Temporal Fluctuations of Moment in Paramagnets by Neutron Diffraction

The inelastic scattering of neutrons by temporal fluctuations of spin in a paramagnet is reviewed. The scattering cross section is directly proportional to the imaginary part of the generalized frequency and wavelength-dependent susceptibility of the scattering material. For localized spins the theory of the scattering is discussed both in terms of exact solutions for the susceptibility in the few cases where this is known and in terms of the moments of the susceptibility. Experimentally the technique provides good measurements of exchange constants at high temperatures for insulators and for rare earth metals where the magnetic electrons are localized. In transition metals the form of the susceptibility is modified by the itinerance of the magnetic electrons and this is reflected in the experimental data. Exchange and correlation effects will enhance the low-frequency component of the susceptibility over that predicted for a noninteracting electron gas. The experimental data available appear to confirm the existence of such enhancement effects.