Magnetic dot arrays with perpendicular magnetic anisotropy were fabricated by patterning Co(80)Pt(20)-alloy continuous films by means of laser interference lithography. As commonly seen in large dot arrays, there is a large difference in the switching field between dots. Here we investigate the origin of this large switching field distribution, by using the anomalous Hall effect (AHE). The high sensitivity of the AHE permits us to measure the magnetic reversal of individual dots in an array of 80 dots with a diameter of 180 nm. By taking 1000 hysteresis loops we reveal the thermally induced switching field distribution SFD(T) of individual dots inside the array. The SFD(T) of the first and last switching dots were fitted to an Arrhenius model, and a clear difference in switching volume and magnetic anisotropy was observed between dots switching at low and high fields.

We carried out solution synthesis of FePt nanoparticles through different chemical methods, using four "hot soap methods", i.e. the particle formation in the presence of surfactant molecules at high temperatures, and one polyol process. Structural and magnetic properties of the as-made particles pointed to a core-shell structure for the particles prepared with hot soap methods, with an irondepleted core surrounded by a pure iron shell. Such a structure has an impact on the magnetic properties of as-made particles since Fe atoms from shell are oxidised and non magnetic. We proved however that iron atoms of this shell are available during the formation of the ordered phase upon annealing: L1 0 phase for small particles, L1 2 for bigger ones. In contrast, the core-shell structure was not observed in the case of nanoparticles synthesised according to the polyol process. This outlines the key role of the stabilising ligands, long alkyl chain surfactants in the former case and tetraethylene glycol in the latter.

The transformation of ≤4 nm equiatomic FePt nanoparticles from the disordered cubic A1 to the ordered tetragonal L1 0 phase was studied by means of high-resolution transmission electron microscopy coupled with in situ heating experiments. In accordance with ex situ annealing experiments, a transition temperature of around 500°C was determined. Diffusion is enhanced at surfaces and plays a dominant role in the ordering process. Hence, the ordering of the crystallographic structure starts at the surface of the nanoparticles and propagates toward their center, resulting in complete ordering within some minutes, for temperatures above 600°C. Unlike the generally assumed lower limit of ordering (3.5 nm), we demonstrate that ultrasmall (less than 3 nm) FePt nanoparticles can also be fully transformed to the L1 0 phase. The well-controlled and precise stoichiometry (Fe 50 at.% Pt 50 at.% ) and the homogeneous composition of these particles both play a major role in their successful phase transformation.