Orbital and spin magnetic moments of transforming one-dimensional iron inside metallic and semiconducting carbon nanotubes

The orbital and spin magnetic properties of iron inside metallic and semiconducting carbon nanotubes are studied by means of local x-ray magnetic circular dichroism (XMCD) and bulk superconducting quantum interference device (SQUID). The iron-nanotube hybrids are initially ferrocene filled single-walled carbon nanotubes (SWCNT) of different metallicities. We show that the ferrocene's molecular orbitals interact differently with the SWCNT of different metallicities with no significant XMCD response. At elevated temperatures the ferrocene molecules react with each other to form cementite nanoclusters. The XMCD at various magnetic fields reveal that the orbital and/or spin magnetic moments of the encapsulated iron are altered drastically as the transformation to the 1D clusters takes place. The orbital and spin magnetic moments are both found to be larger in filled semiconducting nanotubes than in the metallic sample. This could mean that the magnetic polarization of the encapsulated material depends on the metallicity of the tubes. From a comparison between the iron 3d magnetic moments and the bulk magnetism measured by SQUID, we conclude that the delocalized magnetisms dominate the magnetic properties of these 1D hybrid nanostructures.

Figure 1

TEM characterization of the samples. Overall low magnification (a), (b) and intermediate magnification (c) images of the semiconducting-tubes sample. The inset in (b) is a high magnification micrograph of one of the Fe clusters in the main panel. Corresponding overall low magnification (d) and intermediate magnification (e), (f) images of the metallic-tubes sample after annealing.

Figure 2

Observed XAS (a) and XMCD (b) response in the L${}_{2,3}$ edge of Fe for the metallic and the semiconducting FeCp${}_2$@SWCNT samples when a magnetic field of 6 T is applied to the sample at 5 K. All the spectra are offsetted by 0.1 and normalized by $\int ({\mu }^{+}+{\mu }^{-})/2$.

Figure 3

Observed XAS (a) and XMCD (b) response in the L${}_{2,3}$ edge of Fe for the metallic and the semiconducting Fe@DWCNT samples when a magnetic field of 4 T is applied to the sample at room temperature. All the spectra are offset by 0.1 and normalized by $\int ({\mu }^{+}+{\mu }^{-})/2$.

Figure 4

Definition of the integrals used to calculate the orbital [Eq. (1)] and spin [Eq. (2)] magnetic moments. (a) Total XAS and its integral in the whole L${}_{2,3}$ edge ($r$); (b) XMCD response and its integrals in the L${}_3$ ($p$) and the whole L${}_{2,3}$ edge ($q$).

Figure 5

Spin and orbital magnetic moments of the metallic and semiconducting FeCp${}_2$@SWCNT. The data are fitted with the modified Langevin function. The measurements were done at 5 K.

Figure 6

Spin and orbital magnetic moments of the metallic and semiconducting Fe@DWCNT. The data are fitted with the modified Langevin function. The measurements were done at 300 K.

Figure 7

Total magnetic moment measured by SQUID. (a) For pristine semiconducting and metallic SWCNT, FeCp${}_2$@SWCNT and Fe@DWCNT; (b) in the low magnetic field region for semiconducting and metallic Fe@DWCNT. The magnetic moments were calculated per molecular unit with one Fe atom (b). The diamagnetic background of the pristine nanotubes were subtracted from the data of the FeCp${}_2$ and Fe filled nanotubes. The measurements were done at 300 K.