Van der Waals versus dipolar forces controlling mesoscopic organizations of magnetic nanocrystals.

The structure, thermodynamics and dynamics in many physical and chemical systems are determined by interplay of short-range isotropic and long-range anisotropic forces. Magnetic nanoparticles dispersed in solution are ideal model systems to study this interplay, as they are subjected to both isotropic van der Waals and anisotropic dipolar forces. Here we show from experiment an abrupt transition of maghemite nanocrystal organization from chain-like to random structures when nanoparticle solutions are evaporated under a magnetic field. This is explained by brownian dynamics simulations in terms of a variation of the strength of van der Waals interactions with the particle contact distance, which is tuned by the length of the molecules coating the particles. The weak dipole-dipole interactions between the maghemite particles are usually not sufficient to result in the chain formation observed here. However, due to the van der Waals interactions, when the nanocrystal contact distance is short enough, clusters of nanocrystals are formed during the evaporation process. These clusters exhibit large dipole moments compared with a single particle, which explains the formation of chain-like structures. Conversely, when the nanocrystal contact distance is too long, no nanocrystal aggregation occurs, and a random distribution of maghemite nanocrystals is obtained.