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 $sim$ 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 behavior at 300 K. Comparative Mossbauer spectroscopic studies imply that intra-cluster interactions come into play. A 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 suggest that a superspin glass state settles-in at T$_{B}$ $sim$ 160-200 K, while at lower-temperatures, surface spin-glass freezing is established at T$_{f}$ $sim$40- 70 K. In such nanoparticle-assembled systems, with increased $phi$, 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.