PHYSICAL REVIEW B 87, 085402 (2013) Electronic and magnetic properties of Fe clusters inside finite zigzag single-wall carbon nanotubes F. I. Horga and A. Ma˜ nanes * Departamento de F´ ısica Moderna, Universidad de Cantabria, E-39005 Santander, Spain M. J. L ´ opez and J. A. Alonso Departamento de F´ ısica Te´ orica, At ´ omica y ´ Optica, Universidad de Valladolid, E-47011 Valladolid, Spain (Received 15 August 2011; revised manuscript received 7 December 2012; published 4 February 2013) 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 (3–7.4 μ B ) of the spin magnetic moment of the encapsulated clusters with respect to that of the free ones (μ = 38 μ 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 (1–3 μ 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. DOI: 10.1103/PhysRevB.87.085402 PACS number(s): 73.22.−f, 73.63.Fg, 75.75.−c, 85.75.−d I. INTRODUCTION Carbon nanotubes encapsulating atoms, molecules, or clusters have been synthesized, and their properties have been analyzed, due to their fundamental and technological interest. Nanotubes filled with Fe have potential uses in medicine, 1 in spintronic devices, 2 including spin valves, 3 and can also be used as probes for magnetic force microscopy. 4,5 Recent reviews on the preparation of filled carbon nanotubes 6,7 indicate the activity in the field. In general, free metallic magnetic nanoparticles are oxide coated, but the encapsulation of Fe clusters and nanowires in carbon nanotubes, where these metallic nanostructures are protected from the oxidating environment, 8 has been shown to be feasible. 9,10 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 Fe 12 cluster encap- sulated inside finite pieces of single-wall zigzag nanotubes (ZNTs) of indices (11,0) and (10,0). Finite zigzag nanotubes have a peculiar magnetic structure: the ground state is an antiferromagnetic (AFM) spin singlet S = 0. The local spin polarization oscillates (spin up and spin down) along the direction of the nanotube axis, and it is small, except at the two ends of the nanotube, due to the existence of electronic states localized at the edges of the nanotube. The two ends show opposite orientation of the atomic magnetic moments. 12–15 Other states are close in energy to the ground state: magnetic states (M) with net total magnetic moment μ T different from zero (in some of these, the magnetic moments at the two edges show the same orientation 13–15 ), and a nonmagnetic (NM) state showing local spin compensation along the nanotube. The energy difference E(μ 0 T ) − E(AFM) between the lowest-lying magnetic state (with total magnetic moment μ 0 T ) and the ground state (AFM) decreases monotonically when the length of the nanotube increases, up to a point where the difference becomes lower than room temperature. 14 For the shortest zigzag carbon nanotubes, E(μ 0 T ) − E(AFM) increases when the diameter of the nanotube increases, showing an oscillating effect. 16 Our aim is to analyze the interplay between the singular magnetic properties of the finite ZNTs and the large magnetization of the encapsulated Fe clusters. As we have shown in a previous work, 15 the ZNTs present electronic properties associated to edge states similar to those of zigzag graphene ribbons, 17 therefore our study could be of relevance in analyzing the properties of proposed magnetoresistive devices based on zigzag graphene ribbons. 18 Furthermore, 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 organic- ferromagnetic interface, which has been recently described both theoretically and experimentally. 19 Previous calculations of the electronic structure of 3d transition-metal atoms and clusters bonded to car- bon nanostructures have considered infinitely long carbon nanotubes, 20–28 an infinite graphene sheet, 26,29,30 capped finite ZNTs, 31,32 or finite nanotubes with unsaturated open ends. 33 The effects due to the edge states have not been analyzed in those works. The calculations indicate that the magnetism is confined within the Fe aggregates, with a small reduction of the magnetic moment per atom with respect to the free Fe clusters. On the other hand, the magnetic moments per atom result enhanced with respect to those of bulk Fe, due to the reduced coordination of the metal atoms. In the particular case of iron aggregates encapsulated in finite (8,0) nanotubes, the calculations 33 indicate that the cluster is ferromagnetic, with a total magnetic moment which is larger for the capped tubes than for those with unsaturated open ends. Fujima and Oda 32 have studied iron chains encapsulated in finite capped (5,0) and (8,0) ZNTs. They found that the magnetism 085402-1 1098-0121/2013/87(8)/085402(12) ©2013 American Physical Society