Influence of short-range interactions on the mesoscopic organization of magnetic nanocrystals.

Magnetic fluids of 10-nm maghemite, gamma- Fe2 O3 , nanocrystals, subjected or not to an applied field parallel to the substrate, produce, after evaporation, mesoscopic structures. These differ markedly with the surface coating agent used to prevent particles from coalescence. Citrate ions and carboxylic acids with different chain lengths are employed as coating agents. The change in the mesoscopic structure is studied both experimentally and theoretically. The mesoscopic structures obtained by Brownian dynamics simulations are in good agreement with the experimental observations. In particular, the appearance of chainlike organizations in spite of the particles being weakly dipolar is explained by an interplay of van der Waals and magnetic dipolar interactions.