2D architectures of cobalt onto silicon (111) surfaces were elaborated by patterning of magnetic cobalt in the nanometer scale. A continuous cobalt layer of 1, 3 and 10nm thickness, respectively, was first deposited by means of thermoionic vacuum arc technique and then, thermally annealed in vacuum at temperatures ranging from 450 to 800°C. Surface structure was analyzed by atomic force and field emission-scanning electron microscopies. Above 750°C, regular triangular shape cobalt nanostructures are formed with pattern dimensions varying between 10 and 200nm. Good control of shape and packing density could be achieved by adjusting the initial thickness and the thermal and hydrogen plasma treatments. Magnetic properties were investigated using vibrating sample magnetometer technique. The evolution of the coercive field versus packing density and dimensions of the nanostructures was studied and compared to micromagnetic calculations. The observed nanostructures have been modeled by a series of shapes tending to a fractal curve.