  The inelastic scattering of neutrons from magnetic excitations in the
antiferromagnetic phase of the langasite compound Ba3NbFe3Si2O14 is analyzed
theoretically. In the calculations presented, the strongly coupled spin-5/2 Fe
triangles are accounted for as trimerized units. The weaker interactions
between the trimers are included within the mean-field/random-phase
approximation. The theory is compared with linear spin-wave theory, and a model
is developed which leads to good agreement with the published results from
unpolarized and polarized neutron-scattering experiments.
