Face-centered cubic supra-crystals and disordered three-dimensional assemblies of 7.5 nm cobalt nanocrystals: Influence of the mesoscopic ordering on the magnetic properties

Previously, we have shown that we can control the mesoscopic order in three-dimensional (3D) assemblies of 7.5 nm Co nanocrystals to give either face-centered cubic (fcc) supra-crystals or disordered aggregates. These 3D assemblies provide an intermediate between the atomic and the bulk state and have the potential to exhibit many interesting new properties. In this paper we compare the magnetic properties of well-characterized disordered and fcc ordered 3D assemblies and find that there are significant differences arising from the mesoscopic order. We propose that this is due to the difference in anisotropy and distribution of dipolar interaction energies in the two systems and that this represents a new intrinsic property of magnetic supra-crystals.