LOCAL ORDERING IN MULTILAYERS AS SEEN BY X-RAY DIFFRACTION

D. Rafaja, J. Vacínová and J. Lhotka

Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic

The structure of multilayers, which are deposited by other methods than by the molecular beam epitaxy (MBE), is usually far from an ideally ordered state. Nevertheless, multilayers prepared by using the electron beam deposition, for instance, are successfully applied for magnetic and magneto-optic purposes. Therefore, the investigation of the real structure of imperfect multilayers is demanded. However, different techniques must be combined to get a better insight into the real structure of such systems. Concerning the X-ray methods, the reflectivity measurements performed in the low-angle region (0-10^o) are always combined with the diffraction measurements done in the high-angle region (typically above 30^o2q for the CuKa radiation). Both the symmetrical and asymmetrical measurements are performed as the symmetrical measurements yield an information in the perpendicular direction but the insight into the lateral direction can only be obtained in an asymmetrical mode.

Our contribution is devoted to the application of asymmetrical scans in the high-angle region for understanding the local ordering in metallic multilayers. Originally, the asymmetrical measurements in the high-angle region were performed to examine the alloying of adjacent layers in the Co/Ni and Co/Pt/Ni/Pt multilayers, and were also applied for the study of the Fe/Au multilayers. These diffraction experiments were carried out with a constant glancing angle of incidence using the parallel beam technique. Although the presence of satellites corresponding to the multilayer structure was expected near the symmetrical position only, the satellites were detected at a rather large inclination of the scattering vector from the perpendicular direction. The intensity of satellites has decreased with the increasing asymmetry of the diffraction set-up, but the decrease was too slow to be explained by a sole curvature of individual layers.

An explanation of this phenomenon was proposed, which is based on the model of weak correlated atomic chains and clusters. It will be shown, how the degree of correlation influences the decrease of intensity at increasing inclination of the scattering vector from the perpendicular direction. Two-dimensional cuts in the reciprocal space obtained from experiment will be compared with calculated diffraction patterns. The experimental diffraction patterns were taken in a series of off-scans; the calculation of the reciprocal space maps was based on the kinematic theory of diffraction. Two examples of the calculated cut in the reciprocal space map are given in Figure 1.

Figure 1: Two-dimensional cuts in the reciprocal space calculated for the correlation length of 6 nm (a) and 2 nm (b).