Growth of two-dimensional patterns in the Fe–Cu multilayers driven far from equilibrium by ion irradiation

Abstract

In the equilibrium immiscible Fe–Cu system, self-avoiding chains of ferromagnetic polycrystalline Fe grains were grown in the Fe 50Cu 50 multilayers after 200 keV xenon ion irradiation at room temperature to a dose of 5×10 15 Xe + cm −2 and the chains featured self-similarity with a fractal dimension of 1.42. Increasing the irradiation dose up to 1×10 16 Xe + cm −2 induced the Fe grains to organize themselves into a branching tree morphology with an increased fractal dimension of 1.88. In contrast, a new non-fractal pattern consisting of fcc Cu-rich grains was developed in the Fe 30Cu 70 multilayered films at an irradiation dose of 1×10 16 Xe + cm −2 and was named as earthworm-like pattern. It is thought that the observed patterns were grown during the relaxation period immediately after the atomic collision cascade triggered by ion irradiation and that the pattern selection is determined by the combining effects of magnetic and repulsive interactions among the aggregating particles of either Fe or Cu grains.