(FexCo1−x)-alloy filled vertically aligned carbon nanotubes grown by thermal chemical vapor deposition
Introduction
Bulk Fe–Co alloys are known to have attractive soft magnetic properties. In contrast to Fe, Co or Ni, the Fe–Co alloy exhibits a high saturation magnetization, negligible magneto-crystalline anisotropy and a high Curie temperature, which is important for high temperature applications [1], [2]. With the advent of nanofabrication techniques, opportunities to observe unique magnetic properties in nanostructures made from these alloys arose that were previously thought impossible. For example, in-plane arrangements of Fe–Co alloy nanowires exhibit high coercivities (Hc) and a large shape anisotropy with the easy axis of magnetization parallel to the nanowire axis, in contrast to the respective bulk material [3].
The encapsulation of Fe–Co alloy particles inside carbon nanotubes gives the possibility to create dimensionally confined nanowires without the use of elaborate structuring techniques, such as e-beam lithography [4], imprint technology [5], or template methods [6], [7], which are typical for the preparation of arrays. Moreover, the carbon shells provide an effective barrier against oxidation, which ensures a long-term stability of an individual ferromagnetic nanowire, in contrast to most wires extracted in template-based methods. This is a crucial aspect as the high tendency of uncovered magnetic nanowires to oxidize is an appreciable disadvantage, which strongly affects the material properties. Here we report, for the first time, on the synthesis of aligned Fe–Co alloy-filled MWNTs. They were grown on silicon substrates by the pyrolysis of a ferrocene/cobaltocene mixture.
Section snippets
Experimental procedure
The synthesis was carried out in a quartz tube reactor inside a two-stage furnace system [8]. The substrates used in our study comprised thermally oxidized Si (1 0 0) wafers (∼1 μm SiO2). An Ar/H2 mixture was used as carrier gas and ferrocene and cobaltocene were employed as precursors. These materials supply both Co and Fe for the filling inside the carbon nanotubes and C for growing the nanotube shells. The process was performed as follows. A quartz boat with a ferrocene/cobaltocene mixture
Results and discussion
Fig. 1 displays a SEM image of the aligned growth of Fe–Co alloy-filled MWNTs on the silicon substrate. Only a negligible amount of small metal particles encapsulated by graphitic carbon was observed at the periphery of the tubes. The nanotubes are aligned more or less perpendicularly to the substrate surface. Their length may reach values of about 10 μm, with diameters ranging between 30–50 nm.For the TEM studies nanotubes were removed from the substrate and dispersed onto a copper grid. The
Conclusion
We have demonstrated that via pyrolysis of a ferrocene/cobaltocene mixture aligned (FexCo1−x)-alloy filled MWNTs can be grown on a silicon substrate. The nanotubes show higher coercivity and squareness comparing with the already reported values of MWNTs filled with elemental ferromagnets. The aligned Fe–Co alloy-filled MWNTs exhibit a stronger magnetic shape anisotropy, which may be a promising aspect for future applications in magnetic recording.
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Cited by (0)
- 1
Present address: Max Planck Institute of Microstructure Physics, Halle.
- 2
Present address: Institute of Solid State Research IFF-IEE, Research Center Jülich, D-52425 Jülich, Germany.