In recent years, Ni/Cu films and multilayers with perpendicular magnetic anisotropy (PMA) have attracted much interest due to their potential advantage in magneto-optical recording. While the magnetic anisotropy of this system has been intensively investigated and interpreted, a major question concerning the magnitude of the magnetic moments remains unresolved. In this report, using Auger electron spectroscopy (AES) on a Ni/Cu/Si(0 0 1) epitaxied wedge sample, we show that Cu atoms diffuse in the growing Ni layer. We have used a dynamical segregation model to reproduce our experimental results and to calculate the Cu concentration profile. The observed chemical mixing may explain the dramatic decrease of the atomic magnetic moment for ultra-thin films reported in previous experimental works [Phys. Rev. B 55 (17) (1997) 11422].. (C) 2003 Elsevier Science B.V. All rights reserved.