  The spin dynamics in single crystal, electron-doped Ba(Fe1-xCox)2As2 has been
investigated by inelastic neutron scattering over the full range from undoped
to the overdoped regime. We observe damped magnetic fluctuations in the normal
state of the optimally doped compound (x=0.06) that share a remarkable
similarity with those in the paramagnetic state of the parent compound (x=0).
In the overdoped superconducting compound (x=0.14), magnetic excitations show a
gap-like behavior, possibly related to a topological change in the hole Fermi
surface (Lifshitz transition), while the imaginary part of the spin
susceptibility prominently resembles that of the overdoped cuprates. For the
heavily overdoped, non-superconducting compound (x=0.24) the magnetic
scattering disappears, which could be attributed to the absence of a hole
Fermi-surface pocket observed by photoemission.
