  Experimental and simulation studies of superparamagnetic colloids in strong
external fields have systematically shown an irreversible aggregation process
in which chains of particles steadily grow and the average size increases with
time as a power-law. Here we show, by employing Langevin dynamics simulations
the existence of a different aggregation behavior: aggregates form during a
transient period and the system attains an equilibrium distribution of
aggregate sizes. A thermodynamic self-assembly theory supports the simulation
results and it also predicts that the average aggregate size in the equilibrium
state depends only on a dimensionless parameter combining the volume fraction
of colloids phi0 and the magnetic coupling parameter Gamma. The conditions
under which this new behavior can be observed are discussed.
