Confinement-modulated phase transition of Fe-Ni melts in carbon nanotubes.

Fe-Ni nanowires exhibit excellent magnetic properties, making them promising candidates for spintronic and high-density storage applications. Under carbon nanotube (CNT) confinement, their structural evolution and phase behavior remain insufficiently understood. Here, we systematically investigate the solidification and structural evolution of Fe-Ni melts confined within CNTs. Molecular dynamics simulations reveal that nanoconfinement suppresses crystallization and induces amorphous solidification, leading to the formation of coaxial layered nanowires governed by the evolution of common neighbor sub-clusters (CNSs). We demonstrate that structural evolution is controlled by a density-induced structural crossover mechanism. With increasing density, the system transitions from disordered clusters to ordered coaxial structures, reaching an optimal state at ∼7 g cm-3, where packing and distortion are balanced and the potential energy is minimized. In addition, CNT diameter regulates the nanowire architecture, giving rise to distinct structural configurations and a diameter-dependent phase diagram. Finally, first-principles calculations reveal strong spin polarization and robust spin-dependent transport, highlighting the potential of these nanowires for spintronic applications. This work establishes a unified framework in which confinement and density jointly govern structural crossover in nanoconfined metallic systems, providing guidance for the design of functional nanomaterials.