Singlet-triplet dispersion reveals additional frustration in the   triangular dimer compound Ba$_3$Mn$_2$O$_8$
  We present single crystal inelastic neutron scattering measurements of the S=1 dimerized quasi-two-dimensional antiferromagnet Ba$_3$Mn$_2$O$_8$. The singlet-triplet dispersion reveals nearest-neighbor and next-nearest-neighbor ferromagnetic interactions between adjacent bilayers that compete against each other. Although the inter-bilayer exchange is comparable to the intra-bilayer exchange, this additional frustration reduces the effective coupling along the c-axis and leads to a quasi-two dimensional behavior. In addition, the obtained exchange values are able to reproduce the four critical fields in the phase diagram. 
M. B. Stone, M. D. Lumsden, S. Chang, E. C. Samulon, C. D. Batista, I.   R. Fisher