Grain-size dependence of intergranular magnetic correlations in nanostructured metals

Magnetic small-angle neutron scattering experiments were performed on nanos tructured Fe, Co and Ni samples of varying grain size, produced by inert-ga s condensation. The experiments show that the spontaneous spatial magnetic correlations forming in zero-field extend over many individual grains. Thes e 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 = pi delta, whe re delta is the bulk domain-wall width. For grain sizes below L-crit, the r esults are explained on the basis of the random-anisotropy model, which tak es into account that the magnetic alignment between the grains competes wit h the anisotropies of the individual grains. Above L-crit, where domain wal ls can form within one grain, the magnetization direction corresponds to th e anisotropy direction varying from grain to grain, and the magnetic correl ation length increases approximately linearly with grain size. Furthermore, the measurements on Fe, Co and Ni show that the spatial magnetic correlati ons measured by SANS are always larger than L-crit. This is in agreement wi th results of theoretical studies showing that nonuniform magnetization con figurations can only exist in grains with sizes D > L-crit.