Spin-reorientation transitions in cobalt nanowires: Role of the microstructure and finite-size effects
We present self-consistent electronic structure calculations in order to analyze the magnetocrystalline anisotropy energy as well as the local spin Sδ (i) and orbital Lδ (i) magnetic moments for finite-length Co nanowires. We consider monatomic chains, as well as single-crystal and polycrystalline nanowires, assuming two directions of magnetization δ oriented along (δ=x) and perpendicular (δ=z) to the wire axis. The single-crystal wires are cylindrical fragments of the fcc lattice grown along the (111) direction, while the polycrystalline structures are characterized by the presence of various fcc and hcp grains. We obtain that the Co chains are defined by a highly stable magnetization direction oriented along the chain axis. However, when three-dimensional wires are considered, we found a strong influence of the local atomic environment on the determination of the easy axis. In fact, we observe strong oscillations of the low energy orientation of the magnetization between the parallel and perpendicular directions as the length of the wire increases or when the geometrical details of the wire caps are modified. In contrast, in the polycrystalline structures, we found that the electronic perturbation introduced by the presence of internal interfaces is more localized in nature. Actually, no spin reorientation transitions are found for different microstructures; however, reduced energy differences (compared to uniform single-crystal wires with approximately the same length) between the parallel and perpendicular directions are obtained. Finally, the existence of internal interfaces can induce appreciable perturbations in the density of states around the Fermi energy and, as a consequence, they are expected to modify the number of conduction channels as well as the transport properties of the wires. © 2008 The American Physical Society.
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Physical Review B - Condensed Matter and Materials Physics
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