Formation of Co nanodisc with enhanced perpendicular magnetic anisotropy driven by Ga+ ion irradiation on Pt/Co/Pt films

The origin of magnetic phase transition from in-plane to perpendicular magnetic anisotropy (PMA) of Pt/Co/Pt thin film by ${\mathrm{Ga}}⁺$ ion irradiation at fluences of $1--5\ifmmode\times\else\texttimes\fi{}{10}¹⁵\phantom{\rule{0.28em}{0ex}}\mathrm{ions}/{\mathrm{cm}}²$ is investigated by means of x-ray magnetic circular dichroism (XMCD) and extended x-ray absorption fine structure (EXAFS) analyses. We find that Pt and Co atoms are mixed with each other and that Co is oxidized near the surface due to removal of the Pt overlayer. Furthermore, polarization-dependent EXAFS analysis shows that Co is firstly dispersed as separated single-atom-thick sheets in a Pt matrix at $1\ifmmode\times\else\texttimes\fi{}{10}¹⁵\phantom{\rule{0.28em}{0ex}}\mathrm{ions}/{\mathrm{cm}}²$, then the Co sheets are divided into a few $\AA{}$ clusters at $5\ifmmode\times\else\texttimes\fi{}{10}¹⁵\phantom{\rule{0.28em}{0ex}}\mathrm{ions}/{\mathrm{cm}}²$, which are regarded as nanodiscs parallel to the film plane. This process is accompanied by the appearance of an out-of-plane magnetization component and a remanence peak is observed. Because we do not observe an enhancement in anisotropy of Co orbital moment which leads to change in magnetic anisotropy through the transition at about $5\ifmmode\times\else\texttimes\fi{}{10}¹⁵\phantom{\rule{0.28em}{0ex}}\mathrm{ions}/{\mathrm{cm}}²$, it might be possible that such nanodisc formation induces increase of magnetic anisotropy via a shape effect. By comparing with the phase transition observed at lower fluence [Phys. Rev. B 86, 024418 (2012)], we find that the mechanism of two transitions is different, i.e., the transition at lower fluence is caused by anisotropy of orbital moment due to structural strain, while the present transition is possibly by shape effect due to nanodisc formation.