Magnetic nanotubes obtained from atomic layer deposition coated electrospun nanofibers

High-aspect ratio Al₂O₃/Ni/Al₂O₃ core-shell hollow nanotubes were prepared by combining electrospinning, atomic layer deposition, and a subsequent chemical reduction process. Electrospun poly(vinyl alcohol) (PVOH) nanofibers were obtained by electrospinning. Then, these fibers were coated with atomic layer deposition to obtain PVOH/Al₂O₃/NiO/Al₂O₃ core-shell nanowires. Since the NiO must be deposited at 200 °C, the PVOH nanofibers must first be coated with Al₂O₃ at 80 °C, which act as a protective layer of the fibers so that they can withstand higher temperatures. Once PVOH/Al₂O₃/NiO/Al₂O₃ core-shell nanowires are obtained, they are subjected to a chemical reduction process that generates Al₂O₃/Ni/Al₂O₃ core-shell hollow nanotubes. Their morphologies were studied by scanning and transmission electron microscopies, the thickness was determined by ellipsometry, and all magnetic measurements were performed in an alternating-gradient force magnetometer. Finally, assuming that the nanostructures exhibit a curling reversal process, the authors have analytically calculated the coercivity of the nanotubes.