Effects of thermal annealing of thin Au film on Fe₄₀Ni₃₈Mo₄B₁₈ in ultrahigh vacuum (UHV)

Thin films (∼20 nm) of Au were vapour-deposited on melt-spun amorphous ribbon specimens of the alloy Fe₄₀Ni₃₈Mo₄B₁₈ at room temperature. The specimens were subsequently annealed in UHV (∼10⁻⁸ mbar) at 723 and 823 K in order to observe any dispersion of Au as nanoparticles in the alloy matrix. The motivation for these investigations was derived from similar experiments carried out earlier in nitrogen and in low vacuum conditions, wherein a model based on segregation and oxidation of matrix elements was proposed in order to explain the observed dispersion of Au in the alloy matrix. The present investigations in UHV were carried out as a critical test of this model. However, XPS investigations carried out on these specimens in UHV did not show any dispersion of Au particles after annealing at these temperatures. Further examination of annealed specimen surfaces by SEM and AFM revealed the formation of Au-rich islands on the surface. Native oxide film underneath the Au film and crystallization of the alloy during thermal annealing do not seem to have any effect on depth profiles of Au. These results, when compared with those obtained after annealing the specimens in nitrogen and in low vacuum conditions (∼10⁻¹–10⁻³ mbar), are suggestive of the crucial role of the annealing atmosphere during thermal annealing.