Spectra of paramagnetic spin fluctuations in insulating and metallic ferromagnets, obtained by inelastic neutron scattering, are interpreted in terms of appropriate Heisenberg models. A mode coupling approximation is used which takes full account of the lattice structure and the extended range of the exchange interaction, the form of which is obtained from the interpretation of spin wave spectra measured in the ordered state. The favourable outcome of confronting theoretical spectra with experimental data for EuO, Pd 2 MnSn and Fe leads the authors to conclude that the mode coupling theory provides a more than adequate description of paramagnetic spin fluctuations. The dependence of the spectra on an increasing neutron wave vector is a change from a Lorentzian shape to a distinctly squared-up shape, but there is no strong evidence in any of the three systems of a damped collective mode contribution even for zone boundary wavevectors.
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Cuccoli et al. (1990) studied this question.
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