Cobalt aggregates prepared using a cluster beam generator have been deposited on Si(I 0 0) substrate leading to thin films of randomly assembled Co nanoparticles which exhibit a spherical shape with a mono-dispersed diameter distribution centred around 9 nm. Films with thickness ranging from 50 to 550 nm are investigated using magnetic force microscopy (MFM) and results show the presence of twisted magnetic domains. An in-plane magnetic field applied during the growth of the layer leads to the formation of magnetic stripe domains but we observe a similar behaviour if an in-plane magnetic field is applied after the deposition. This indicates that probably the magnetic field applied during the film growth does not drive its magnetic structure. Finally, the measured variation of magnetic domain width D reveals a root t dependence, where t is the film thickness, and is independent of the magnetic history of the films. (c) 2005 Elsevier B.V. All rights reserved.