Dipolar effects on the magnetic phases of superparamagnetic clusters

We report a theoretical study of the impact of dipolar interactions on the room temperature magnetic phases of superparamagnetic nanoparticles confined in spherical and ellipsoidal clusters. We consider Fe3O4 nanoparticles with size ranging from 9 nm to 12 nm, arranged with uniform density in hundred nanometer-sized clusters. We show that one may have a large enhancement of the initial susceptibility for ellipsoidal clusters of high eccentricity, as required for most biomedical applications. Spherical clusters display a reduction of the initial susceptibility, due to the early nucleation of new magnetic phases. In densely packed systems, the dipolar interaction may lead to thermal stabilization of the individual nanoparticle moments, while keeping the cluster superparamagnetic, with a vanishingly small magnetic moment in the absence of an external field. The theoretical model is used to discuss recent findings on quasi-one-dimensional arrays of superparamagnetic Fe and Co nanoparticles, and on spherical c...