Magnetization and AC susceptibility measurements have been performed on ∼3 nm NiO nanoparticles in powder form. The results indicate that the structure of the particles can be considered as consisting of an antiferromagnetically ordered core, with an uncompensated magnetic moment, and a magnetically disordered surface shell. The core magnetic moments block progressively with decreasing temperature, according to the distribution of their anisotropy energy barriers, as shown by a broad maximum of the low field zero-field-cooled magnetization (M(ZFC)) and in the in-phase component χ' of the AC susceptibility, centred at ∼70 K. On the other hand, surface spins thermally fluctuate and freeze in a disordered spin-glass-like state at much lower temperature, as shown by a peak in M(ZFC) (at 17 K, for H = 50 Oe) and in χ'. The temperature of the high temperature χ' peak changes with frequency according to the Arrhenius law; instead, for the low temperature maximum a power law dependence of the relaxation time was found, τ = τ(0)(T(g)/(T(ν)-T(g)))(α), where α = 8, like in spin glasses, τ(0) = 10(-12) s and T(g) = 15.9 K. The low temperature surface spin freezing is accompanied by a strong enhancement of magnetic anisotropy, as shown by the rapid increase of coercivity and high field susceptibility. Monte Carlo simulations for core/shell antiferromagnetic particles, with an antiferromagnetic core and a disordered shell, reproduce the qualitative behaviour of the temperature dependence of the coercivity. Interparticle interactions lead to a shift to a high temperature of the distribution of the core moment blocking temperature and to a reduction of magnetization dynamics.

We have used the Monte Carlo simulation technique to investigate the effect of a ferrimagnetic disordered shell on the hysteresis behavior of composite magnetic nanoparticles with ferromagnetic core/ferrimagnetic disordered shell morphology. We have examined the cooling field $\left(H_{\text{cool}}\right)$ dependence of the exchange bias $\left(H_{\text{ex}}\right)$, the coercive field $\left(H_c\right)$, and the remanent magnetization $\left(M_r\right)$ and we find that an increase in $H_{\text{cool}}$ results initially in an increase in $H_{\text{ex}}$, $H_c$, and $M_r$, while further increase in $H_{\text{cool}}$ causes a reduction in $H_{\text{ex}}$ and $H_c$, but $M_r$ remains constant. Our simulations show that an increase in the shell thickness for a given core size enhances the exchange bias field and reduces the remanent magnetization, the vertical shift, and the training effect. We also find that a reduction in the core size for a given particle size results in an increase in both exchange bias and coercive field. This behavior is reversed as the temperature increases. Our results are in good agreement with experimental findings.

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 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 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.

The intimate relationship between stoichiometry and physicochemical properties in transition-metal oxides makes them appealing as tunable materials. These features become exacerbated when dealing with nanostructures. However, due to the complexity of nanoscale materials, establishing a distinct relationship between structure-morphology and functionalities is often complicated. In this regard, in the Fe_xO/Fe₃O₄ system a largely unexplained broad dispersion of magnetic properties has been observed. Here we show, thanks to a comprehensive multi-technique approach, a clear correlation between the magneto-structural properties in large (45 nm) and small (9 nm) Fe_xO/Fe₃O₄ core/shell nanoparticles that can explain the spread of magnetic behaviors. The results reveal that while the Fe_xO core in the large nanoparticles is antiferromagnetic and has bulk-like stoichiometry and unit-cell parameters, the Fe_xO core in the small particles is highly non-stoichiometric and strained, displaying no significant antiferromagnetism. These results highlight the importance of ample characterization to fully understand the properties of nanostructured metal oxides.