  Spin wave calculations demonstrate that the macroscopic continuous degeneracy
associated with the two-dimensional kagome Heisenberg spin lattice persists in
the case of the stacked fcc structure giving rise to zero energy modes in three
dimensions. The addition of an effective local cubic anisotropy is shown to
remove this continuous degeneracy and introduce a gap in the spectrum as well
as modify the inelastic scattering function $S({\bf q},\omega)$. This scenario
supports earlier Monte Carlo simulations which indicate that the phase
transition to long range $q=0$ magnetic order is driven to be discontinuous by
critical fluctuations associated with the large degeneracy in the absence of
anisotropy, but becomes continuous with the addition of anisotropy. The results
are relevant to Ir-Mn alloys which are widely used in the magnetic storage
industry in thin film form as the antiferromagnetic pinning layer in spin-valve
structures.
