Grazing-incidence x-ray-scattering study of (001)-oriented high-quality epitaxial Co/Cr superlattices

The structural properties of Co/Cr(001) superlattices grown by molecular-beam epitaxy have been studied by means of grazing-incidence x-ray-scattering techniques. From the in-plane diffraction pattern, we find that hcp-Co(11.0) is epitaxially grown on Cr(001) with the Co[00.1] axis parallel to Cr[110]. This epitaxial relation is equivalent to the Pitsch-Schrader orientational relationship. The Co layers exhibit a precursive structural phase transition from hcp to bcc with decreasing Co layer thickness ${\mathit{t}}_{\mathrm{Co}}$. First, the Co in-plane unit cell changes its shape from rectangular (hcp) to square (bcc), due to the epitaxial strain. Second, the intensity I of the hcp-Co{1¯1.1} spots shows a continuous decrease with decreasing ${\mathit{t}}_{\mathrm{Co}}$. The intensity I is proportional to the square of the displacement δ of the Co atoms from the bcc center sites, which is the order parameter for this structural transition. This intensity decrease therefore implies that the Co lattice continuously changes its symmetry from hcp to bcc. The experimentally determined ${\mathit{t}}_{\mathrm{Co}}$ dependence of δ is in good agreement with a model calculation describing the competition of the surface potential on Cr(001) and the bulk hcp potential of Co. Finally, we find that the observed anomalous out-of-plane expansion of the Co layers is a precursor of the structural transition. The non-Poisson-like behavior can be explained by a rigid-atom model which takes into account the atomic displacement during the structural transition.