Spin dynamics in Ni nearTC: A high-resolution neutron scattering study

We have studied the magnetic cross section in Ni below and above $T_C$ using unpolarized and polarized neutron scattering techniques. For temperatures near the Curie temperature ($T_C$=631 K) and for a range of momentum transfer 0.2<q<0.4 A${̊}^{\mathrm{-}1}$ (Brillouin zone width≃1.54 A${̊}^{\mathrm{-}1}$), the spin waves renormalize as expected from extrapolating the results at smaller momentum transfer (q<0.2 A${̊}^{\mathrm{-}1}$). At $T_C$ the spin waves merge into a broad single peak centered at zero-energy transfer, which is well described by a double-Lorentzian scattering function. Our results, below and above $T_C$, agree with the predictions of the dynamic scaling theory in the area of (q,ω) space and the range of temperatures investigated. This is in contrast with recent measurements which reported magnetic excitations at finite energies above $T_C$.