Stabilization of γ-/α-Fe nanocrystalline phases inside carbon nanotube buckypapers and functionalization with Gd

The stabilization of γ-Fe inside carbon nanotubes (CNTs) and its magnetic-coupling with α-Fe or Fe₃C has attracted a great interest for the possible control of the hysteresis-loss processes in magnetic hyperthermia. Being interested in exploring further the stabilization-dynamics of this phase (γ-Fe), we have performed novel investigations on the fabrication and manipulation of filled CNTs films (buckypapers) with major focus on the effects of vapour-flow- and cooling-rate synthesis-parameters. X-ray diffractograms (XRD) and Rietveld refinements evidence an unusual stabilization of γ-Fe upon cooling, with a maximum relative abundance of 26 % after a slow-cooling step of 30 min. In-situ post-growth manipulation of the CNT-structure, in presence of tris(tetramethylcyclopentadienyl)‑gadolinium (III), allows for a particularly efficient functionalization of the CNT-walls, with in-situ surface-deposition of an amorphous Gd-phase. The rapid-cooling approach employed in the post-growth Gd-functionalization-step is found to have beneficial effects on the γ-Fe stabilization, with an enhanced relative abundance of 30 %, as extracted from Rietveld refinements. Extended characterization through magnetometry, evidence a dependence of the coercive-field value on both cooling and flow-rate parameters. The reported dataset confirms the existence of a magnetic-coupling within the encapsulated nanocrystals, with the unit-cell-volume being in the order of 45.9 ų.