Theory of enhanced magnetic anisotropy induced by Pt adatoms on two-dimensional Co clusters supported on a Pt(111) substrate

Abstract

Calculations are reported of the magnetic anisotropy energy of two-dimensional (2D) Co nanostructures on a Pt(111) substrate. The perpendicular magnetic anisotropy (PMA) of the 2D Co clusters strongly depends on their size and shape, and rapidly decreases with increasing cluster size. The PMA calculated is in reasonable agreement with experimental results. The sensitivity of the results to the Co-Pt spacing at the interface is also investigated and, in particular, for a complete Co monolayer we note that the value of the spacing at the interface determines whether PMA or in-plane anisotropy occurs. We find that the PMA can be greatly enhanced by the addition of Pt adatoms to the top surface of the 2D Co clusters. A single Pt atom can induce in excess of $5\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ to the anisotropy energy of a cluster. In the absence of the Pt adatoms the PMA of the Co clusters falls below $1\phantom{\rule{0.3em}{0ex}}\mathrm{meV}∕\mathrm{Co}$ atom for clusters of about 10 atoms whereas, with Pt atoms added to the surface of the clusters, a PMA of $1\phantom{\rule{0.3em}{0ex}}\mathrm{meV}∕\mathrm{Co}$ atom can be maintained for clusters as large as about 40 atoms. The effect of placing Os atoms on the top of the Co clusters is also considered. The addition of $5d$ atoms and clusters on the top of ferromagnetic nanoparticles may provide an approach to tune the magnetic anisotropy and moment separately.