  The linear spin-wave theory study of PbFeBO4 spin dynamics is presented. It
is shown that the modes observed in Raman scattering experiments below Neel
temperature are optical magnon and two-magnon excitations. Based on the magnon
energy, two-magnon band lineshape, and Weiss temperature, the consistent set of
the exchange coupling constants up the third neighbor is derived and compared
with the results of ab initio calculations. The small deviation of the observed
two-magnon band from the one-magnon density of states suggests a surprisingly
negligible role of magnon-magnon interactions.
