Carbon nanotubes (CNTs) filled with iron nanowires (Fe@CNTs) were synthesized by chemical vapor deposition on substrates with catalyst thin film coatings. The Fe@CNTs showed magnetic shape anisotropy due to the high aspect ratio shape of the nanowires. Here, the authors examined how the conditions of catalyst films (Fe and Ni), i.e., crystal structures and oxidation states, influenced the growth characteristics and the magnetic properties of the Fe@CNTs. The conditions of the catalyst films depended on whether vacuum deposition or sputter deposition was used. The coercivity of the Fe@CNTs was influenced by the conditions of the catalyst films. Their results showed that the sputter-deposited Ni film, which had the smallest grain size and lowest oxidation, showed the highest coercivity.

Carbon nanotubes (CNTs) filled with iron nanowires show high coercivity owing to their shape anisotropy originating from the high-aspect-ratio shapes of the iron nanowires. In this study, CNTs filled with iron were prepared by the thermal chemical vapor deposition (T-CVD) method using ferrocene as a precursor, and the magnetic properties of the synthesized CNTs were examined in detail. It was found that the CVD temperature and the amount of the ferrocene introduced into the CVD reactor influence the amount of CNT growth and the magnetic properties. The high coercivity of approximately 1.6 kOe or higher was obtained under certain CVD conditions. The selected-area electron diffraction analysis showed that the magnetic properties of the CNTs filled with iron are determined by the crystal structure (α-Fe or Fe3C) of the iron nanowires, which can be controlled by the amount of the ferrocene supplied during CVD.

Carbon nanotubes filled with iron (Fe-filled CNTs) show shape anisotropy on account of the high aspect ratio of magnetic nanowires, and are promising candidates for various applications, such as magnetic recording media, probes for scanning force microscopy, and medical treatment for cancer. The ability to appropriately control the magnetic properties of CNTs for those applications is desirable. In this study, the authors investigated magnetic properties of Fe-filled CNTs synthesized by thermal chemical vapor deposition for the purpose of tuning their coercivity. Here, the authors implemented hydrogen plasma irradiation of catalyst film that was previously deposited on a substrate as a catalyst layer. This treatment activates the catalyst film and thus enhances the growth of the Fe-filled CNTs. It was confirmed that the H2 plasma irradiation enhances the growth of the CNTs in terms of increasing their length and diameter compared to CNTs without irradiation. On the other hand, the coercivity of Fe-filled CNTs dropped to approximately half of those without H2 plasma irradiation. This is probably due to a decrease in the aspect ratio of the Fe nanowires, which results from the increase in their diameter. Furthermore, the crystal structure of the Fe nanowires may affect the coercivity.

Carbon nanotubes filled with iron nanowires (Fe@CNTs) show magnetic anisotropy because of the high aspect ratio of the nanowires and are expected to find numerous applications such as magnetic recording media, probes for scanning force microscopy and medical treatment for cancer (hyperthermia). These applications require precise control of Fe@CNT growth. This growth depends on the condition of the substrate surface on which the CNTs are grown. In this study, Fe@CNTs were synthesized by thermal chemical vapor deposition on various thin metal films (Fe, Ni, Co, Al, Pt, Ti, or Mo) deposited on SiO2/Si(100) substrates using ferrocene as a precursor, and their growth characteristics were investigated. The growth clearly depends on the metal species incorporated in the thin films. Magnetic properties correlated with growth morphologies. Therefore, the appropriate selection of an underlayer is important for controlling magnetic characteristics of Fe@CNTs.