Magnetic properties of FeCo nanoclusters on Cu(100):Ab initiocalculations

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We present ab initio calculations of the magnetic moments and magnetic anisotropy energies of small FeCo clusters of varying composition on top of a Cu(100) substrate. Three different cluster layouts have been considered, namely, $2\ifmmode\times\else\texttimes\fi{}2$, $3\ifmmode\times\else\texttimes\fi{}3$, and crosslike pentamer clusters. The ratio of Co atoms with respect to the total number in a chosen cluster (``concentration'') was varied and all possible arrangements of the atomic species were taken into account. Calculations have been performed fully relativistic using the embedded-cluster technique in conjunction with the screened Korringa-Kohn-Rostoker method and the magnetocrystalline anisotropy energy (MAE) has been evaluated by means of the magnetic force theorem. A central result of the investigations is that the size of the magnetic moments of the individual Fe and Co atoms and their contributions to the anisotropy energy depend on the position they occupy in a particular cluster and on the type and the number of nearest neighbors. The MAE for the $2\ifmmode\times\else\texttimes\fi{}2$ and $3\ifmmode\times\else\texttimes\fi{}3$ clusters varies with respect to the concentration of Co atoms in the same manner as the corresponding monolayer case, whereas the pentamer clusters show a slightly different behavior. Furthermore, for the clusters with an easy axis along a direction in the surface plane, the MAE shows a significant angular dependence.