Abstract
We present observations of highly frustrated quasi-two-dimensional (2D) magnetic correlations in the honeycomb lattice layers of the Seff =1/2 compound γ−BaCo2(PO4)2 (γ-BCPO). Specific heat shows a broad peak comprised of two weak kink features at TN1∼6 K and TN2∼3.5 K, the relative weights of which can be modified by sample annealing. Neutron powder diffraction measurements reveal short range quasi-2D order that is established below TN1 and TN2, at which two separate, incompatible, short range magnetic orders onset: commensurate antiferromagnetic correlations with correlation length ξc=60±2 Å (TN1) and in quasi-2D helical domains with ξh=350±11 Å (TN2). The ac magnetic susceptibility response lacks frequency dependence, ruling out spin freezing. Inelastic neutron scattering data on γ-BCPO is compared with linear spin wave theory, and two separate parameter regions of the XXZ J1−J2−J3 model with ferromagnetic nearest-neighbor exchange J1 are favored, both near regions of high classical degeneracy. High energy coherent excitations (∼10 meV) persist up to at least 40 K, suggesting strong in-plane correlations persist above TN. These data show that γ-BCPO is a rare highly frustrated, quasi-2D Seff =1/2 honeycomb lattice material which resists long range magnetic order and spin freezing.
