We present quenched molecular dynamics simulations of the vacancy behaviour near the surface of an L1₀ ordered CoPt alloy.

We calculate the following parameters:
- amount of relaxation for the two possible terminations and surface formation energies,
- formation and migration energies of a vacancy within the different planes,
- formation energies of the antisites,
- migration energies of a vacancy between the different planes,
- formation and migration energies of adatoms on the different surfaces.

We find that as expected the alloy surface is preferentially a Pt plane. There are typically 2 planes where the migration and formation energies are different from the bulk. Whereas the activation energy of the vacancies is higher than 2.5 eV in the bulk, near to the surface some mechanisms have an activation energy smaller than 1 eV. 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 analyse this behaviour in terms of chemical ordering using a classical phenomenological diffusion law with the values of the energies deduced by quenched molecular dynamics.

The energy profiles in bulk are also used to extend the model developed by Schober [1] to the L1₀ structure and to deduce from the phonon spectra measured using neutron inelastic scattering on a single variant single crystal CoPt [2] a migration energy that is compared to the calculated values.

[1] H.R. Schober et al, J. Phys.: Cond. Matter, 4 (1992) 9321 ; E. Kentzinger and H.R. Schober, J. Phys.: Cond. Matter, 12 (2000) 8145.

[2] K. Tanaka et al, Mater. Sci. Eng. A 312 (2001) 118.

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