Magnetic, thermal and martensitic phase transitions in Ni-Co nanoparticles.

The nickel-cobalt system is an important alloy in nanotechnology because of its ferromagnetic properties, high thermal stability and shape memory effect. At the nanoscale, its physico-chemical properties become size-, shape- and composition- dependent. However, those properties are still mostly unknown at the nanoscale. Therefore, this manuscript fills this gap in knowledge. Indeed, when the size of the alloy is reduced, the entire phase diagram undergoes a vertical shift downward to lower temperatures, and a tilt caused by a larger size effect affecting cobalt than nickel. Among the most commonly observed shapes, the thermal stability of nickel-cobalt nanoparticles was determined as a function of their size and composition. Furthermore, the ferromagnetic-paramagnetic phase transition and martensitic transformation for those shapes were calculated as a function of the nickel-cobalt alloy composition. As expected, the ferromagnetic and martensitic regions shrunk at the nanoscale; but the martensitic transformation was less affected by size effects compared to magnetic and thermal (melting) transitions.