title: Assembly-mediated interplay of dipolar interactions and surface spin disorder in colloidal maghemite nanoclusters.
abstract: 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 effects are examined against the increasing volume fraction, φ, of the inorganic magnetic phase that goes from individual colloidal nanoparticles (φ = 0.47) to clusters (φ = 0.72). The frozen-liquid dispersions of the latter exhibit weak ferrimagnetic behaviour at 300 K. Comparative Mössbauer 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 χ''(T), with strong frequency dispersion. Scaling-law analysis together with the observed memory effects suggests that a superspin-glass state settles-in at TB ∼ 160-200 K, while at lower-temperatures, surface spin-glass freezing is established at Tf ∼ 40-70 K. In such nanoparticle-assembled systems, with increased φ, 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.
authors.affiliation: Institute of Electronic Structure and Laser, Foundation for Research and Technology-Hellas, Vassilika Vouton, Heraklion 71110, Greece. lappas@iesl.forth.gr.
authors.forename: A
authors.initials: A
authors.lastname: Kostopoulou
authors.forename: K
authors.initials: K
authors.lastname: Brintakis
authors.forename: M
authors.initials: M
authors.lastname: Vasilakaki
authors.forename: K N
authors.initials: KN
authors.lastname: Trohidou
authors.forename: A P
authors.initials: AP
authors.lastname: Douvalis
authors.forename: A
authors.initials: A
authors.lastname: Lascialfari
authors.forename: L
authors.initials: L
authors.lastname: Manna
authors.forename: A
authors.initials: A
authors.lastname: Lappas