Structural changes in Co/Zr multilayers during annealing below the crystallisation temperature

Abstract

The local structure of two multilayer systems Co/Zr of different layer thickness during annealing well below the crystallisation temperature was determined by means of extended X-ray absorption fine structure (EXAFS), performed at Co K edge as well as at Zr K edge. By taking additionally wide angle X-ray scattering (WAXS) measurements into consideration, the averaged EXAFS signal summing up contributions of all exited atoms in the sample could be separated into parts belonging to the crystalline and amorphous phases which occurred in the intermediate states of the system Co/Zr simultaneously. The existence and conservation of fibre textures within the crystalline layers and the conservation of periodicity in chemical composition perpendicular to the layer plane could be proved. Moreover, intensities of the 0002 and 10–11 reflections of Zr provided reasonable starting models for fitting crystalline Zr to a subshell model of close lying shells. Coordination numbers obtained for the crystalline phases of Co and Zr agreed well with the bulk data. Looking from the position of either absorbing species the atomic radial distribution was evaluated. Zr neighbours dominated for Zr as well as for Co central atoms. Around Co atoms we found Co at typical distances of 2.45, 2.78 and 3.06 Å; around Zr the Zr neighbours occupied neighbour shells at 3.07 and 3.30 Å. Distances between unlike atoms were found to be 2.65 and 2.81Å. These values describe single symmetric contributions of asymmetric pair distributions derived on the basis of multiple Gauss analysis. From EXAFS and WAXS we concluded that the amorphous part of the layer system is initially dominated by a Zr-rich phase which competes with a second amorphous phase of an approximate composition Co 50Zr 50 during the the subsequent stages. As the reaction proceeds the latter grows while the Zr-rich phase is reduced.