Magnetization profile for impurities in graphene nanoribbons (original) (raw)

Suppression of spin polarization in graphene nanoribbons by edge defects and impurities

2008

We investigate the effect of edge defects (vacancies) and impurities (substitutional dopants) on the robustness of spin-polarization in graphene nanoribbons (GNRs) with zigzag edges, using density-functional-theory calculations. The stability of the spin state and its magnetic moments is found to decrease continuously with increasing concentration of defects or impurities. The system generally becomes non-magnetic at the concentration of one edge defect (impurity) per ∼ 10Å. The spin suppression is shown to be caused by reduction and removal of edge states at the Fermi energy.

Carrier density and magnetism in graphene zigzag nanoribbons

Physical Review B, 2009

The influence of carrier density on magnetism in a zigzag graphene nanoribbon is studied in a pi\pipi-orbital Hubbard-model mean-field approximation. Departures from half-filling alter the magnetism, leading to states with charge density variation across the ribbon and parallel spin-alignment on opposite edges. Finite carrier densities cause the spin-density near the edges to decrease steadily, leading eventually to the absence of magnetism. At low doping densities the system shows a tendency to multiferroic order in which edge charges and spins are simultaneously polarized.

Stability of edge states and edge magnetism in graphene nanoribbons

Physical Review B, 2011

We critically discuss the stability of edge states and edge magnetism in zigzag edge graphene nanoribbons (ZGNRs). We point out that magnetic edge states might not exist in real systems and show that there are at least three very natural mechanisms-edge reconstruction, edge passivation, and edge closure-which dramatically reduce the effect of edge states in ZGNRs or even totally eliminate them. Even if systems with magnetic edge states could be made, the intrinsic magnetism would not be stable at room temperature. Charge doping and the presence of edge defects further destabilize the intrinsic magnetism of such systems.

Edge state magnetism of single layer graphene nanostructures

Chemical Physics, 2008

We study edge state magnetism in graphene nanostructures using a mean field theory of the Hubbard model. We investigate how the magnetism of the zigzag edges of graphene is affected by the presence of other types of terminating edges and defects. By a detailed study of both regular shapes, such as polygonal nanodots and nanoribbons, and irregular shapes, we conclude that the magnetism in zigzag edges is very robust. Our calculations show that the zigzag edges that are longer than three to four repeat units are always magnetic, irrespective of other edges, regular or irregular. We, therefore, clearly demonstrate that the edge irregularities and defects of the bounding edges of graphene nanostructures does not destroy the edge state magnetism.

Magnetic States of the Zigzag Edge of a Graphene Nanoribbon

Physics of the Solid State, 2020

Using a simple structural model and the multicenter Anderson Hamiltonian, Green's functions are obtained for the atoms of the zigzag edge of an epitaxial graphene nanoribbon. The electronic structure of the free nanoribbon is discussed in detail. Specifically, expressions for the band spectrum and density of states are found and estimates of the occupation numbers and magnetic moments are given. For a nanoribbon strongly bonded to a metal substrate, the criteria for the appearance of magnetic moments are determined. As it is shown for both free and epitaxial nanoribbons, the probability of the appearance of magnetic moments and their magnitude for zigzag edge atoms that have two nearest neighbors is higher than for atoms with three nearest neighbors.

Edge-functionalized and substitutionally doped graphene nanoribbons: Electronic and spin properties

Physical Review B, 2008

Graphene nanoribbons are the counterpart of carbon nanotubes in graphene-based nanoelectronics. We investigate the electronic properties of chemically modified ribbons by means of density functional theory. We observe that chemical modifications of zigzag ribbons can break the spin degeneracy. This promotes the onset of a semiconducting-metal transition, or of a half-semiconducting state, with the two spin channels having a different band gap, or of a spin-polarized half-semiconducting state, where the spins in the valence and conduction bands are oppositely polarized. Edge functionalization of armchair ribbons gives electronic states a few eV away from the Fermi level and does not significantly affect their band gap. N and B produce different effects, depending on the position of the substitutional site. In particular, edge substitutions at low density do not significantly alter the band gap, while bulk substitution promotes the onset of semiconducting-metal transitions. Pyridinelike defects induce a semiconducting-metal transition.

Broken Edge Spin-Symmetry Induces Spin-polarized Current in Graphene Nanoribbon

Zig-zag graphene nanoribbons (ZGNRs) are known to possess spin moments at the hydrogen- terminated edge carbon atoms, thus the spin-polarized electron transmission is expected while the current is longitudinally passed through the ZGNRs. However, in pristine ZGNRs, the spin polarized transmission is not observed due to symmetric anti-parallel distributions of the spin densities between the edges. Here, the hypothesis is, any physical or chemical process that breaks such anti-parallel spin-symmetry can induce spin-polarized transmission in the ZGNRs. In this work, we have established this proof-of-concept by depositing the trimethylenemethane (TMM) radical on 6ZGNRH and investigating the quantum transport properties by employing the density functional theory in conjunction with nonequilibrium Green’s function (DFT-NEGF) method. Although TMM has a high magnetic moment (2 µB ), it does not induce magnetization in 6ZGNRH when TMM is physisorbed. But, during the chemisorption of TMM, it ...

Edge magnetization and local density of states in chiral graphene nanoribbons

Physical Review B, 2014

We study the edge magnetization and the local density of states of chiral graphene nanoribbons using a π-orbital Hubbard model in the mean-field approximation. We show that the inclusion of a realistic next-nearest hopping term in the tight-binding Hamiltonian changes the graphene nanoribbons band structure significantly and affects its magnetic properties. We study the behavior of the edge magnetization upon departing from half filling as a function of the nanoribbon chirality and width. We find that the edge magnetization depends very weakly in the nanoribbon width, regardless of chirality as long as the ribbon is sufficiently wide. We compare our results to recent scanning tunneling microscopy experiments reporting signatures of magnetic ordering in chiral nanoribbons and provide an interpretation for the observed peaks in the local density of states, that does not depend on the antiferromagnetic interedge interaction.

Unveiling the Magnetic Structure of Graphene Nanoribbons

Physical Review Letters, 2011

We perform magnetotransport measurements in lithographically patterned graphene nanoribbons down to a 70 nm width. The electronic spectrum fragments into an unusual Landau levels pattern, characteristic of Dirac fermion confinement. The two-terminal magnetoresistance reveals the onset of magnetoelectronic subbands, edge currents and quantized Hall conductance. We bring evidence that the magnetic confinement at the edges unveils the valley degeneracy lifting originating from the electronic confinement. Quantum simulations suggest some disorder threshold at the origin of mixing between chiral magnetic edge states and disappearance of quantum Hall effect.

Emergence of local magnetic moments in doped graphene-related materials

Physical Review B, 2009

Motivated by recent studies reporting the formation of localized magnetic moments in doped graphene, we investigate the energetic cost for spin polarizing isolated impurities embedded in this material. When a well-known criterion for the formation of local magnetic moments in metals is applied to graphene we are able to predict the existence of magnetic moments in cases that are in clear contrast to previously reported Density Functional Theory (DFT) results. When generalized to periodically repeated impurities, a geometry so commonly used in most DFT-calculations, this criterion shows that the energy balance involved in such calculations contains unavoidable contributions from the long-ranged pairwise magnetic interactions between all impurities. This proves the fundamental inadequacy of the DFT-assumption of independent unit cells in the case of magnetically doped low-dimensional graphene-based materials. We show that this can be circumvented if more than one impurity per unit cell is considered, in which case the DFT results agree perfectly well with the criterion-based predictions for the onset of localized magnetic moments in graphene. Furthermore, the existence of such a criterion determining whether or not a magnetic moment is likely to arise within graphene will be instrumental for predicting the ideal materials for future carbon-based spintronic applications.