Reaction-Diffusion-Driven Stoichiometric Gradient in Coevaporated Superconducting NiBi3 Thin Films

The intermetallic compound NiBi3 belongs to the interesting class of superconductors that are based on strong ferromagnetic elements. Although NiBi3 has been grown in single-crystal form and has also been demonstrated to form at the interface of Ni and Bi layers, uniform NiBi3 thin films have yet to appear in the literature. In this work, we have explored the codeposition method, wherein high-purity Ni and Bi metals are evaporated simultaneously at controlled rates, to study the dynamics involved in the formation of NiBi3 films. We have used the Rutherford backscattering spectrometry (RBS) technique to make a quantitative stoichiometric comparison of codeposited films grown at two different evaporation rates of Bi, while keeping the evaporation rate of Ni fixed. We find that, due to the highly diffusive nature of the Bi atoms in this system, the growth dynamics favors the formation of almost stoichiometric NiBi3 only at the top surface (few tens of nanometers) of the films, whereas there is a nonlinear stoichiometric variation toward the substrate. The average local bonding environment around Ni, studied by synchrotron-based X-ray absorption spectroscopy, agreed with the overall findings of RBS. Since the physical properties of NiBi3 are sensitive to the ratio of Ni and Bi, we find the superconducting transition and magnetic coercivity to be consistent with the above observations.