Dispersive magnetic excitations in the S=1 antiferromagnet   Ba$_3$Mn$_2$O$_8$
  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 $\Delta \approx 1.0$ meV and a dispersive spectrum with a bandwidth of approximately 1.5 meV. Comparison to coupled dimer models describe the dispersion and scattering intensity accurately and determine the exchange constants in Ba$_3$Mn$_2$O$_8$. 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. 
M. B. Stone, M. D. Lumsden, Y. Qiu, E. C. Samulon, C. D. Batista, I.   R. Fisher