Structural evolution of Fe-Al multilayer thin films for different annealing temperatures

The phase formation during thermal annealing of Fe/Al multilayer thin films prepared by electron-beam evaporation, with an overall atomic concentration ratio of Fe:Al = 1:1, has been studied by Rutherford backscattering spectrometry (RBS), x-ray diffraction spectroscopy (XRD), and conversion-electron Mossbauer spectroscopy (CEMS). At the annealing temperature of 473 K some degree of atomic mixing between Fe and Al layers is revealed only by GEMS. At 573 K a large degree of atomic mixing is indicated also by RES, leading to the nucleation and growth of the B2 FeAl intermetallic phase, as detected by means of XRD and GEMS. At 673 K all Fe atoms have reacted and the multilayer film is transformed into a defective B2 phase. Annealing at higher temperature increases the structural order of the B2 phase. We suggest that the observed phase formation occurs in three stages: (1) formation of a thin intermixed layer between Fe and Al in the as-deposited sample; (2) Al migration into the initial intermixed layer; (3) B2 phase growth at the interface between the intermixed layer and the Fe layer.