Diffusion piloted ordering in codeposited CoPt epitaxial layers: Experiment and quenched molecular dynamics simulations

This paper presents quenched molecular dynamics simulations of the vacancy behavior near the surface of an ordered CoPt alloy. The formation, migration, and activation energies are deduced for the different atoms (Co and Pt), moving in and between the different planes parallel to the surface. The migration of an adatom at the surface is also studied. The alloy surface is preferentially a Pt plane. There are typically two planes where the migration and formation energies are different from the bulk. The anisotropy of the structure involves an anisotropy of the diffusion. The values of the energies obtained for the different processes are used to understand the chemical ordering in CoPt(001) thin films grown at different temperatures. These films present indeed a concentration modulation along the growth direction with 2 temperature ranges. We analyze this behavior in terms of chemical ordering using a classical phenomenological diffusion law with the values of the energies deduced by quenched molecular dynamics.

Figure 1

Nearly face-centered-cubic cells generally used to describe the $\text{L}{1}_0$ structure ($c/a\approx 1$, thin lines). All lattice parameters and directions are given for this system in the text. The Bravais lattice is in fact a simple tetragonal structure (thick lines) with a Co atom at each edge and a unique Pt at the center (with a cross). The Bravais lattice is the unit cell that we have used to describe the slices as the atoms are placed on a simple square lattice in the (001) planes.

Figure 2

Variation of the total energy when a vacancy migrates between two successive (001) planes for both the (a) Co and the (b) Pt surfaces. The labels near the arrows give the type of planes (pure Co or pure Pt planes). The curves have not been corrected for the concentration change for clarity reasons.

Figure 3

(Color online) Temperature dependence of the CLRO parameter $\eta$ in the CoPt(001) films. The dashed (blue online) thick line shows the surface contribution; the thin (red online) lines show the variation of $\eta$ with the bulk contribution (2 eV: full, 2.5 eV: dashed). The thick black line is the variation of $\eta$ in the bulk as calculated in the cluster-variation method (Ref. 20).

Figure 4

(Color online) Temperature dependence of the CMAGA order parameter $\eta$ in the ${\text{Co}}_3\text{Pt}\left(111\right)$ and ${\text{Co}}_3\text{Ru}\left(0001\right)$ films. The thick (blue online) line shows the surface contribution; the thin (red online) line show the bulk contribution. The full and dashed lines, circles and squares correspond, respectively, to the ${\text{Co}}_3\text{Pt}\left(0001\right)$ and ${\text{Co}}_3\text{Ru}\left(0001\right)$ films.