Reactive diffusion and superconductivity of Nb3Al multilayer films

Thin films of A15 Nb3Al have been prepared by reactive diffusion of sputter-deposited Nb/Al multilayers. The diffusion reactions were studied by in situ annealing x-ray diffraction in the temperature range 50–950 °C. Initially the Nb and Al sublayers react to form the phase NbAl3. This interface reaction prevents the formation of the σ-phase Nb2Al, frequently found as a second phase in A15 Nb3Al materials; NbAl3 reacts with the remaining Nb to form the A15 phase. The highest Tc, 16.2 K measured resistively and 15.2 K inductively, was found in a Nb/Al multilayer with an A15 cell parameter a0=5.195 Å which corresponds to ∼20 at. % Al. From a comparison with previous investigations of the Tc dependence on Al concentration and A15 cell parameter, it is concluded that a small amount of the A15 phase has a higher composition of 22–23 at. % Al.