Disordered to ordered phase transformation: Correlation between microstructure and magnetic properties in Fe-Pd thin films

The kinetics of the transformation from the disordered to the ordered phase has been studied in Fe56Pd44 thin films deposited on silicon substrates by Joule evaporation. Subsequent thermal treatments at 550 degrees C in vacuum were performed at different annealing times to promote the formation and the optimization of the ordered L1(0) FePd phase. X-ray diffraction allowed us to investigate the microstructure of the samples (grain size, volume fraction of the transformed phase, order parameter of the ordered phase, and average order of the alloy), whereas magnetic measurements allowed us to evaluate coercivity and to independently estimate the volume fraction of the ordered L1(0) phase. The kinetics of the transformation turned out to reach completion after 20 min of annealing time with the grain size growing with a power law with an exponent equal to 8.24. The resulting monophasic film has a high density of defects, which contribute to the magnetic coercivity. A linear relationship between the coercive field and the average order parameter of the alloy has been established. Finally, the evolution of the transformed phase obeys Avrami's law with an exponent of similar to 0.4, indicating that the transformation is controlled by the nucleation mechanism without grain growth. Published under an exclusive license by AIP Publishing.