Grain-size dependence of intergranular magnetic correlations in nanostructured metals

Magnetic small-angle neutron scattering experiments were performed on nanostructured Fe, Co and Ni samples of varying grain size, produced by inert-gas condensation. The experiments show that the spontaneous spatial magnetic correlations forming in zero-field extend over many individual grains. These correlations depend strongly on grain size. In Fe, they have a minimum at a grain size of around 35 nm and increase sharply for smaller grain sizes. The crossover occurs at grain sizes comparable with L_crit = πδ, where δ is the bulk domain-wall width. For grain sizes below L_crit, the results are explained on the basis of the random-anisotropy model, which takes into account that the magnetic alignment between the grains competes with the anisotropies of the individual grains. Above L_crit, where domain walls can form within one grain, the magnetization direction corresponds to the anisotropy direction varying from grain to grain, and the magnetic correlation length increases approximately linearly with grain size. Furthermore, the measurements on Fe, Co and Ni show that the spatial magnetic correlations measured by SANS are always larger than L_crit. This is in agreement with results of theoretical studies showing that nonuniform magnetization configurations can only exist in grains with sizes D > L_crit.