FexPt100-x films were grown on MgO(111) by co-sputtering Fe and Pt. Composition of the films was determined by Rutherford backscattering spectrometry with an accuracy of 1%. Epitaxy and alloy ordering were quantified by x-ray diffraction and the order parameter was determined to be 0.97 for a film with x=30 and 0.99 for a film with x=25. Neutron diffraction measurements established the presence of an antiferromagnetic phase at T= 100 K in 500 nm FePt3 samples grown on MgO(111). Since FePt3 can be grown as an ordered antiferromagnet and a disordered ferromagnet, these films provide a pathway to grow lattice matched interfaces for exchange bias studies. (c) 2005 American Vacuum Society.