LONGITUDINAL SPIN FLUCTUATIONS IN NICKEL

The longitudinal, transverse, and paramagnetic spin fluctuations in Ni have been measured near T(c) by means of polarized neutron scattering in the momentum range 0.06 < q < 0.18 angstrom-1. In transverse scans, spin-wave peaks at omega-q = Dq2 appear as expected from previous measurements performed with unpolarized neutrons. The longitudinal fluctuations are quasielastic, in agreement with predictions of a recent mode-mode coupling theory and renormalization-group theory. The data indicate that the longitudinal dynamical scaling function is smaller than 1 just below T(c). The scaling function for the paramagnetic scattering is shown to be in agreement with the Resibois-Piette scaling function for energy scales up to k(B)T(C). The measured field dependence of the scattering is rather weak, the indicating that the internal fields H are rather large. Therefore it was impossible to observe the H-1/2 divergence of chi-L(q --> 0) that is predicted for the isotropic Heisenberg model in three dimensions. In contrast, we found chi-L(q --> 0) is proportional to (1-T/T(c))-y, which also appears to be a result of the internal fields. The q dependence of chi-L) is Lorentzian-like. The T dependence of the correlation length indicates critical behavior. These features closely resemble the behavior of the paramagnetic fluctuations, and they are in agreement with results obtained with use of expansion techniques.