  Exchange interactions up to fourth nearest neighbor are shown within a
classical local-moment Heisenberg approach to be important to model inelastic
neutron scattering data on the fcc kagome antiferromagnet IrMn$_3$. Spin wave
frequencies are calculated using the torque equation and the magnetic
scattering function, $S({\bf Q},\omega)$, is determined by a Green's function
method, as an extension of our previous work, LeBlanc et al, Phys. Rev. B 90,
144403 (2014). Results are compared with intensity contour data on powder
samples of ordered IrMn$_3$, where magnetic Mn ions occupy lattice sites of ABC
stacked kagome planes. Values of exchange parameters taken from DFT
calculations used in our model provide good agreement with the experimental
results only if further-neighbor exchange is included. Estimates of the
observed energy gap support the existence of strong cubic anisotropy predicted
by DFT calculations.
