Berlin 2005 – wissenschaftliches Programm

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MA: Magnetismus

MA 28: Magnetische Partikel/Cluster I

MA 28.10: Vortrag

Dienstag, 8. März 2005, 17:30–17:45, TU H1028

Monte Carlo Simulations of Thermodynamic Properties of FePt Nanoparticles — •Michael Müller and Karsten Albe — TU Darmstadt, FB Materialwissenschaft, Petersenstr. 23, 64287 Darmstadt

To date, a number of experimental routes for synthesizing FePt nanoparticles have been developed. In many cases, the disordered fcc phase instead of the desired L10 ordered phase is obtained even at low temperatures and it is still unclear whether this is due to kinetic or thermodynamic reasons. Therefore, a detailed investigation of the thermodynamic properties of nanoalloys as a function of particle size, temperature and composition can provide important information.

In our work we have chosen a lattice Monte-Carlo model combined with a spin-1/2 Ising model including nearest and next nearest neighbor interactions. The parameters of the model were fitted to the FePt bulk phase diagram and experimental data on Pt surface segregation. By using this model, we have systematically explored the influence of particle size and chemical composition on the ordering temperature as well as the segregation of Fe and Pt in ordered particles. In general, we find no evidence for a considerable decrease of the ordering temperature with particle size. On the other hand, a lowering of the ordering temperature with increasing deviation from stoichiometry could clearly be observed. This effect was found to be strongly assymetric with respect to compositions enriched with Fe or Pt, which can be explained by differences in the segregation behaviors of the two elements at the surface.