Reversible electric-field manipulation of the adsorption morphology and magnetic anisotropy of small Fe and Co clusters on graphene

First-principles electronic calculations show how the adsorption morphology, orbital magnetism, and magnetic anisotropy energy (MAE) of small ${\mathrm{Co}}_N$ and ${\mathrm{Fe}}_N$ clusters ($N\ensuremath{\le}3$) on graphene (G) can be reversibly controlled under the action of an external electric field (EF). A variety of cluster-specific and EF-induced effects are revealed, including (i) perpendicular or canted adsorption configurations of the dimers and trimers, (ii) significant morphology-dependent permanent dipole moments and electric susceptibilities, (iii) EF-induced reversible transitions among the different metastable adsorption morphologies of ${\mathrm{Fe}}₃$ and ${\mathrm{Co}}₃$ on graphene, (iv) qualitative changes in the MAE landscape driven by structural changes, (v) colossal values of the magnetic anisotropy $\mathrm{\ensuremath{\Delta}}E\ensuremath{\simeq}45$ meV per atom in ${\mathrm{Co}}₂/\mathrm{G}$, (vi) EF-induced spin-reorientation transitions in ${\mathrm{Co}}₃/\mathrm{G}$, and (vii) reversibly tunable coercive field and blocking temperatures, which in some cases allow a barrierless magnetization reversal of the cluster. These remarkable electric and magnetic fingerprints open new possibilities of characterizing and exploiting the size- and structural-dependent properties of magnetic nanostructures at surfaces.