The magnetic anisotropy determines the orientation and the stability of the magnetization which is a crucial parameter for a large part of the technological applications. The possibility of modify the magnetic anisotropy of small clusters by changing the shape, composition and dimensions makes such systems of evident applicative interest. A study of the anisotropy and magnetic momenta of metallic clusters constituted by 1÷100 atoms of Co and deposited onto a crystalline substrate of Pt(111) allowed to determine either the magnetic anisotropy in function of the clusters dimension and the quantum origin of the anisotropy through magnetometric measurements of the orbital momenta. It is observed that small clusters deposited onto metallic surfaces posses strong magnetic anisotropy, with characteristic energies of the order of 0.001÷0.01 eV/atom values that exceed of several orders of magnitude those characteristic of thin films and bulky materials. The anisotropy of Co atoms isolated on the Pt(111) surface results to be higher than in whatever else magnetic material (9 meV/atom). Among the implications of these results, there is the possibility of obtaining stable magnetic bits at 300 K with 400 atoms/bit (or of 1 atom at 2 K).S
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