Phase evolution in thermally annealed Ni/Bi multilayers studied by X-ray absorption spectroscopy

The thin films of Ni and Bi are known to form NiBi₃ and NiBi compounds spontaneously at the interface, which become superconducting below 4.2 K and show ferromagnetism either intrinsically or due to Ni impurities. Formation of NiBi₃ and NiBi is a slow diffusion reaction, which means the local environment around Ni and Bi atoms may vary with time and temperature. In this report, we assess the feasibility of using X-ray Absorption Spectroscopy (XAS) as a tool to track the changes in local bonding environment in NiBi₃ and NiBi. Thermal annealing at temperatures up to 500 °C was used to induce changes in the local environment in NiBi₃ system. Consequent decomposition of NiBi₃ into NiO and Bi has been tracked through changes in structural and magnetization behavior, which matched well with the findings of XAS. In addition, the magnetic hysteresis measurements indicated that NiO should be the dominant phase when NiBi₃ is annealed at 500 °C. This was corroborated from XAS and was found to be >90%. The shift in K-edge of Ni in annealed samples was attributed to increasing charge state on Ni atom, which was ascertained by Bader charge analysis using Density Functional Theory (DFT). This study correlating macroscopic properties of NiBi₃ with local bonding environment of the system indicates that XAS can be a very reliable tool for studying dynamics of diffusion in the NiBi₃ system.