Time-dependent spin-autocorrelation functions at T=∞ for classical Heisenberg magnets in dimensionalities d=1, 2, and 3 are investigated by means of a computer simulation. These functions are shown to exhibit power-law long-time tails of form t−αd with characteristic exponents αd which differ significantly from the values αd(SD)=d/2 predicted by the phenomenological spin-diffusion theory: α1=0.609±0.005, α2=1.050±0.025, α3≃1.6. The method to employ computer simulations for this problem differs from methods previously employed. Anomalous spin diffusion is confirmed by existing proton spin-lattice relaxation data for the quasi-1D s=5/2 Heisenberg antiferromagnet (CH3)4NMnCl3.
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