Magnetic correlations of fine ferromagnetic particles studied by small-angle neutron scattering

We have performed small-angle x-ray and neutron scattering of fine Fe particles embedded in an alumina matrix. For the sample with the lowest Fe content (volumic fraction 20%), x-ray scattering reveals a well-defined peak, characterizing a liquidlike short-range order between particles of similar size, with a chemical radius of 10 Angstrom. The magnetic radius of the particles and their magnetization, as measured by neutron scattering, increases with decreasing temperature, due to the alignment of spins at the surface. Below 100 K, ferromagnetic correlations start to develop between near-neighbor particles. For higher particle densities (volumic fraction 35% and 47%), the nanoparticles coexist with larger aggregates, yielding two typical particle sizes in the same sample. Here, the interparticle correlations persist up to the highest measured temperature (500 K). Such correlations could arise from the dipolar field, which increases with decreasing temperature, as observed by inelastic neutron scattering.