Various magnetic states for novel layered cobalt oxides CaCo6O11 and BaCo6O11†
Novel layered cobalt oxides ACo6O11 (A = Ca, Ba) have been synthesized under high-pressure and high-temperature conditions of 8 GPa and 1373 K, respectively. Both oxides crystallize in magnetoplumbite-related structures with hexagonal symmetry of the space group (P63/mmc, no. 194), as well as SrCo6O11. The magnetic ground state systematically transforms from antiferromagnetic-like (CaCo6O11) to ferrimagnetic (SrCo6O11) to ferromagnetic (BaCo6O11) by changing the A-site ion. These magnetic transitions are dominated by the superexchange interactions between magnetic CoO5 trigonal bipyramids. Furthermore, CaCo6O11 exhibits a magnetic-field-driven phase transition from an antiferromagnetic-like to a ferrimagnetic state. Rietveld analysis indicates that the overall magnetic phase diagram for the ACo6O11 series provides a wide variety of electronic states induced by structure and the magnetic field.