Pt-Co Films and Alloys

The initial step in the preparation of the mixed-metal films consisted of vapor deposition, one metal at a time, onto a refractory substrate sequential deposition was necessary in order to track the doser-calibration conditions. Alloy formation was then carried out by a high-temperature treatment. Figme 5 shows LEISS spectra of a Mo(llO) substrate on which ten monolayers of a Pt-Co mixtrrre, deposited in a 1 4 monolayer ratio, was heated to selected temperatures. In Fig. 5a, 2 ML of Pt were deposited first, followed by 8 ML of Co in Fig. 5b, the reverse was done in which 8 ML of Co were generated first. 

Additional Pt-Co films were prepared, each consisting of a total of ten monolayers but with variable Pt Co ML ratios. The results, in terms of LEISS spectra, are summarized in Fig. 6. It is not shown in the Fig. 6 summary, but it was found that stable top most-layer alloys were always formed when the deposits were annealed at 1000 K. However, as can be seen in Fig. 6, the peak-intensity ratios were not invariant and depended upon the initial Pt Co vapor-deposition (ML) proportion. 

Surface Elemental Composition and Surface Phase Diagrams 

Qualitatively, it can be readily gleaned from the data in Fig. 5 and Fig. 6 that, in the alloyed state generated at 1000 K, the elemental composition at the outermost layer, as measured by the LEISS Pt Co peak-intensity ratio, is vastly different from that in the bulk. The peak intensities can be quantitatively converted to surface concentrations through the use of Eqs. (6) and (7) where the value offit-co was determined from experiment to be equal to 0.5. 

The divergence between the elemental composition at the topmost layer and that in the bulk is appreciated best when the atom-percent composition of Pt (or Co) at the alloy surface is plotted as a function of the monolayer-percent composition of Pt (or Co) in the bulk. Such a plot, which represents the phase diagram of the outermost-layer Pt-Co alloy, is shown in Fig. 7 the open circles are data for when Ptwas deposited initially, whereas the 

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