In order to optimize their magnetic properties, Fe/Au multilayers were treated by pregrowth and postgrowth ion-beam bombardments. The ferromagnetic resonance linewidth was used as our main figure of merit. The pregrowth treatment of the MgO substrate using a 60 eV atomic oxygen beam resulted in a reduction of the inhomogeneous linewidth broadening in comparison with a sample grown on an untreated substrate. This homogeneity increase is linked to the removal of substrate carbon contamination by the chemically active oxygen. It correlates with the reduced interface roughness. The postgrowth sample irradiation using 30 keV He(+) ions also reduces the inhomogeneous broadening in the linewidth. Fe and Au have a miscibility gap, but the demixing is kinetically quenched at room temperature. Ion collisions locally minimize the interface energy by providing the energy necessary for localized demixing, resulting in a smoothing effect. Combined, the pregrowth and the postgrowth irradiations lead to the lowest observed linewidth. (c) 2008 American Institute of Physics.