Hysteresis shift in Fe-filled carbon nanotubes due to γ-Fe

The phase distribution of high aspect ratio, Fe-filled carbon nanotubes prepared by pyrolyzing a mixture of powered ferrocene and ${\mathrm{C}}_{60}$ has been determined by means of M\"ossbauer spectroscopy. Our results for that characterization are closely related to the observation, after field cooling processes, of a hysteresis loop shift and clearly suggest a spatial phase distribution which includes the presence of a \ensuremath{\gamma}-Fe/\ensuremath{\alpha}-Fe interface. The temperature dependence of the hysteresis loop shift is discussed in terms of localized regions at that interface exhibiting uncompensated antiferromagnetism within reduced dimensions.