Monte Carlo study of the superspin glass behavior of interacting ultrasmall ferrimagnetic nanoparticles

The magnetism of a dense assembly of ultrasmall ferrimagnetic nanoparticles exhibits unique features due to the combination of intraparticle and strong interparticle interactions. To model such system we need to account for the internal particle structure and the short- and long-range interparticle interactions. We have developed a mesoscopic model for the particle assembly that includes three spins (two for the surface and one for the core) for the description of each nanoparticle, interparticle dipolar interactions and the interparticle exchange interactions for the nanoparticles in contact. The temperature dependence of the observed exchange bias effect, due to exchange coupling at the interface between core/surface spins and the interparticle exchange coupling, and the zero-field-cooled--field-cooled magnetization vs temperature curves have been investigated using the Monte Carlo simulation technique with the implementation of the Metropolis algorithm. Our simulations reproduce well the experimental data of ultrasmall $\ensuremath{\sim}2$-nm $\mathrm{Mn}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$ nanoparticles, confirming the close relationship between the superspin glass state and the exchange-bias effect in dense nanoparticle systems, owing to the interplay between the intraparticle structure and the interparticle effects.