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.

A numerically solved two-level Stoner-Wohlfarth model with thermal agitation is used to simulate Zero Field Cooling (ZFC)–Field Cooling (FC) curves of monosize and polysize samples and to determine the best method for obtaining a representative blocking temperature T_B value of polysize samples. The results confirm a technique based on the T derivative of the difference between ZFC and FC curves proposed by Micha et al. (the good) and demonstrate its relation with two alternative methods: the ZFC maximum (the bad) and the inflection point (the ugly). The derivative method is then applied to experimental data, obtaining the T_B distribution of a polysize Fe₃O₄ nanoparticle sample suspended in hexane with an excellent agreement with TEM characterization.