Chemical order and size effects on the magnetic anisotropy of FePt and CoPt nanoparticles

We investigate the consequence of the dimension reduction on the magnetic anisotropy of FePt and CoPt nanoparticles. Using an extension of the magnetic anisotropy model of N'eel, we show that, due to a statistical finite size effect, chemically disordered clusters can display a magnetic anisotropy energy (MAE) as high as 0.5\times10^6 J/m3, more than one order of magnitude higher than the bulk MAE. Concerning L10 ordered clusters, we show that the surface induces a reduction of the MAE as compared to the bulk, due to the symmetry breaking at the cluster surface, which modifies the chemical order.