  We have studied the properties of clusters of colloidal magnetic particles
generated from a 2D aggregation model with dipolar interparticle interactions.
Particles diffuse off-lattice, experiencing dipolar interactions with the
already attached particles until either they stick to the cluster or wander far
away and are removed. Our results are interpreted in terms of a fractal
dimension that is a monotonically decreasing function of the temperature,
varying between a definite value close to 1 at T=0, and the limit $T\to\infty$,
corresponding to free diffusion-limited aggregation. By analyzing orientational
correlation functions, an ordered state is found at low temperatures; this
state is destroyed by the fractal disorder generated at high $T$. Our study
could be relevant in understanding aggregation of dipolar colloids and phase
transitions in Langmuir monolayers.
