Effect of interface morphology on intermetallics formation upon annealing of Al–Ni multilayer
Introduction
Alloys of nickel and aluminium are important in industrial applications and research fields because of their attractive properties like light weight, high strength, high melting point, good oxidation resistance and high mechanical strength [1], [2], [3]. Intermetallics 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 multilayer 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. Al3Ni, Al3Ni2, AlNi, and AlNi3 in order of increasing Ni concentration. 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 × 1022 cm−3 and of Al as 6.02 × 1022 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 Al3Ni 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 demonstrated 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 mm2). 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 showed asymmetric roughness at interfaces as measured by reflectometry data. Ni on Al (Ni/Al) interfaces showed much higher roughness compared to Al on Ni (Al/Ni) interfaces. We annealed the sample at 160 °C from 1 h to 8 h and looked for the alloy composition at the interfaces. Our analysis shows that upon annealing the sample at 160 °C, Al3Ni alloy formed at the Al/Ni (Al on Ni) whereas Al3Ni2 alloy layer has been found at the Ni/Al interface (Ni on Al) interface. Effective heat of formation rule predicts that the first interface alloy layer should be Al3Ni [16]. According to Colgan et al. [13] formation of interface alloy will also depend on kinetics as well as interface composition. Our work specifically highlights this issue. According to the PNR analysis these interface alloy layers carry zero magnetic moment and hence are considered to be magnetically dead layers.
Section snippets
Experimental details
A multilayer sample Si/[Al(25 Å)/Ni(50 Å)] × 10 was prepared by ion beam sputtering at a base pressure of 2 × 10−9 Torr [11]. The deposition rate for both the elements was 0.1 Å/sec. During deposition the thickness of the layers were calibrated using a water cooled quartz crystal monitor. The samples were annealed at 160 °C for time intervals of 1 h, 4 h and 8 h. XRR and PNR data were collected after successive annealing. The XRR data were taken in a Bruker’s D8 advanced laboratory source and the PNR data
Results and discussion
Fig. 1 shows the PNR (R+ and R−) data from as-deposited and annealed Al-Ni multilayer. Closed (red)1 and open (blue) circles in Fig. 1 depict the experimental spin dependent reflectivities R+ and R−, respectively. Fig. 1a shows the PNR profile for as-deposited multilayer sample. PNR profiles for sample annealed at 160 °C for 1 h, 4 h and 8 h are shown in Fig. 1b–d, respectively. Reflectivity plots were
Summary
We carried out depth dependent structure and magnetic properties of a Al–Ni multilayer (with thickness ratio of d(Al)/d(Ni) = 1:2) in as-deposited and annealed conditions (at 160 °C for 1–8 h) using XRR and PNR. The as-deposited multilayer showed asymmetric roughness at the interfaces. On annealing the sample at 160 °C for 1h we observed asymmetric alloy formation at interfaces, which might have resulted from asymmetric roughness in the as-deposited sample. Detailed analysis of XRR and PNR suggested
Acknowledgements
SB acknowledges the help received from Chuck Majkrzak of NCNR, NIST during the PNR experiment.
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