Nanoscale dynamics - an atomistic picture

AbstraktHow do metal atoms diffuse in ordered intermetallic alloys? What is the consequence in new material types, like thin films, or epitaxial ultra thin layers? The accepted view is that diffusion takes place by random jumps of atoms between nearest-neighbor sites. But what happens when the structure is highly anisotropic like a tetragonal L10 phase? We have carried out extensive studies on epitaxial L10-ordered FePt thin films which show exemplary and exceptional behavior. Firstly, we have studied thin films of an isotopic altering multilayer superstructure containing alternate layers Fe56 and Fe57 isotopes using synchrotron radiation Mössbauer spectroscopy - nuclear resonant scattering (NRS) under grazing incidence. The measurements reveal the decay of the isotopic superstructure due to diffusion driven by successive annealing steps [1,2]. This superstructure decay can be seen with atomic resolution. The well-defined orientation of the lattice in FePt-thin films allows us to resolve the strong anisotropy of diffusion in the tetragonal structure, see Fig. 1 left. Secondly, we have studied order-order transitions again on L10-ordered FePt thin films using CEMS [3]. We found that the films exhibiting the c-axis of the tetragonal structure perpendicular to the surface orient towards a-axis perpendicular to the film surface by annealing at 500°C. This process is again driven by atomic diffusion. The ordering starts from the film surface, while a c-axis oriented re-ordering starts at the same time from the epitaxial interface, see Fig 1, right. The total film ordering runs towards an equilibrium state specified by the annealing temperature. We could analyze the transitions using an advanced Johnson-Mehl-Avrami formalism [3], showing the time constant, activation energies and ordering dynamics. The activation energies we find for the c-variant ordering are the same we found from the atomic diffusion studies.
NameSepiol, Bogdan
RolleInvited speaker
Externe OrganisationMediterranean Conference on the Applications of the Mössbauer Effect, MECAME 2015
Datum2015.6.7 - 2015.6.10
Keywords:103015 Kondensierte Materie, 103008 Experimentalphysik, 103018 Materialphysik
Keywords:1209 Experimentalphysik, 1210 Festkörperphysik