Epitaxial-strain-stabilized ordering in Au1-xNix alloy thin films grown by MBE

The influence of the epitaxial strain on the structural evolution with temperature of AuNi metastable alloys thin films is investigated. Samples with different initial configurations (codeposited Au1-xNix solid solutions and artificially layered structures) were grown by molecular-beam epitaxy on different (001)-oriented buffer layers (Au, Pt, and Pd). The epitaxial strain was varied by changing (i) the Ni content of the AuNi layer for a given kind of buffer layer, (ii) the nature of the buffer layer for a fixed Ni content, and (iii) the AuNi layer thickness for a fixed Ni content and buffer layer. The structural evolution upon annealing in the 180-300 degrees C temperature range was studied by in situ temperature x-ray diffraction as well as high-resolution electron microscopy. It is shown that a modulated structure develops along the growth direction of the AuNi layer, when the temperature reaches 200-240 degrees C, provided that the residual strain is high enough (>2%). This structure consists of a periodic stacking of 1 Ni-rich plane and 2 or 3 Au-rich planes, depending on the Ni content. The results are explained in terms of a strain-stabilized ordering effect, as supported by energetic calculations based on semiempirical interatomic potentials within the tight-binding scheme.