novel cobalt oxides, CaCoO and BaCoO, 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 CaCoO and BaCoO crystallize in a magnetoplumbite structure with a hexagonal space group of 6/ (No. 194) as well as SrCoO. The magnetic study demonstrates that itinerant and localized 3d electrons coexist in all CoO ( = Ca, Sr, Ba) and the magnetic ground state transforms from antiferromagnetic ( = Ca) to ferrimagnetic ( = Sr) to antiferromagnetic ( = Ba), which is in stark contrast to the systematic change in the magnetoplumbite-related cobalt oxides of CoO from antiferromagnet ( = Ca) to ferrimagnet ( = Sr) to ferromagnet ( = Ba). The nonmonotonic magnetic evolution with isoelectronic -site substitution in CoO is probably attributed to changes in the interactions between two magnetic sublattices of localized 3d electrons at trigonal-bipyramidal and tetrahedral sites for CoO. 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.