Co-Pt films were deposited on GaAs(001) substrates at room temperature or 200 degreesC by dc plasma sputtering with an application of a bias of 0 V or - 150 V to the substrate. The Co-Pt films retain the face-centered cubic (fcc) structure grown with the 111-texture but the atomic composition in the films depends on both the substrate bias and the substrate temperature. The crystalline quality of the film is improved mainly by application of the substrate bias. However, the atomic composition of the film is affected by both the substrate bias and the substrate temperature, by applying the substrate bias, the averaged atomic ratio decreases drastically on the unheated substrate but only a little on the heated substrate. These phenomena are explained in terms of two factors, first, preferential re-sputtering of Co by the incident particles generated under the bias voltage and, second, preferential atomic diffusion of Co into the substrate enhanced by elevating the substrate temperature. (C) 2003 Elsevier B.V. All rights reserved.