We have investigated the magnetic properties of epitaxially grown Fe/NiO/Fe(0 0 1) trilayers, for different thicknesses of the NiO spacer. Magneto Optical Kerr Effect has been exploited to study the in-plane magnetization reversal processes in the iron layers. We found that the NiO thickness tAFM has a critical value tC for the magnetic coupling between the Fe layers: for tAFM<tC the magnetization directions align perpendicularly, with zero applied field, while the alignment is collinear for thicker spacers. A phenomenological model has been developed to reproduce and discuss the results. Complementary information has been obtained by means of spin polarized low energy electron microscopy.