Sputtered Co/Cu films were deposited on silicon either directly, using an argon plasma, or through an intermediate step involving the deposition of CoN/Cu3N by reactive sputtering in a nitrogen plasma. The nitride composite was pyrolyzed to release the nitrogen. The effect of preparation route and different annealing treatments on the resistivity of these films as a function of magnetic field and temperature was established. As in previous studies particle size is of primary importance in determining the giant magnetoresistance (GMR) characteristics of a film; thus optimizing the magnitude of the GMR effect requires close control of the duration and temperature of annealing treatments. The microstructure of these films was examined by means of atomic force microscopy. The intermediate nitride step produced a film in which the GMR effect was degraded apparently through the inclusion of voids that are not eliminated in the pyrolysis stage.