Hard Magnetic Co-Pt-Based Nanotubes Produced via Direct Electroplating

Co-Pt nanostructures having a tubular geometry, a diameter of 200 nm and a length of 2 mum were produced via direct electroplating into polycarbonate-based templates. The nanotube-formation mechanism was explained using the relative rates of deposition and the diffusion of the metal ions. The as-deposited Co-Pt nanotubes are magnetically soft, with magnetocrystalline anisotropy prevailing over the shape anisotropy. The Co-Pt nanotubes were prepared in two different stoichiometries. While the Co₃₃Pt₆₇ nanotubes exhibit hard magnetic properties with a coercivity of 100 kA/m, a coercivity of 680 kA/m was achieved with the Co₄₅Pt₅₅ nanotubes. This increase in the coercivity was attributed to the transformation of the FCC Co-Pt to the L1₀ phase, which happens upon annealing at 700degC for 60 min. In the first case the low coercivity is attributed to the composition being slightly out of range for the transformation to the L1₀ phase to occur.