We have achieved (001) oriented L10 (Fe1-xCux)55Pt45 thin films, with magnetic anisotropy up to 3.6x10^7 erg/cm^3, using atomic-scale multilayer sputtering and post annealing at 400 {\deg}C for 10 seconds. By fixing the Pt concentration, structure and magnetic properties are systematically tuned by the Cu addition. Increasing Cu content results in an increase in the tetragonal distortion of the L10 phase, significant changes to the film microstructure, and lowering of the saturation magnetization and anisotropy. The relatively convenient synthesis conditions, along with the tunable magnetic properties, make such materials highly desirable for future magnetic recording technologies. ; Comment: 16 pages, 6 figures, to appear in APL