Finite-size effects in highly ordered ultrathin FePt films

Granular epitaxial L1(0) ordered FePt films of different thickness are used to study finite size effects in this hard magnetic material. Heat absorption and material evaporation have been identified as finite size effects that strongly affect ordering. For film thickness below the absorption length heating by radiation is strongly reduced, which hinders ordering. When using equilibrium heating instead, ordering in subnanometer thick films is obtained. Furthermore preferential evaporation of Fe in comparison to Pt results in composition shifts. Since the fraction of surface to volume increases with reduced particle size, this effect limits the annealing temperature in particular for small grain sizes. The control of these finite size effects allows to achieve a high degree of order (S >= 0.95) also in ultrathin films. In this way, a large coercivity of up to 7.3 T at room temperature is obtained in 10 nm thick films. In films with particle sizes down to a few nanometer high L1(0) order is proven by a similar coercivity at low temperature, where thermal fluctuations are negligible. These films are superparamagnetic above a blocking temperature of 160 K. These highly ordered and textured nanostructures present an ideal model system to study superparamagnetism.