9 research outputs found
Colloidal Assemblies of Oriented Maghemite Nanocrystals and their NMR Relaxometric Properties
Elevated-temperature polyol-based colloidal-chemistry approach allows for the
development of size-tunable (50 and 86 nm) assemblies of maghemite iso-oriented
nanocrystals, with enhanced magnetization. 1H-Nuclear Magnetic Resonance (NMR)
relaxometric experiments show that the ferrimagnetic cluster-like colloidal
entities exhibit a remarkable enhancement (4 to 5 times) in the transverse
relaxivity, if compared to that of the superparamagnetic contrast agent
Endorem, over an extended frequency range (1-60 MHz). The marked increase of
the transverse relaxivity r2 at a clinical magnetic field strength (1.41 T),
which is 405.1 and 508.3 mM-1 s-1 for small and large assemblies respectively,
allows to relate the observed response to the raised intra-aggregate magnetic
material volume fraction. Furthermore, cell tests with murine fibroblast
culture medium confirmed the cell viability in presence of the clusters. We
discuss the NMR dispersion profiles on the basis of relaxivity models to
highlight the magneto-structural characteristics of the materials for improved
T2-weighted magnetic resonance images.Comment: Includes supporting informatio
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 effects are
examined against the increasing volume fraction, , of the inorganic
magnetic phase that goes from individual colloidal nanoparticles (= 0.47)
to clusters (= 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 ''(T), with strong frequency dispersion. Scaling-law
analysis, together with the observed memory effects suggest that a superspin
glass state settles-in at T 160-200 K, while at
lower-temperatures, surface spin-glass freezing is established at T
40- 70 K. In such nanoparticle-assembled systems, with increased ,
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
Vacancy-Driven Noncubic Local Structure and Magnetic Anisotropy Tailoring in FeₓO-Fe₃-{δ}_O₄ Nanocrystals
In contrast to bulk materials, nanoscale crystal growth is critically influenced by size- and shape-dependent properties. However, it is challenging to decipher how stoichiometry, in the realm of mixed-valence elements, can act to control physical properties, especially when complex bonding is implicated by short- and long-range ordering of structural defects. Here, solution-grown iron-oxide nanocrystals (NCs) of the pilot wüstite system are found to convert into iron-deficient rock-salt and ferro-spinel subdomains but attain a surprising tetragonally distorted local structure. Cationic vacancies within chemically uniform NCs are portrayed as the parameter to tweak the underlying properties. These lattice imperfections are shown to produce local exchange-anisotropy fields that reinforce the nanoparticles’ magnetization and overcome the influence of finite-size effects. The concept of atomic-scale defect control in subcritical-size NCs aspires to become a pathway to tailor-made properties with improved performance for hyperthermia heating over defect-free NCs
Vacancy-Driven Noncubic Local Structure and Magnetic Anisotropy Tailoring in Fe_{x}O-Fe_{3−δ}O_{4} Nanocrystals
In contrast to bulk materials, nanoscale crystal growth is critically influenced by size- and shape-dependent properties. However, it is challenging to decipher how stoichiometry, in the realm of mixed-valence elements, can act to control physical properties, especially when complex bonding is implicated by short- and long-range ordering of structural defects. Here, solution-grown iron-oxide nanocrystals (NCs) of the pilot wüstite system are found to convert into iron-deficient rock-salt and ferro-spinel subdomains but attain a surprising tetragonally distorted local structure. Cationic vacancies within chemically uniform NCs are portrayed as the parameter to tweak the underlying properties. These lattice imperfections are shown to produce local exchange-anisotropy fields that reinforce the nanoparticles’ magnetization and overcome the influence of finite-size effects. The concept of atomic-scale defect control in subcritical-size NCs aspires to become a pathway to tailor-made properties with improved performance for hyperthermia heating over defect-free NCs