1Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, P.O. Box 49, Budapest H-1525, HungaryThe interface of Fe-B multilayers was studied by Mossbauer spectroscopy, X-ray diffraction and transmission electron microscopy. The interface region is amorphous and consumes 2 nm from the iron layer. The same hyperfine field distribution is observed within the amorphous interface when the individual layer thicknesses exceed the width of the interface. The width of this distribution is much larger than those measured in amorphous Fe-B alloys and the excess width is attributed to concentration distribution along the interface. On the contrary, the amorphous component in samples with smaller layer thicknesses is more homogeneous as deduced from the hyperfine field distributions, and the hyperfine parameters depend on the ratio of the iron and boron layer thicknesses, Solid state amorphization during the sample deposition is assumed to account for this behavior. Diffusion amorphization is also detected when low-temperature heat treatment of the large layer thickness samples is performed. Single-phase amorphous alloy is formed in both cases in a limited concentration range around 40 at% boron in agreement with the predictions of free-energy diagrams.