Unusual stabilization of γ-Fe/α-Fe nanocrystalline junctions and antiferromagnetic transition within cm-scale buckypapers of entangled multiwall carbon nanotubes

Abstract

We report the observation of an unusual weak antiferromagnetic coupling arising within μm long Fe-based nanowires encapsulated within cm-scale buckypapers of entangled multiwall carbon nanotubes (CNTs). The samples were produced in-situ by employing a custom-designed Cl-assisted chemical vapour deposition (CVD) approach, in which the chlorine-radicals have the crucial role of slowing-down the CNTs growth and enhancing the filling-ratio of the CNT-capillary. Interestingly, an enhancement of the Ar-vapour flow rate from 20 mL/min to 130 mL/min was found to have significant effects towards the stabilization of the FCC nanoscale γ-Fe phase, with a maximum relative abundance of 31% estimated by means of X-ray diffraction (XRD) and theoretical Rietveld refinement analyses. TEM, HRTEM and Fourier transform analytical methods allowed for the identification of sharp magnetic junctions established between the coexisting Fe3C, α-Fe and γ-Fe phases. Magnetometry and (field-dependent) zero-field-cooled (ZFC)/field cooled (FC) analytical methods, allowed the identification of a weak Néel temperature signal at T∼ 130 K. The origin of the antiferromagnetic component was investigated by tuning the applied field from 300 Oe to 50,000 Oe. The obtained results were further corroborated by means of magnetisation vs field acquisitions, with a significant enhancement in the coercive parameter in proximity of the Néel T-point.