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 $\mu$ m 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 Co $_{33}$Pt $_{67}$ nanotubes exhibit hard magnetic properties with a coercivity of 100 kA/m, a coercivity of 680 kA/m was achieved with the Co$_{45}$ Pt$_{55}$ nanotubes. This increase in the coercivity was attributed to the transformation of the FCC Co-Pt to the ${\rm L}1_{0}$ phase, which happens upon annealing at 700$^{\circ}$C 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 ${\rm L}1_{0}$ phase to occur.