Two-staged magnetoresistance driven by the Ising-like spin sublattice in SrCo6O11

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.

Effect of the on-site interaction on the magnetic properties of an exactly solvable spin-electron system

A hybrid lattice-statistical model on doubly decorated planar lattices, which have localized Ising spins at their nodal lattice sites and two itinerant electrons at each pair of decorating sites, is examined within the framework of an exact analytical method based on the generalized decoration-iteration mapping transformation. A particular emphasis is laid on the investigation of the effect of the on-site interaction U on the ground-state properties and critical behaviour of the system. It is shown that the ground state of the investigated hybrid system consists of a remarkable four-sublattice quantum antiferromagnetic phase, which manifests itself through a non-trivial criticality at non-zero temperatures. Moreover, the thermal variations of the staggered magnetization and the specific heat provide unambiguous evidence that the investigated spin-electron system exhibits the critical behaviour from the standard Ising universality class.