  The spin fluctuation spectra from nonsuperconducting Cu-substituted, and
superconducting Co-substituted, BaFe2As2 are compared quantitatively by
inelastic neutron scattering measurements and are found to be indis-
tinguishable. Whereas diffraction studies show the appearance of incommensurate
spin-density wave order in Co and Ni substituted samples, the magnetic phase
diagram for Cu substitution does not display incommensu- rate order,
demonstrating that simple electron counting based on rigid-band concepts is
invalid. These results, supported by theoretical calculations, suggest that
substitutional impurity effects in the Fe plane play a signifi- cant role in
controlling magnetism and the appearance of superconductivity, with Cu
distinguished by enhanced impurity scattering and split-band behavior.
