Abstract
Density functional calculations of the electronic structure of the Fe${}_{12}$ cluster encapsulated inside finite single-wall zigzag carbon nanotubes of indices $(11,0)$ and $(10,0)$ have been performed. Several Fe${}_{12}$ isomers have been considered, including elongated shape isomers aimed to fit well inside the nanotubes, and the icosahedral minimum energy structure. We analyze the structural and magnetic properties of the combined systems, and how those properties change compared to the isolated systems. A strong ferromagnetic coupling between the Fe atoms occurs both for the free and the encapsulated Fe${}_{12}$ clusters, but there is a small reduction (3\char21{}$7.4\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{B}$) of the spin magnetic moment of the encapsulated clusters with respect to that of the free ones ($\ensuremath{\mu}=38\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{B}$). The reduction of the magnetic moment is mostly due to the internal redistribution of the spin charges in the iron cluster. In contrast, the spin magnetic moment of the carbon nanotubes, which is zero for the empty tubes, becomes nonzero (1\char21{}$3\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{B}$) because of the interaction with the encapsulated cluster. We have also studied the encapsulation of atomic Fe and the growth of small Fe${}_{n}$ clusters ($n=2$, 4, 8) encapsulated in a short $(10,0)$ tube. The results suggest that the growth of nanowires formed by distorted tetrahedral Fe${}_{4}$ units will be favorable in $(10,0)$ nanotubes and nanotubes of similar diameter.
Highlights
Carbon nanotubes encapsulating atoms, molecules, or clusters have been synthesized, and their properties have been analyzed, due to their fundamental and technological interest
As we have shown in a previous work,[15] the zigzag nanotubes (ZNTs) present electronic properties associated to edge states similar to those of zigzag graphene ribbons,[17] our study could be of relevance in analyzing the properties of proposed magnetoresistive devices based on zigzag graphene ribbons.[18]
As far as finite ZNTs can be considered as organic molecules containing π electrons, our results can be useful in the interpretation of the local spin polarization at the organicferromagnetic interface, which has been recently described both theoretically and experimentally.[19]
Summary
Molecules, or clusters have been synthesized, and their properties have been analyzed, due to their fundamental and technological interest. The aim of this work is to present a theoretical study of the electronic and magnetic properties of iron clusters encapsulated in carbon nanotubes, using density functional theory (DFT).[11] We shall focus on the Fe12 cluster encapsulated inside finite pieces of single-wall zigzag nanotubes (ZNTs) of indices (11,0) and (10,0). The two ends show opposite orientation of the atomic magnetic moments.[12,13,14,15] Other states are close in energy to the ground state: magnetic states (M) with net total magnetic moment μT different from zero (in some of these, the magnetic moments at the two edges show the same orientation13–15), and a nonmagnetic (NM) state showing local spin compensation along the nanotube. As far as finite ZNTs can be considered as organic molecules containing π electrons, our results can be useful in the interpretation of the local spin polarization at the organicferromagnetic interface, which has been recently described both theoretically and experimentally.[19]
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