  In this paper we summarize the effects of magnetic and nonmagnetic impurities
on the spin dynamics in Febased superconductors and their parent compounds. The
effects of chemical substitution, vacancies, and disorder on the suppression or
stabilization of superconductivity and spin-density-wave phases are reviewed in
the context of recent neutron-spectroscopy measurements of spin excitations. We
also present new results on the structure of magnetic fluctuations in BaFe2As2
single crystals doped with Mn local moments and discuss them in relationship to
the previously reported (pi, pi) branch of checkerboard magnetic excitations.
