Enhanced magnetism in Fe-filled carbon nanotubes produced by pyrolysis of ferrocene

By optimization of the synthesis of ferromagnetic-filled carbon nanotube ensembles on Si substrates (catalytic decomposition of ferrocene) and following annealing at $645°C$⁠, marked hysteresis loops can be measured by the alternating-gradient method. Unusually high coercivities and strong anisotropies with an easy magnetic axis parallel to the alignment of the nanotubes are observed from the as-grown samples, whereas an enhanced magnetic saturation moment (up to a factor of 2) and a decreased anisotropy are realized after annealing at $645°C$⁠. The increase of the magnetic saturation moment of the Fe-filled carbon nanotube ensembles is caused by the entire transformation within the tubes of the $γ-Fe$ and $Fe3C$ phases to ferromagnetic $α-Fe$ and graphite. X-ray diffraction with different glancing incidence shows that the $γ-Fe$ is predominantly at the tips of the nanotubes, while the iron carbide resides closer to the substrate. However, after the annealing process only $α-Fe$ is found. At an annealing temperature of $675°C$ the nanotube structures are destroyed and the magnetic characteristics are dramatically altered (viz., the disappearance of anisotropy and reduction in coercivity).