Crystallization and Magnetic Properties of 3D Micrometer-Scale Simple-Cubic Maghemite Superlattices

A reproducible method is proposed for the crystallization of three-dimensional (3D), micrometer-scale, simple-cubic superlattices (SCSLs) via the evaporation of a dispersion of 14.7 nm maghemite nanocubes in hexane. It is shown that the formation of the maghemite SCSLs is critically dependent on the presence of excess oleic acid (OA) in the nanocube dispersion. Specifically, the dynamic light-scattering results show that, given a relative concentration of approximately 15-20 vol %, the OA acts as a mild destabilization agent and prompts the homogeneous nucleation and growth of maghemite SCSLs. The magnetic properties of the SCSLs are investigated and compared with those of isolated maghemite nanocubes diluted in paraffin. It is shown that the coercivity force (Hc) and remanence ratio (M-r/M-s) of the 3D SCSLs at a temperature of 5 K are significantly lower than those of the isolated nanocubes at the same temperature. The magnetostatic energy-to-anisotropy energy (E-ms/E-K) ratio of the SCSLs is around 0.47. Thus, it is inferred that the reduction in the Hc and M-r/M-s properties of the SCSLs is the result primarily of assembling effects.