Control of the supercrystallinity, nanocrystallinity, morphology and magnetism of cobalt nanoparticle assemblies. Towards novel colloidal crystals with high magnetic anisotropy

Abstract

Starting from a colloidal solution of dodecanoic acid-coated Co nanoparticles (NP) of 8.1 nm in size and of low anisotropy and using a solvent-mediated ligand-ligand interaction strategy, we obtain either colloidal crystals or supercrystalline films. The comparison between the structural and magnetic properties of these two different architectures of supercrystals evidences improved magnetic properties for the colloidal crystals. This is interpreted as a result of both an increase in the mesoscopic coherence length and a decrease in the interparticle distance. In this work we extend this comparative study between the two types of supercrystals after annealing treatment. A common batch of NP is used for the native and the annealed samples in order to optimize the accuracy of the comparison. A complete set of structural characterizations and magnetic measurements shows that upon annealing the NP evolve from soft to hard magnetic NP, with changes of both the anisotropy constant and saturation mangnetization, while the supercrystal structures remain unchanged. Both native and annealed colloidal crystals remain characterized by a smaller interparticle distance compared to the corresponding films, inducing higher dipolar interactions, which is evidenced by an increase in the blocking temperature. In this study, we highlight the first fcc colloidal crystals made of hcp-Co single crystals resulting from the annealing of their native counterparts made of fcc-Co polycrystals. In addition, the comparative magnetic study between both the annealed supercrystalline films and colloidal crystals evidences enhanced magnetic properties for the latter ones.