Structure and magnetic properties of iron nanowires encased in multiwalled carbon nanotubes

By using X-ray diffraction, transmission electron microscopy, ferromagnetic and electron paramagnetic resonances, Fe-filled multiwalled carbon nanotubes were studied. The iron within the nanotubes was found to be in three phases: the austenite γ-Fe is located at the top of the nanotubes, while the ferrite α-Fe and cementite θ-Fe₃C are found close to the substrate. The morphology of the α + θ mixture suggests that the eutectoid reaction γ → α + θ occurs. Two ferromagnetic signals were observed and identified as those belonging to ferrite and cementite. Ferromagnetic signals revealed a surprising temperature dependence: with decreasing temperature, their integral intensity decreases nearly linearly, and the signals disappear at temperatures below 70 K. Possible reasons for this behaviour are discussed. Two paramagnetic signals appear at low temperatures. Based on their temperature behaviour and on the values of the g-factors, the first signal is related to localized spin moments and the second one to free electrons.