Effect of interface morphology on intermetallics formation upon annealing of Al-Ni multilayer

Following Benès rule, the initial phase formed at interface of Al-Ni multilayer on annealing is Al3Ni. On further annealing it should give rise to intermetallics Al3Ni2, AlNi and AlNi3, gradually increasing in Ni content. Using X-ray and polarized neutron reflectivity (PNR) we studied the depth dependent structure and magnetization of as-deposited and annealed Al-Ni multilayer, with thickness ratio of Al:Ni equal to 1:2 giving an overall atomic stoichiometry of 1:3 for Al:Ni. We identified asymmetric initial alloy phase formation at two interfaces, i.e. Ni on Al (Ni/Al) and Al on Ni (Al/Ni), of Al-Ni multilayer on annealing. The phases are identified as Al3Ni and Al3Ni2 at Al/Ni and Ni/Al interfaces respectively. Our results indicate that the asymmetric alloy formation at interfaces is directly correlated with the interface morphology of as-deposited multilayer. Higher roughness at Ni/Al interface changes the local concentration of elements at interface, which results into change in effective heat of formation of the alloys and thus makes Al3Ni2 more favourable at Ni/Al interfaces. PNR results also suggested that alloy layers are magnetically dead.