Distribution of the magnetic anisotropy energy of an array of self-ordered Co nanodots deposited on vicinal Au(111):: X-ray magnetic circular dichroism measurements and theory

The magnetic anisotropy energy (MAE) of a self-assembled Co nanodot array on a Au vicinal surface is studied by x-ray magnetic circular dichroism. The mean dot diameter is 2 nm, i.e., 120 atoms, and each dot is aligned on a 3.8x7.2 nm(2) lattice. The hysteresis cycles have been determined for various temperatures below and above the blocking temperature. The cycles are well fitted by a rate equation model based on the Stoner-Wohlfarth Hamiltonian. Dynamical hysteresis cycles are calculated with this model for various MAE distributions. It is shown that the blocking temperature indicates the highest MAE in the assembly and that the coercive field indicates the mean MAE. In our self-ordered sample, we find a mean MAE of 26 meV per dot and a full width at half maximum of 53 meV. The determination of experimental distributions from cycles published in the literature generally shows negligible interactions among dots and a MAE distribution broader than the size distribution. The latter is interpreted in terms of atomic relaxations inside the Co dots.