Single crystalline FeMn/Ni bilayer was epitaxially grown on Cu(001) substrate and investigated by photoemission electron microscopy (PEEM). The FeMn and Ni films were grown into two cross wedges to facilitate an independent control of the FeMn (0-20 ML) and Ni (0-20 ML) film thicknesses. The Ni magnetic phases were determined by Ni domain images as a function of the Ni thickness (dNi) and the FeMn thickness (dFeMn). The result shows that as the Ni thickness increases, the Ni film undergoes a paramagnetic-to-ferromagnetic state transition at a critical thickness of dFM and an in-plane to out-of-plane spin reorientation transition at a thicker thickness dSRT. The phase diagram shows that both dFM and dSRT increase as the FeMn film establishes its antiferromagnetic order.