  A combined experimental, using X-ray Magnetic Circular Dichroism and
theoretical investigation, using full-potential Korringa-Kohn-Rostoker (KKR)
Green function method, is carried out to study the spin structure of small
magnetic Cr adatom--clusters on the surface of 3 monolayers of $fcc$ Fe
deposited on Cu(001). The exchange interaction between the different Cr adatoms
as well as between the Cr atoms and the Fe atoms is of antiferromagnetic nature
and of comparable magnitude, leading due to frustration to complex
non-collinear magnetic configurations. The presence of non-collinear magnetic
coupling obtained by {\it ab initio} calculations is confirmed by the
experimental results.
