  Electron spin resonance measurements in EuFe2As2 single crystals revealed an
absorption spectrum of a single resonance with Dysonian lineshape. Above the
spin-density wave transition at T_SDW = 190 K the spectra are isotropic and the
spin relaxation is strongly coupled to the CEs resulting in a Korringa-like
increase of the linewidth. Below T_SDW, a distinct anisotropy develops and the
relaxation behavior of the Eu spins changes drastically into one with
characteristic properties of a magnetic insulating system, where dipolar and
crystal-field interactions dominate. This indicates a spatial confinement of
the conduction electrons to the FeAs layers in the SDW state.
