Abstract
The sixth frequency moment of the frequency-wavevector dependent relaxation-shape function F(k, omega ), at elevated temperatures is presented for a model applicable to RbMnF3. The exact value of this moment is compared to the approximate values predicted by the semi-phenomenological representations of the relaxation-shape function based on a knowledge of only the second and fourth moments. Finally, a Gaussian choice for the memory function associated with the spin-velocity relaxation function is made which preserves all the frequency moments of F(k, omega ), up to and including the sixth. The form of F(k, omega ) obtained with this memory function is compared to the first principle calculations of Blume and Hubbard and of Reiter. The value of the diffusion constant is found to be very close to that given by Mori and Kawasaki and by Bennett and Martin.
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