  The kinetics of aggregation in a monolayer of magnetic particles are studied
using stochastic dynamics computer simulations. At low densities (<8% coverage)
the equilibrium structure is made up of chains and rings; the primary
mechanisms by which these motifs form are described. At higher densities (>15%
coverage), we observe large transient concentrations of Y-shaped and X-shaped
defects that ultimately give way to an extended, labyrinthine network. Our
results suggest that a defect mechanism -- such as that proposed by Tlusty and
Safran, Science 290, 1328 (2000) -- could drive a metastable phase separation
in two dimensions.
