Invasion percolation universality class and fractal geometry of magnetic domains

The magnetization reversal process of thin epitaxied FePt/Pt(001) and FePt/MgO(001) layers occurs through rare nucleation events followed by domain-wall propagation. Whereas low-scale observations show that these systems possess very different domain-wall geometries, the structures of the reversed magnetic domains appear to be similar at larger scales: at the beginning of the magnetization reversal, their geometry is that of a fractal percolating cluster. This similarity is analyzed and explained using percolation theory tools and standard domain-growth models, and appears to be an experimental illustration of the universality associated with two-dimensional percolation. We study, in particular, the influence of the demagnetizing field on critical properties and discuss the extent to which these results can be applied to other magnetic thin layers.