Paramagnetic neutron scattering from the Heisenberg ferromagnet EuO

Paramagnetic neutron scattering from the insulating Heisenberg ferromagnet EuO has been studied through the entire Brillouin zone. The measured linewidth Γ of the quasielastic scattering at $T_c$ follows the predictions of dynamic scaling Γ=${\mathrm{Aq}}^{2.5}$ up to momentum transfers q=0.4 A${̊}^{\mathrm{-}1}$. The new results disagree with an early work by Passell et al., where a smaller exponent and smaller linewidths have been reported, but agree perfectly with recent spin-echo measurements performed by Mezei at very small q. The $q^{2.5}$ dependence of Γ is now established over nearly four decades in energy. At large momentum transfers the energy distribution evolves into a three-peaked structure with much more spectral weight near E=0, in disagreement with the two-peaked structure reported by Mook. The cross sections near the zone boundary are in quantitative agreement with recent calculations by Young and Shastry. We conjecture that peaks at finite energy occur in EuO and EuS because of the short-ranged exchange interactions.