Beyond Simple Bilayers in the Triangular Lattice Dimer compound Ba$_{3}$Mn$_{2}$O$_{8}$

We present single crystal inelastic neutron scattering measurements of the $S=1$ dimerized quasi-twodimensional antiferromagnet Ba$_{3}$Mn$_{2}$O$_{8}$. The singlet-triplet dispersion reveals nearest-neighbor and nextnearest- neighbor ferromagnetic interactions between adjacent bilayers that compete against each other. Although the interbilayer exchange is comparable to the intrabilayer exchange, this additional frustration reduces the effective coupling along the $c$ axis and leads to a quasi-two-dimensional behavior. We also present magnetic field-dependent neutron scattering measurements examining long range order and excitations within the field-induced ordered phases in Ba$_{3}$Mn$_{2}$O$_{8}$.