The present work focuses on the study of size effects on the thermodynamical behaviour of CoPt nanoparticles, synthesized by a tuned Pulsed Laser Deposition (PLD) technique. Bulk equiatomic CoPt exhibits a phase transition at 825°C, between a tetragonal ordered phase (L10) at low temperature and a disordered Face Centered Cubic (FCC) structure at high temperature. Order-disorder phenomena in CoPt nanoparticles are investigated using Transmission Electron Microscopy (TEM).
CoxPt1-x nanoparticles on amorphous alumina are synthesized by PLD. By means of an alternate deposition experiment, we show that the composition of the nanoparticles varies linearly with their lattice parameter following the Végard law.
Since the substrate temperature is a key parameter for the growth of nanoparticules, an in situ heating experiment in a TEM using disordered nanoparticles is performed. By means of this experiment we determine the temperature range of ordering within the particles and coalescence mechanisms between the particles. We can then optimize the experimental parameters of the synthesis process in order to obtain isolated as-grown ordered Co50Pt50 particles of controlled size, without post-synthesis annealing.
To study the size effects on the order-disorder phenomena, disordered particles with a mean size of 2 and 3 nm are annealed over the temperature range of ordering. Consistent with previous work, we do not find ordered particles smaller than 2.9 nm. Moreover, particles having the same size in projection (3 to 4.5 nm) exhibit either one of the two structural states (L10, FCC). We show, using focal series in high resolution TEM and electron tomography that the order-disorder phenomenon is strongly dependent on the 3D morphology of the CoPt nanoparticles.
Finally, quantitative measurements of the long range order (LRO) parameter on single nanoparticles are presented. The experimental intensity profiles of surstructure and fundamental reflections, on zero loss energy filtered diffraction, are compared to dynamical diffraction simulations in order to deduce the LRO parameter.
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