Abstract
The magnetic excitations in the paramagnetic-tetragonal phase of underdoped
Ba(Fe0.953Co0.047)2As2, as measured by inelastic neutron scattering, can be well described
by a phenomenological model with purely di usive spin dynamics. At low energies, the spec-
trum around the magnetic ordering vector Q_AFM consists of a single peak with elliptical shape
in momentum space. At high energies, this inelastic peak is split into two peaks across the
direction perpendicular to Q_AFM. We use our fittings to argue that such a splitting is not due
to incommensurability or propagating spin-wave excitations, but is rather a consequence of the
anisotropies in the Landau damping and in the magnetic correlation length, both of which are
allowed by the tetragonal symmetry of the system. We also measure the magnetic spectrum deep
inside the magnetically-ordered phase, and find that it is remarkably similar to the spectrum of the
paramagnetic phase, revealing the strongly overdamped character of the magnetic excitations.
