Assembly-mediated interplay of dipolar interactions and surface spin disorder in colloidal maghemite nanoclusters

Controlled assembly of single-crystal, colloidal maghemite nanoparticles is facilitated via a high-temperature polyol-based pathway. Structural characterization shows that size-tunable nanoclusters of 50 and 86 nm diameters (D), with high dispersibility in aqueous media, are composed of similar to 13 nm (d) crystallographically oriented nanoparticles. The interaction effects are examined against the increasing volume fraction, phi, of the inorganic magnetic phase that goes from individual colloidal nanoparticles (phi = 0.47) to clusters (phi = 0.72). The frozen-liquid dispersions of the latter exhibit weak ferrimagnetic behaviour at 300 K. Comparative Mossbauer spectroscopic studies imply that intra-cluster interactions come into play. New insight emerges from the clusters' temperature-dependent ac susceptibility that displays two maxima in chi ''(T), with strong frequency dispersion. Scaling-law analysis together with the observed memory effects suggests that a superspin-glass state settles-in at T-B similar to 160-200 K, while at lower-temperatures, surface spin-glass freezing is established at T-f similar to 40-70 K. In such nanoparticle-assembled systems, with increased 4, Monte Carlo simulations corroborate the role of the inter-particle dipolar interactions and that of the constituent nanoparticles' surface spin disorder in the emerging spinglass dynamics.