  We calculate the effective charge for multimagnon infrared (IR) absorption
assisted by phonons in a perovskite like antiferromagnet and we compute the
spectra for two magnon absorption using interacting spin-wave theory. The full
set of equations for the interacting two magnon problem is presented in the
random phase approximation for arbitrary total momentum of the magnon pair. The
spin wave theory results fit very well the primary peak of recent measured
bands in the parent insulating compounds of cuprate superconductors. The line
shape is explained as being due to the absorption of one phonon plus a new
quasiparticle excitation of the Heisenberg Hamiltonian that consists off a long
lived virtual bound state of two magnons (bimagnon). The bimagnon states have
well defined energy and momentum in a substantial portion of the Brillouin
zone. The higher energy bands are explained as one phonon plus higher
multimagnon absorption processes. Other possible experiments for observing
bimagnons are proposed. In addition we predict the line shape for the spin one
system La$_2$NiO$_4$.
