Two-magnon bound state in fcc ferromagnets

We show that an optically active two-magnon bound state exists for fcc Heisenberg ferromagnets. There is a repulsive interaction between magnons with equal and opposite wave vectors near the zone edge which in most lattices does not lead to any dramatic effects. However, in the fcc lattice there is a logarithmic divergence in the density of states at the zone boundary, which causes a bound state to split off from above the band. We discuss the possibility of observing such a state experimentally by Raman scattering, possibly in EuO.