Modification of Pt/Co/Pt film properties by ion irradiation

Abstract

We studied the structural modifications of a Pt/Co/Pt trilayer epitaxial film under ${\mathrm{Ga}}^{+}$ 30-keV ion irradiation by means of classical molecular dynamics and Monte Carlo simulations. The semiclassical tight-binding second-moment approximation potential was adjusted to reproduce the enthalpies of formation, the lattice constants, and the order-disorder transition temperatures for Co-Pt alloys. We found that during irradiation, the sandwich-type Pt(fcc)/Co(hcp)/Pt(fcc) film structure underwent a transition to the new solid solution $\ensuremath{\alpha}\ensuremath{-}\mathrm{Co}/\mathrm{Pt}(\mathrm{fcc})$ phase. Our analysis of the short-range order indicates the formation, within a nanosecond time scale, of a homogeneous chemically disordered solution. The longer time-scale simulations employing a Monte Carlo algorithm demonstrated that the transition from the disordered phase to the ordered $L{1}_{0}$ and $L{1}_{2}$ phases was also possible but not significant for the changes in perpendicular magnetic anisotropy (PMA) observed experimentally. The strain analysis showed that the Co layer was under tensile strain in the lateral direction at the fluences of $1.5\ifmmode\times\else\texttimes\fi{}{10}^{14}--3.5\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{0.28em}{0ex}}{\mathrm{ions}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}}^{\ensuremath{-}2}$; this range of fluences corresponds to the appearance of PMA. This strain was induced in the initially relaxed hcp Co layer due to its partial transformation to the fcc phase and to the influence of atomic layers with larger lattice constants at upper/lower interfaces.