Ferromagnetically filled carbon nanotubes: Radial structures and tuning of magnetic properties through new synthesis methods.
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Multiwall carbon nanotubes filled with continuous single-crystals of the ferromagnetic phase -Fe were produced with two new synthesis methods: the boundary layer chemical vapour synthesis and the perturbed vapour chemical vapour deposition. In the first method, the nanotubes nucleate and grow radially from a central agglomeration of homogeneously nucleated spherical particles in a randomly fluctuating vapour created in the viscous boundary layer between a rough surface and a laminar pyrolyzed-ferrocene/Ar vapour flow. In the second method, the nanotubes nucleate and form in a flower-like arrangement departing from homogeneously nucleated particles. These particles are produced by the creation of a local perturbation in a vapour with a high density of Fe and C species obtained from the pyrolysis of ferrocene in a laminar Ar flow. Electron microscopy investigations revealed that the continuous single crystals obtained with both methods exhibit diameters much lower than the critical diameter for a single magnetic domain of -Fe (~ 66 nm). In the radial structures, the single-crystal diameter is in the range of ~ 17-37 nm, while in the flower-like structures the single crystals show mainly a diameter of ~ 30 nm and ~ 55 nm. The average single crystals length is 7-8 m in the case of the radial structures and 19-21 m in the case of the flower-like structures. DC magnetization measurements at 5 K show different magnetic behaviours. The flower-like structures present a very high saturation magnetization of 189.5 emu/g and a high coercivity of 580 Oe. The radial structures exhibit an exchange-coupled ferromagnetic/antiferromagnetic system despite only 2% of -Fe is present inside the nanotubes. The radial structures obtained at flow-rates of 3.5 ccm and 20 ccm, show saturation-magnetizations of 31emu/g and 13 emu/g, and coercivities of 790 Oe and 843 Oe respectively.
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