Magnetic diffusion-limited aggregation

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Abstract
An extra degree of freedom is introduced in the well-known diffusion-limited aggregation model. The growth entities are ‘‘spins’’ taking, e.g., two states that are coupled via a physically relevant interaction potential responsible for a competition process between the two components. The presence of an external field favoring one spin species over the other is also considered. This model leads to a wide variety of kinetic processes and morphologies distributed in a ‘‘phase diagram’’ of both growth control parameters, i.e., the coupling energy and the field strength. The Brownian motion of the spins leads to fractal-like structures with a fractal dimension varying from 1.68±0.02 to 1.99±0.01 depending upon the growth parameters. A physical basis is presented to describe the new kinetic processes. The spreading and geometry of the two components in fractal clusters have also been investigated. The earlier stages of growth are driven by a dominating spin component. This finite-size process is found to imply a drastic change of the physical and geometrical properties of the cluster during growth history. For large clusters the fractal branches can be divided into segments of the same spin species having a characteristic coherence length ξ. This length ξ is found numerically to scale as a power law of the segment mass with a critical exponent μ=1.2±0.1.