  Understanding stabilization and aggregation in magnetic nanoparticle systems
is crucial to optimizing the functionality of these systems in real
physiological applications. Here we address this problem for a specific, yet
representative, system. We present an experimental and analytical study on the
aggregation of superparamagnetic liposomes in suspension in the presence of a
controllable external magnetic field. We study the aggregation kinetics and
report an intermediate time power law evolution and a long time stationary
value for the average aggregate diffusion coefficient, both depending on the
magnetic field intensity. We then show that the long time aggregate structure
is fractal with a fractal dimension that decreases upon increasing the magnetic
field intensity. By scaling arguments we also establish an analytical relation
between the aggregate fractal dimension and the power law exponent controlling
the aggregation kinetics. This relation is indeed independent on the magnetic
field intensity. Despite the superparamagnetic character of our particles, we
further prove the existence of a population of surviving aggregates able to
maintain their integrity after switching off the external magnetic field.
Finally, we suggest a schematic interaction scenario to rationalize the
observed coexistence between reversible and irreversible aggregation.
