Structural characterization of epitaxial cobalt-chromium superlattices.

A series of epitaxial Co-Cr superlattices has been grown by molecular beam epitaxy and characterized with x-ray diffraction and in-situ RHEED. Using some x-ray techniques newly developed in my laboratory I have been able to provide complete descriptions of both the growth process and an analysis of epitaxial phase transitions. The superlattices are epitaxially ordered such that the in-plane crystallographic orientations of the individual layers are preserved during the growth process as indicated by the RHEED studies. This epitaxial ordering is confirmed (for Cr layers up to 6A) by in-plane x-ray scattering measurements. We have accomplished this work with only standard laboratory x-ray equipment. While synchrotron radiation offers several advantages, e.g., for higher resolution measurements, the techniques developed here are convenient for a laboratory setting which combines growth and structural characterization. X-ray scans along 101l are used to distinguish hcp and fcc stacking sequences in both Co and Cr layers. We find that the Co layers and the first 2-3 monolayers of Cr are stacked according to an hcp sequence. The data clearly show that we were able to synthesize a new close-packed metastable phase of Cr up to a layer thickness of 6A, providing an example of epitaxial stabilization of a structure which does not occur in the bulk. In addition, we showed that beyond this thickness there is an abrupt structural transition to bcc (110) Cr in the classical Nishiyama-Wasserman and Kurdjumov-Sachs orientations.