  We study the dynamics of a suspension of magnetic nanoparticles. Their
relaxation times are strongly size-dependent. The dominant mode of relaxation
is also governed by the size of the particles. As a result the dynamics is
greatly altered due to polydispersity in the sample. We study the effect of
polydispersity on the response functions. These exhibit significant changes as
the parameters characterizing polydispersity are varied. We also provide a
procedure to extract the particle size distribution in a polydisperse sample
using Cole-Cole plots. Further the presence of attractive interactions causes
aggregation of particles leading to the formation of clusters. Repulsive
interactions along with thermal disorder not only hinder aggregation, but also
introduce the possibility of removal of particles or "fragmentation" from
clusters. The competing mechanisms of aggregation and fragmentation yield a
distribution of cluster sizes in the steady-state. We attempt to understand the
formation of clusters and their distributions using a model incorporating the
phenomena of aggregation and fragmentation. Scaling forms for quantities of
interest have been obtained. Finally we compare our numerical results with
experimental data. These comparisons are satisfactory.
