  A formalism is developed to treat magnetic impurities in a Mott-Hubbard
antiferromagnetic insulator within a representation involving multiple orbitals
per site. Impurity scattering of magnons is found to be strong when the number
of orbitals N' on impurity sites is different from the number N on host sites.
The impurity-scattering-induced softening of magnon modes leads to enhancement
in thermal excitation of magnons, and hence to a lowering of the Neel
temperature in layered or three dimensional systems. Weak impurity scattering
of magnons is obtained in the case N'=N, where the impurity is represented in
terms of modified hopping strength, and a momentum-independent, multiplicative
renormalization of magnon energies is obtained. Split-off magnon modes are
obtained when the impurity-host coupling is stronger, and implications are
discussed for two-magnon Raman scattering. The mapping between antiferromagnets
and superconductors is utilized to contrast formation of impurity-induced
states.
