We use a Monte Carlo approach to simulate the influence of the dipolar interaction on assemblies of monodisperse superparamagnetic $\gamma {\text{-Fe}}_2{\text{O}}_3$ nanoparticles. We have identified a critical concentration $c^{\ast }$, that marks the transition between two different regimes in the evolution of the blocking temperature $\left(T_B\right)$ with interparticle interactions. At low concentrations $\left(c<c^{\ast }\right)$, magnetic particles behave as an ideal noninteracting system with a constant $T_B$. At concentrations $c>c^{\ast }$, the dipolar energy enhances the anisotropic energy barrier and $T_B$ increases with increasing $c$ so that a larger temperature is required to reach the superparamagnetic state. The fitting of our results with classical particle models and experiments supports the existence of two differentiated regimes. Our data could help to understand apparently contradictory results from the literature.

• Received 28 September 2009

DOI:https://doi.org/10.1103/PhysRevB.82.134433