Spontaneous morphology and phase modification driven by sequence of deposition in superconducting Ni-Bi bilayers

Abstract We report how the sequence of deposition of Ni and Bi affects the final morphology of Ni-Bi bilayers. Two samples have been prepared by sputtering with the same thickness of Ni (4 nm) and Bi (40 nm) but reversed sequence of deposition. The sample with Bi deposited first formed a smooth layer of NiBi3 intermetallic, while the sample Ni deposited first formed mainly NiBi3 nanoparticles with its out-of-plane dimensions bigger than the thickness of the total deposited bilayer. The NiBi3 phase in the two samples results from differing nucleation and growth processes leading to differing morphologies. The volume of NiBi3 is smaller than the pure Bi volume needed for the reaction and this volume contraction is a thermodynamic factor that favors the NiBi3 formation. Both samples are superconducting with the critical temperature Tc of ∼ 4.0 K. The Ni-Bi system offers an example of the formation of a superconducting compound under a volume contraction of its constituents, thus being of interest for both researchers in physics and material sciences.