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

TLDR

The authors optimized ferrocene decomposition on Si substrates and annealed the resulting Fe‑filled carbon nanotubes at 645 °C, enabling measurable hysteresis loops via alternating‑gradient magnetometry. Annealing at 645 °C transforms γ‑Fe and Fe₃C into ferromagnetic α‑Fe and graphite, doubling the saturation magnetization, reducing anisotropy, and aligning the easy axis along the nanotube direction, while higher temperatures (675 °C) destroy the tubes and eliminate anisotropy and coercivity.

Abstract

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).