COLLECTIVE EXCITATIONS IN PARAMAGNETS - CONFRONTATION OF THEORETICAL AND EXPERIMENTAL RESULTS FOR GADOLINIUM

Recent high-resolution data for the spin response function S(k, omega) of gadolinium at T greater-than-or-equal-to 1.5 T(c), obtained by neutron scattering, is compared with results derived from the coupled-mode theory of a Heisenberg exchange interaction on an HCP lattice. A well-defined collective mode is obtained in a calculation which uses the long-range interaction derived from an analysis of the low temperature spin-wave dispersions. However, the peak position is at a much lower energy than the corresponding structure in the data, even though the estimated T(c) agrees very well with the experimental value. Good overall agreement between theoretical and experimental results is found by using a nearest neighbour exchange deduced from T(c) and the measured second-frequency moment.