Crossover in the magnetic properties of nanostructured metals

Magnetization measurements and small-angle neutron scattering (SANS) were performed on nanostructured Fe, Co and Ni samples, produced by inert-gas condensation. The grain size, which is 10–20 nm in the as-prepared state, was increased incrementally up to 100 nm by thermal annealing. The coercive field shows a pronounced variation with grain size. In Fe, it passes a maximum at around 35 nm and shows a steep decrease towards smaller grain sizes. Small-angle neutron scattering experiments show that magnetic correlations which extend over several grains form spontaneously in zero-field. The correlation length depends strongly on grain size and has in Fe a minimum at around 35 nm. We explain the results at low grain sizes on the basis of an extended random-anisotropy model. Within this model, we take into account that the intergrain exchange coupling constant is smaller than the intragrain exchange constant. We further demonstrate that the random-anisotropy model will break down for grain sizes exceeding a critical value Lcrit=πδ, with δ approximately the bulk domain-wall width. For grain sizes above Lcrit, the coercive field follows a 1/D behavior and the magnetic correlation length increases approximately linearly with grain size, i.e. the magnetic properties approach those of coarse-grained ferromagnetic materials.