  We present observations of highly frustrated quasi two-dimensional (2D)
magnetic correlations in the honeycomb lattice layers of the S$_{eff}$ = 1/2
compound $\gamma$-BaCo$_2$(PO$_4$)$_2$ ($\gamma$-BCPO). Specific heat shows a
broad peak comprised of two weak kink features at $T_{N1} \sim$ 6 K and $T_{N2}
\sim$ 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 $\xi_c = 60\pm2$ \AA \ ($T_{N1}$) and in
quasi-2D helical domains with $\xi_h = 350 \pm 11$ \AA \ ($T_{N2}$). 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 ($\sim 10$ 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.
