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 lowes t Fe content (volumic fraction 20%), x-ray scattering reveals a well-d efined peak, characterizing a liquidlike short-range order between par ticles of similar size, with a chemical radius of 10 Angstrom. The mag netic 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 correlat ions start to develop between near-neighbor particles. For higher part icle densities (volumic fraction 35% and 47%), the nanoparticles coexi st with larger aggregates, yielding two typical particle sizes in the same sample. Here, the interparticle correlations persist up to the hi ghest measured temperature (500 K). Such correlations could arise from the dipolar field, which increases with decreasing temperature, as ob served by inelastic neutron scattering.