Abstract
The spin dynamics of the semiclassical Heisenberg model on the fcc lattice, with ferromagnetic interactions up to the second-nearest-neighbor shell, is studied in the paramagnetic phase at the temperatures 1.68T/sub c/ and 2.0T/sub c/ using the Monte Carlo--molecular-dynamics technique. The important quantities calculated are the dynamic structure function S(q,..omega..), the spin autocorrelation function , and the static correlation functions. Our results for S(q,..omega..) show the existence of purely diffusive modes in the low-q regime. For q close to the zone boundary, our calculated S(q,..omega..) shows a three-peak structure, signifying damped propagating modes. This result disagrees with the earlier experimental observation of Mook and the theoretical result of Lindgard, where a two-peak structure was obtained near the zone boundary. Our results for S(q,..omega..) in the entire q space are in good qualitative and quantitative agreement with the recent neutron scattering experiments by Boeni and Shirane and also with the predictions of Young and Shastry. Our calculated autocorrelation function shows a diffusive behavior temporally.
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