This paper addresses the in situ growth stress evolution and post-growth stress relaxation during the phase separation of immiscible Fe0.51Cu0.49 thin films at various in situ deposition temperatures. Each film was sputter-deposited onto a 10 nm Si3N4 underlayer that was grown on top of Si [0 0 1] substrate at 25 degrees C, 145 degrees C, 205 degrees C, 265 degrees C or 325 degrees C. The thin film stress was measured using a wafer curvature technique. The in situ growth stress increased in compression with increasing substrate temperature. The stress relaxation of the Fe0.51Cu0.49 was found to have a linear increase with the inverse grain size for films deposited at temperatures greater than 205 degrees C. The stress state was correlated to the films' phase and morphology by X-ray diffraction, (scanning) transmission electron microscopy and atomic force microscopy techniques. (C) 2010 Elsevier B.V. All rights reserved.