  We have performed Raman-scattering measurements on high-quality single
crystals of A$_{0.8}$Fe$_{1.6}$Se$_2$ superconductors of several compositions.
We find a broad, asymmetric peak around 1600 cm$^{-1}$ (200 meV), which we
identify as a two-magnon process involving optical magnons. The intensity of
the two-magnon peak falls sharply on entering the superconducting phase. This
effect, which is entirely absent in the non-superconducting system
KFe$_{1.5}$Se$_2$, requires a strong mutual exclusion between
antiferromagnetism and superconductivity arising from proximity effects within
regions of microscale phase separation.
