In situ fast kinetic ellipsometry, X-ray and polarized neutron scattering are used to investigate the growth of sputtered Fe89C11-Si multilayers with atomic resolution and to determine interface thickness and composition. The values of these properties depend on the sputtering parameters. Under the conditions of minimal thickness of the interface layers we find on top of the silicon layer 15 Angstrom (Fe89Co11)(0.5)Si-0.5 and on top of the iron cobalt layer 19 Angstrom (Fe89Co11)(0.67)S-0.33. During the growth of the iron cobalt layer a phase transition occurs. After the formation of the interface layer on top of the silicon layer the iron cobalt alloy starts to grow in an amorphous phase until a thickness of 25 Angstrom is reached. Then it crystallizes down to the interface layer whereby it shrinks to 18.5 Angstrom, the atomic density changes nearly proportionally to the imaginary part of the index of refraction. As will be shown, the growth of the layers and the formation of the interface layers can be followed on an atomic scale. (C) 2003 Elsevier Science B.V. All rights reserved.