Fabrication of $$\text {Ni-NiBi}_3$$ ferromagnet-superconductor nano particles

Magnetic nanoparticles with an additional functional component are appealing systems for fundamental and applied research. Although a wide range of multifunctional magnetic nanoparticles have been reported in the literature, ferromagnet-superconductor nanoparticle systems are rare. Here, we report the fabrication of a ferromagnet-superconductor nanoparticle system via a fully physical route. As the intermetallic superconducting compound NiBi(_3) forms spontaneously at the interface between Ni and Bi layers, we first formed Ni nanoparticles, with sizes ranging between 50 and 200 nm, by thermal dewetting of Ni films on a Si/SiO(_2) substrates. A very thin layer of Bi was deposited on top to allow the formation of NiBi(_3) at the Bi-Ni interface by reaction-diffusion process. The formation of NiBi(_3) on the boundary of Ni particles was confirmed via high-resolution transmission electron microscopy and superconducting transition in the NiBi(_3) was verified by magnetometry. The as dewetted, bare Ni nanoparticles showed superparamagnetic behavior, typical of a collection of single-domain ferromagnetic nanoparticles. Temperature-dependent magnetization showed an evolution of diamagnetic response below 4 K over a period of 2 weeks, indicating the gradual formation of the superconducting NiBi(_3).