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 13 nm ( d ) crystallographically oriented nanoparticles. The interaction e ff ects are examined against the increasing volume fraction, 4 , of the inorganic magnetic phase that goes from individual colloidal nanoparticles ( 4 ¼ 0.47) to clusters ( 4 ¼ 0.72). The frozen-liquid dispersions of the latter exhibit weak ferrimagnetic behaviour at 300 K. Comparative M ̈ ossbauer 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 c 00 ( T ), with strong frequency dispersion. Scaling-law analysis together with the observed memory e ff ects suggests that a superspin-glass state settles-in at T B 160 – 200 K, while at lower-temperatures, surface spin-glass freezing is established at T f 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 spin- glass dynamics.