EPITAXY OF CO ON FEAL(001) .2. C(2X2) FILMS

The c (2 X 2) low-energy electron-diffraction (LEED) pattern observed on Co films thicker than about 13 angstrom on FeAl{001} is shown to be caused by a strained hexagonal Co(1120BAR) film. The film is strained differently in the two orthogonal directions within the plane of the interface: it is expanded by 0.91% in one and compressed by 5.4% in the other direction. As a consequence, the bulk interlayer spacing, as determined by quantitative LEED, is expanded by 2.9% with respect to the spacing in the unstrained hexagonal-close-packed phase (1.2535 angstrom). This is a more general case of pseudomorphic epitaxy than when film and substrate have similar unit meshes, and requires a more elaborate strain analysis. Such strain analysis finds an interlayer strain of 3.35%, in satisfactory agreement with the quantitative LEED result. It is noted that the strained Co(1120BAR) film does not grow directly onto the FeAl{001} substrate, but rather on a strained Co{001} film, 8-10 layers thick, which grows first on the substrate and is analyzed in a previous paper.