Simulation of spin diffusion in a disordered system

We simulated the effect of dilution on the transport of magnetization between classical gyromagnets, coupled by a dipole-dipole interaction in a high external magnetic field, on a simple cubic lattice for a system with quenched disorder. We studied the dependence of the spin-diffusion coefficient on the concentration of dipoles on the lattice for various system sizes. For small concentrations we find that the diffusion coefficient depends linearly on concentration. We extrapolated the simulation results to the thermodynamic limit, i.e., to the limit of large system sizes and wavelengths. We also tested the consequences of a strong exponential cutoff of the dipole interactions. In this case our results show indications of a percolation transition at a finite value of the concentration. {} 1996 The American Physical Society.