We present a detailed study of static and dynamic magnetic behavior of ${\mathrm{Fe}}_3{\mathrm{O}}_4$ nanoparticles with average particle sizes 〈d〉 ranging from 5 to 150 nm. Bulk-like properties such as saturation magnetization, hyperfine parameters, coercive field, and Verwey transition are observed in 150 nm particles. For decreasing particle size, the Verwey temperature, $T_V,$ shifts down to ∼20 K for $\text{〈d〉=50\hspace{0.17em}}\mathrm{nm}$ and is no longer observable for smaller particles. The smallest particles $\text{(〈d〉=5\hspace{0.17em}}\mathrm{nm})$ display superparamagnetic behavior at room temperature, with transition to a blocked state at $T_B\text{∼45\hspace{0.17em}}\mathrm{K},$ which depends on the applied field. The existence of surface spin disorder can be inferred from the decrease of saturation magnetization $M_S$ at low temperatures, as the average particle size is reduced. This disordered surface did not show effects of exchange coupling to the particle core, as observed from hysteresis loops after field cooling in a 7 T magnetic field. For particles with $\text{〈d〉=5\hspace{0.17em}}\mathrm{nm},$ dynamic ac susceptibility measurements show a thermally activated Arrhenius–Néel dependence of the blocking temperature with applied frequency. The interparticle interactions are found to influence the energy barriers yielding an enhancement of the estimated magnetic anisotropy. From the calculus of the magnetic anisotropy, it is inferred that there is no structural transition from cubic to triclinic symmetry for $\text{〈d〉=5\hspace{0.17em}}\mathrm{nm},$ in agreement with the absence of the Verwey transition. A value $K_1{\text{=4.68×10}}^5\hspace{0.17em}{\mathrm{erg/cm}}^3$ is obtained for the magnetocrystalline anisotropy constant of the cubic phase.

We have performed a detailed characterization of the magnetic properties of Ni nanoparticles embedded in a ${\mathrm{SiO}}_2$ amorphous matrix. A modified sol-gel method was employed which resulted in Ni particles with average radius $\sim 3\mathrm{nm},$ as inferred by TEM analysis. Above the blocking temperature $T_B\approx 20\mathrm{K}$ for the most diluted sample, magnetization data show the expected scaling of the ${M/M}_S$ vs $H/T$ curves for superparamagnetic particles. The hysteresis loops were found to be symmetric about zero field axis with no shift via exchange bias, suggesting that Ni particles are free from an oxide layer. For $T<\mathrm{}{T}_B$ the magnetic behavior of these Ni nanoparticles is in excellent agreement with the predictions of randomly oriented and noninteracting magnetic particles, as suggested by the temperature dependence of the coercivity field that obeys the relation $H_C{\mathrm{}\left(T\right)=H}_{C0\mathrm{}}\left[1-{(T/T}_B{)}^{1/2}\right]$ below $T_B$ with $H_{C0\mathrm{}}\sim 780\mathrm{Oe}.$ The obtained values of $H_{C0\mathrm{}},$ considering the temperature dependence of the magnetic anisotropy constant, are discussed within the scenario of isolated randomly oriented and noninteracting single-domain particles.