Magnetic structure and excitations of the frustrating perovskite oxide PbCoO₃

The perovskite oxide PbBO₃ (B: transition metal) shows interesting charge disproportionation of Pb ions. Very recently, it was found that in PbCoO_3, an unusual charge distribution occurs with charge ordering of both Pb (Pb²⁺/ Pb⁴⁺) and Co (Co²⁺/ Co³⁺).[1] The Co³⁺ and Co²⁺ ions are expected to have the low spin (S=0) and the high spin state (S=3/2), respectively. Magnetic succeptibility and specific heat measurements show a two-step transition at 4 K and 7.8 K. Since the Co²⁺ ions form a pyrochlore lattice, the magnetic states are expected to originate from frustrating interactions. The magnetic structure and excitations of PbCoO₃ were investigated using elastic and inelastic neutron scattering experiments and the magnetic propagation vector was found to be q_m = (0.5, 0.5, 0.5). Possible magnetic structures in the two magnetic phases will be discussed. Inelastic neutron scattering experiments show two magnetic excitations centered at 0.9 meV and 1.55 meV. A possible spin Hamiltonian that explains the neutron scattering data will also be presented. [1] J. Am. Chem. Soc. 2017, 139, 4574−4581