ON THE CHEMICAL DIFFUSION IN LAYERED THIN-FILMS CONTAINING AMORPHOUS CO-ZR, NI-ZR, AND FE-ZR

The chemical diffusion in thin trilayer films of TM-TM100-xZrx-TM with an amorphous middle layer where TM = Co, Ni, or Fe and in amorphous Fe-Zr and Ni-Zr films with composition gradients has been investigated using Rutherford backscattering spectrometry. The growth of the amorphous layer in the trilayers, due to in-diffusion of cobalt and nickel, is initially found to be proportional to the square root of the time, t1/2, and subsequently found to level off before the compositions corresponding to metastable equilibria are reached. Irradiation, with 500 keV Xe+ ions, is found to promote the in-diffusion. This behavior is discussed in terms of structural relaxation effects and their influence on the metastable equilibrium. In amorphous Fe-Zr the chemical diffusivity is observed to be very sluggish. Contrary to the behavior in Co-Zr and Ni-Zr trilayers, the direction of the iron diffusion in Fe-Zr trilayers suggests a broad positive peak in the Gibbs free energy at a composition around 50 at. % Zr. It is argued that the sluggish chemical diffusivity of iron is directly related to the unusual composition-dependence of the Gibbs free energy for amorphous Fe-Zr.