  The dipolar interaction is known to substantially affect the properties of
magnetic nanoparticles. This is particularly important when the particles are
kept in a fluid suspension or packed inside nano-carriers. In addition to its
usual long-range nature, in these cases the dipolar interaction may also induce
the formation of clusters of particles, thereby strongly modifying their
magnetic anisotropies. In this paper we show how AC susceptibility may be used
to obtain important information regarding the influence of the dipolar
interaction in a sample. We develop a model which includes both aspects of the
dipolar interaction and may be fitted directly to the susceptibility data. The
usual long-range nature of the interaction is implemented using a mean-field
solution, whereas the particle-particle aggregation is modeled using a
distribution of anisotropy constants. The model is then applied to two samples
studied at different concentrations. One consists of spherical magnetite
nanoparticles dispersed in oil and the other of cubic magnetite nanoparticles
embedded on PLGA nanospheres. We also introduce a simple technique to access
the importance of the dipolar interaction in a given sample, based on the
height of the AC susceptibility peaks for different driving frequencies. Our
results help illustrate the important effect that the dipolar interaction has
in most nanoparticle samples.
