Theoretical study of carbon-coated iron nanowires Mariana Weissmann, 1 G. García, 2 Miguel Kiwi, 2 and R. Ramírez 2 1 Departamento de Física, Comisión Nacional de Energía Atómica, Avenida del Libertador 8250, (1429) Buenos Aires, Argentina 2 Facultad de Física, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 6904411, Chile (Received 18 August 2004; published 1 November 2004) Several properties of hybrid systems made of iron nanowires coated with carbon are computed from first principles. In particular, we focus on how the presence of carbon determines the magnetic ordering. A quasi- one-dimensional fcc (or hcp) Fe structure favors ferromagnetic ordering, but when encapsulated into a C tube, antiferromagnetic ordering can become favorable. The spin polarization at the Fermi level is large for the bare nanowires, but it decreases due to hybridization with the carbon coating. Implications of these results for the fabrication of nanodevices, as well as for the appearance of exchange bias, are discussed. DOI: 10.1103/PhysRevB.70.201401 PACS number(s): 71.10.-w, 71.20.Tx, 72.80.Le I. INTRODUCTION Transition-metal carbon-coated nanowires constitute an active and attractive field of research, 1 as these systems are promising materials for use in nanodevices and in the mag- netic storage industry. In addition, their potential use in spin- tronics provides a strong motivation to develop a full under- standing of them, since the combination of C nanotubes and ferromagnetic transition metals (TM) has the right ingredi- ents of small size and tailorable magnetic and transport prop- erties. In particular, recent experimental work has shown that it is possible to produce iron-filled carbon nanotubes and that these have very interesting properties, such as a shift in the hysteresis loop. 2 However, not all the samples present the same properties; 3–5 in fact, these properties appear to depend on the fabrication procedure. All these reasons provide the motivation to study theoretically the effect of carbon coating of quasi-one-dimensional Fe systems. Previous works on this subject studied carbon nanotubes and only a small number of Fe atoms inside or close to them. 6–8 In this Rapid Communication our purpose is to study Fe nanowires of a reasonable diameter. In order to make the calculation feasible we have not coated them with complete carbon nanotubes but with a smaller number of carbon atoms. We have investigated from first principles the magnetic structure, the conduction-band polarization, and other features of uncoated Fe nanowires and compared them with the carbon-coated ones and also with previous results for other C-encapsulated TM hybrid structures. II. THE SYSTEM The nanosystem we have chosen to study is small enough to allow for an ab initio calculation but has a larger diameter 5Å than that of previous works, and it is therefore more appropriate to compare with experimental results. The nanowires contain two types of Fe atoms, external or periph- eral ones with very few nearest neighbors (nn), and interior Fe atoms, with a number of nn that is close to that of bulk Fe. With respect to the atomic structure of these wires we notice that each Fe atom in a bcc structure nanowire would have a smaller number of neighbors than in a fcc or hcp structure nanowire, thus making it less stable. We must re- member that the stability of the bulk bcc structure is due to the presence of six second nearest neighbors, located at al- most the same distance as the eight first nn, but these would be absent in systems of a diameter accessible to our calcula- tions. For this reason we decided to use nanowires following the (111) direction of an hcp structure that repeats itself ev- ery two planes (ABABA…). This structure is illustrated in Fig. 1. The TM system has six atoms per layer, arranged as an equilateral triangle, with TM atoms on the vertices (which we will refer to as peripheral atoms) and at the midpoints of each side of the triangle (interior atoms); two successive TM layers are rotated relative to each other by 60°. The coating carbon atoms are located outside the TM nanowire and equi- distant to two contiguous TM layers, forming straight C chains parallel to the wire axis. This is of course not a carbon nanotube, but we may consider it as a first approximation, to study the influence of carbon coating. The interior TM atoms have ten nn and the peripheral ones have only four nn. The total number of atoms in the unit FIG. 1. The unit cell used in the calculation, viewed parallel and perpendicular to the tube axis. The dark circles represent carbon atoms and the two shades of gray represent the two atomic layers of iron. The Fe- Fe nearest-neighbor distances are 2.48 Å and the Fe-C and C-C ones are 2.03 Å. PHYSICAL REVIEW B 70, 201401(R)(2004) RAPID COMMUNICATIONS 1098-0121/2004/70(20)/201401(4)/$22.50 ©2004 The American Physical Society 70 201401-1