Reaction kinetics and biasing in Al/Ni multilayers

The phase evolution from annealing ion-beam-sputtered Al/Ni multilayers was studied using X-ray diffraction and differential scanning calorimetry measurements. The sequence of phase formation depends on the modulation wavelength Λ and the average composition 〈c〉. The initial phase formed may also depend on the composition gradient. Annealing short Λ samples with a composition of either Al0.40Ni0.60 or Al0.75Ni0.25 produced only AlNi or Al3Ni, i.e. the stable phase with the same 〈c〉. At intermediate Λ, Al9Ni2 was often the initial phase formed. Experiments were performed on multilayer samples in which 20 nM AlNi layers, denoted as biasing layers, were placed between each Al and Ni layer. The AlNi modified the kinetics, increasing the nucleation temperatures and changing the amounts of the product phases. The AlNi layer appears to dissociate in the later stages of annealing. Composition gradients and biasing layers may be used to control the phases present in technological materials.