Ion-beam-induced interface mixing: unified atomistic simulations of collisional and thermal processes

Ion beam processing of materials at elevated temperature, which can be room temperature for metals, is controlled simultaneously by both, collisional and thermally activated processes. So far, large scale process simulations separate these processes, calculating all collision cascades of the complete ion irradiation in the binary collision approximation like the TRIDYN code, and simulating then the defect relaxation/impurity nucleation by kinetic Monte Carlo.

Here we present and apply a unified computer simulation program package which unifies both programs, i.e. after each calculated 3D collisional cascade some kinetic Monte Carlo steps allow relaxation, diffusion, phase separation etc. in the whole volume. Contrary to molecular dynamics simulations, our approach allows studies on experimental spatiotemporal scales. In this presentation we focus on the evolution of interfaces under ion irradiation, where collisional mixing is in competition with thermally activated diffusion and phase separation. He⁺ irradiations for two extreme cases were studied: (i) Irradiation of interfaces made by immiscible elements, here Al and Pb. Ballistic interface mixing is accompanied by phase separation. Al and Pb nanoclusters form and show self-ordering (banding) parallel to the interface. (ii) Irradiation of interfaces made by intermetallics forming species, here Pt and Co. Well-ordered layers of phases of intermetallics appear in the sequence Pt/Pt₃Co/PtCo/PtCo₃/Co, resulting in a stepwise changing Pt/Co concentration depth profile. Novel magnetic properties of such sandwiched phases are predicted and can explain the transition between out of plane and in-plane magnetic anisotropy caused by ion irradiation.