Abstract
We present powder inelastic neutron scattering measurements of the $S=1$ dimerized antiferromagnet Ba$_3$Mn$_2$O$_8$. The $T=1.4$~K~magnetic spectrum exhibits a spin-gap of $\Delta = 0.9$~meV and a dispersive spectrum with a bandwidth of approximately $1.74$~meV. Comparison to coupled dimer models describe the dispersion and scattering intensity accurately and determine the exchange constants in Ba$_3$Mn$_2$O$_8$. Interdimer exchange between triangular lattice dimer bilayers in Ba$_3$Mn$_2$O$_8$ is two orders of magnitude weaker than intradimer exchange. Ba$_3$Mn$_2$O$_8$ is thus a
quasi-two-dimensional frustrated bilayer triangular lattice of $S=1$ dimers.
The wave vector dependent scattering intensity confirms the proposed $S=1$ dimer bond. Temperature dependent measurements of the magnetic excitations indicate the presence of both singlet-triplet and thermally activated
triplet-quintet excitations.
