Short-range order in the quantum XXZ honeycomb lattice material BaCo2(PO4)2!

We present observations of highly frustrated quasi-two-dimensional (2D) magnetic correlations in the honeycomb lattice layers of the S-eff = 1/2 compound. gamma-BaCo2(PO4)(2) (gamma-BCPO). Specific heat shows a broad peak comprised of two weak kink features at T-N1 similar to 6 K and T-N2 similar to 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 T-N1 and T-N2, at which two separate, incompatible, short range magnetic orders onset: commensurate antiferromagnetic correlations with correlation length xi(c) = 60 +/- 2 angstrom (T-N1) and in quasi-2D helical domains with xi(h) = 350 +/- 11 angstrom (T-N2). The ac magnetic susceptibility response lacks frequency dependence, ruling out spin freezing. Inelastic neutron scattering data on gamma-BCPO is compared with linear spin wave theory, and two separate parameter regions of the XXZ J(1)-J(2)-J(3) model with ferromagnetic nearest-neighbor exchange J(1) are favored, both near regions of high classical degeneracy. High energy coherent excitations (similar to 10 meV) persist up to at least 40 K, suggesting strong in-plane correlations persist above T-N. These data show that gamma-BCPO is a rare highly frustrated, quasi-2D S-eff = 1/2 honeycomb lattice material which resists long range magnetic order and spin freezing.