We have investigated the effect of an expansion of the lattice parameter on the magnetic properties of fcc FeNi films. Epitaxial FexNi1-x ultrathin films (0.58 < x < 0.92) were grown on a Cu90Au10(100) single crystal, a substrate with lattice parameter 1.4% larger than the pure copper. The magnetism of the FeNi films was explored by using linear magnetic dichroism in the angular distribution of the photoelectrons (LMDAD). The LMDAD measurements were carried out at 160 K, using synchrotron radiation with an energy of 140 eV. Although we have observed a gradual decrease of the Fe 3p LMDAD asymmetry in the 67-80 at% Fe concentration range, as already reported for FexNi1-x on Cu(100), for the Fe-rich region (x > similar to 0.80) we have obtained a Fe 3p asymmetry of 6%, suggesting that the lattice expansion of the FeNi due to the epitaxy on Cu90Au10(100) stabilizes the high-spin ferromagnetic state up to higher Fe concentration values. (c) 2006 Elsevier B.V. All rights reserved.3102322742276