Effect of interface morphology on intermetallics formation upon annealing of Al–Ni multilayer Mitali Swain a,⇑ , Surendra Singh a , Saibal Basu a , Mukul Gupta b a Solid State Physics Division, Bhabha Atomic Research Center, Mumbai 400 085, India b UGC DAE Consortium for Scientific Research, Khandwa Road, Indore 452 017, India article info Article history: Received 27 February 2013 Received in revised form 18 April 2013 Accepted 21 April 2013 Available online 29 April 2013 Keywords: Intermetallics Magnetic films and multilayers Surfaces and interfaces Diffusion Neutron reflectometry X-ray reflectometry abstract Following Benès rule, the initial phase formed at interface of Al–Ni multilayer on annealing is Al 3 Ni. On further annealing it should give rise to intermetallics Al 3 Ni 2 , AlNi and AlNi 3 , 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 Al 3 Ni and Al 3 Ni 2 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 ele- ments at interface, which results into change in effective heat of formation of the alloys and thus makes Al 3 Ni 2 more favourable at Ni/Al interfaces. PNR results also suggested that alloy layers are magnetically dead. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Alloys of nickel and aluminium are important in industrial applications and research fields because of their attractive proper- ties like light weight, high strength, high melting point, good oxidation resistance and high mechanical strength [1–3]. Interme- tallics can be produced by several methods like cold rolling [4,5], ball milling or mechanical alloying [6,7], shock compaction and self sustained reactions [8]. But special care has to be taken during new phase evolution in the system. Thermal annealing in vacuum is one of the suitable methods for inducing solid state reactions between pure Ni and Al layers to produce nickel aluminides [9,10] in a mul- tilayer thin film with alternating Ni and Al layers. Ni and Al both are iso-structural (fcc) at room temperature with cell parameters 3.52 Å and 4.05 Å, respectively. Al atomic size is 13% larger compared to Ni, which also reflects in their respective unit cell parameters. They form several ordered intermetallics viz. Al 3 Ni, Al 3 Ni 2 , AlNi, and AlNi 3 in order of increasing Ni concen- tration. Interface diffusion in thin films and multilayer systems is mediated through several external parameters like temperature [9] and reaction time [11]. In case of multilayer stacks, we find that thickness ratio of the respective components is also an important parameter [12]. Considering the number density (no. of scatterers per unit volume) of Ni as 9.1  10 22 cm 3 and of Al as 6.02  10 22 cm 3 , in bulk, to get an alloy with 1:1 atomic ratio in a multilayer stack we should have a thickness ratio d(Al)/d(Ni) of 1.5:1 [9]. Similarly to get a stoichiometry of 1:3 in Al:Ni we should have a thickness ratio d(Al)/d(Ni) of 1:2. Thickness of the present system is based on these calculations. Between Al and Ni, Al with a lower melting point is the more mobile species. Usually during alloy formation at the interfaces Al 3 Ni is the first phase that forms at lower temperatures of annealing typically (about 250–300 °C) [13]. Thickness of the component layers also might dictate the first phase formed at the interfaces, which further decides the kinetics of phase formation at the interfaces [12]. Earlier we had studied the structure and magnetism of a Al–Ni multilayer film and the effect of annealing on the system with thickness ratio, d(Al)/d(Ni) of 1:1 [11]. Using X-ray reflectivity (XRR) [14] and polarized neutron reflectivity (PNR) [15], we dem- onstrated that it is possible to quantify the composition of binary alloy formed at the interface of Al–Ni multilayer on annealing [11]. XRR and PNR are two non-destructive techniques that provide quantitative measures of the chemical and magnetic depth profiles of films with nanometer resolution averaged over the lateral dimensions of the entire sample (typically 100 mm 2 ). Here we study the structural and magnetic properties of a Al–Ni multilayer and effect of annealing on the multilayer with a thickness ratio, d(Al)/d(Ni), of 1:2, which gives an atomic stoichiometry of Al:Ni equal to 1:3, making it rich in Ni. Also the as-deposited sample 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.04.140 ⇑ Corresponding author. Tel.: +91 22 25594070. E-mail address: mitali.@barc.gov.in (M. Swain). Journal of Alloys and Compounds 576 (2013) 257–261 Contents lists available at SciVerse ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom