Fractal properties of clusters of colloidal magnetic particles

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.