For pt.I see ibid., vol.9, no.7, p.1421 (1979). At low temperature the sperimagnetic amorphous alloy ErCo₂ gives rise to a very large small-angle neutron scattering of magnetic origin. Evidence has been given for a crossover in the diffusion pattern, with two different regimes: a k^-2 law below the crossover and a k^-3 law above it. These two regimes are explained by the nature of the magnetic correlations between the erbium moments in such a sperimagnetic alloy.