Edge Magnetism Research Articles

Spin motive force driven by skyrmion dynamics in magnetic nanodisks

The spin motive force driven by the dynamics of the skyrmion structure formed in a nanomagnetic disk is numerically investigated. Due to the existence of the magnetic structure along the disk edge, the collective mode of the magnetization is modified from that of the bulk skyrmion lattice obtained by using the periodic boundary condition. For a single-skyrmion disk, the dynamics of the skyrmion core and the edge magnetization induce the spin motive force, and a measurable AC voltage is obtained by two probes on the disk. For a multi-skyrmions disk, the phase-locked collective mode of skyrmions is found in the lowest resonant frequency where the amplitude of the AC voltage is enhanced by the cascade effect of the spin motive force. We also investigate the effect of the Rashba spin-orbit coupling on the spin motive force.

Two-dimensional hexagonal V2O nanosheet and nanoribbons

A novel two-dimensional hexagonal V2O was proposed and investigated through first-principles calculations. The hexagonal V2O nanosheet exhibited a V–O–V sandwich structure. The stability of monolayer V2O was verified by formation energy and phonon frequency calculations. The calculations of electronic properties using a hybrid density functional method indicate that monolayer V2O is a nonmagnetic metal. However, the zigzag nanoribbon exhibited robust edge magnetism, while the armchair nanoribbon was found to be a nonmagnetic semiconductor with a direct band gap. The variety of electronic properties of monolayer V2O nanosheets and nanoribbons opens up tremendous opportunities for nano-electronic and spintronic devices.

Role of Chemical Potential in Flake Shape and Edge Properties of Monolayer MoS2

The precise control of the edge geometry and crystal shape of monolayer MoS2 is of particular importance for their application in nanoelectronics and photoelectro catalysts. Here, we reveal a crucial role of chemical potential in the determination of equilibrium shape (ES) and edge structure of monolayer MoS2 by using density-functional theory calculations. Applying the Wulff construction rule, our results demonstrate the shape evolution of monolayer MoS2 flake from the dodecagonal shape, then to the hexagonal shape, to the triangular shape with the variation of chemical potential from the Mo-rich to the S-rich condition, and the edge structure of ES changes correspondingly from mixed zigzag/armchair edges to purely zigzag edges. This finding can be applied to explain extensive experimental observations about the morphology of MoS2 domains. Likewise, the edge magnetism and electronic structures of monolayer MoS2 domains are found to be dependent on their edge structure, which provides specific guidance for the magnetic modulation of monolayer MoS2 and the design of more effective MoS2-based catalysts.

Electric and geometric controlling of the magnetic coupling in Kane-Mele nanoribbons

In this paper, we show that indirect spin interaction J between two magnetic impurities located on honeycomb Kane-Mele zigzag/armchair ribbons (KMZR/KMAR) is easily controlled by staggered potential and geometry. We demonstrate that J in periodic-boundary KMZR reaches maximum at the edges, and oscillates between antiferromagnetic and ferromagnetic couplings when tuning the sublattice staggered potential Δ. The odd-even length effect of J in KMZR and the width dependence of J in KMAR are also presented. These results clearly demonstrate the unique role of topological edge states and finite-size effect in magnetic coupling of quantum spin Hall (QSH) ribbons, and the controllability of the edge magnetism, hence favoring the fabrication of the spintronic devices in two-dimensional buckled honeycomb materials, e.g., silicene and germanene.

Strain-induced edge magnetism at the zigzag edge of a graphene quantum dot

We study the temperature dependent magnetic susceptibility of a strained graphene quantum dot using the determinant quantum Monte Carlo method. Within the Hubbard model on a honeycomb lattice, our unbiased numerical results show that a relative small interaction $U$ may lead to an edge ferromagneticlike behavior in the strained graphene quantum dot. Around half-filling, the ferromagnetic fluctuations at the zigzag edge are strengthened both by the on-site Coulomb interaction and the strain, especially in the low temperature region.

Magnetic Centres in Functionalized Graphene

Discussion of the origin of paramagnetic centres observed by electron paramagnetic resonance in graphene oxide (GO) and reduced graphene oxide (rGO) is done on the assumption that GO can be considered as a functionalized graphene. This leads to the conclusion that the narrow signal with g close to 2, observed for GO and thermally reduced GO, is due to paramagnetic centres localized on defects and exchange coupled to conduction electrons. Randomness of graphene modi cation results in variety of parameters of EPR signals. The broad signals observed in GO and rGO and ascribed to magnetic clusters on the zig-zag edge states indicate that the edge magnetism can be preserved by functionalization.

Electromechanical response at polar zigzag boundaries in hybrid monolayers

First-principles calculations are used to demonstrate electromechanical control of charge and spin at zigzag-edged interfaces between graphene and boron-nitride domains in hybrid monolayers. We show how, through a direct piezoelectric effect, the interfacial bound charges and associated electric fields can be tuned by application of an external mechanical force (stress) on the system. This results in mechanical control of the edge magnetization (piezomagnetic effect), and the possibility to transform a semiconducting heterostructure into a half-metal. The inverse effect (application of an external electric field to induce a mechanical deformation) goes together with a magnetoelectric response, which under ideal conditions we estimate to be comparable to that of prototypical ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$. These effects originate from the magnetic properties of graphene's zigzag edges and the dielectric properties of the boron-nitride domain, and can also be expected in any other coplanar heterostructures with polar discontinuities.

Limited robustness of edge magnetism in zigzag graphene nanoribbons with electrodes

It is shown that apart from well-known factors, like temperature, substrate, and edge reconstruction effects, also the presence of external contacts is destructive for the formation of magnetic moments at the edges of graphene nanoribbons (GNRs). The edge magnetism gradually decreases when graphene/electrode interfaces become more and more transparent for electrons. In addition to the graphene/electrode coupling strength, the aspect ratio parameter, i.e. a width/length ratio of the GNR, is also crucial for the suppression of edge magnetism. The present theory uses a tight-binding method, based on the mean-field Hubbard Hamiltonian for π electrons, and the Greenʼs function technique within the Landauer–Büttiker approach.

Electrical Control of Edge Magnetism in Two-Dimensional Buckled Honeycomb Lattice

We theoretically study indirect spin coupling strength between two magnetic impurities located on honeycomb Kane—Mele zigzag ribbon (KMZR) with periodic and open boundary (PB and OB). We show that spin interaction J in PB ribbons displays an AFM-FM oscillating behavior with increasing the staggered potential and electron density, and approaches to maximum at the edges. While the spin coupling in OB KMZR shows a trivial smooth AFM coupling with varying staggered potential. Such a novel J(Δ) behavior is the combining effect of finite size, topological edge states and inversion symmetry breaking induced by the staggered potential. We propose that one could control the edge magnetism electrically in two-dimensional buckled honeycomb materials, e.g., silicence, germanene and stanene.

Effect of Zn doping on the magneto-caloric effect and critical constants of Mott insulator MnV2O4

X-ray absorption near edge spectra (XANES) and magnetization of Zn doped MnV2O4 have been measured and from the magnetic measurement the critical exponents and magnetocaloric effect have been estimated. The XANES study indicates that Zn doping does not change the valence states in Mn and V. It has been shown that the obtained values of critical exponents β, γ and δ do not belong to universal class and the values are in between the 3D Heisenberg model and the mean field interaction model. The magnetization data follow the scaling equation and collapse into two branches indicating that the calculated critical exponents and critical temperature are unambiguous and intrinsic to the system. All the samples show large magneto-caloric effect. The second peak in magneto-caloric curve of Mn0.95Zn0.05V2O4 is due to the strong coupling between orbital and spin degrees of freedom. But 10% Zn doping reduces the residual spins on the V-V pairs resulting the decrease of coupling between orbital and spin degrees of freedom.

Preserving the edge magnetism of graphene nanoribbons by iodine termination: a computational study

First-principles computations revealed that iodine (I) is an ideal terminal group for zigzag graphene nanoribbons (zGNRs) in terms of stabilizing the pure sp2 coordinated edges and preserving the edge magnetism. Due to the strong steric effect of I atoms, the unfavorable sp3 coordination can be efficiently suppressed and the pure sp2 coordinated edges can be stabilized at rather feasible experimental conditions. Interestingly, the electronic structures of I-terminated zGNRs (I-zGNRs) with different edge configurations can be well rationalized by employing the Clar’s model. I-zGNRs can well reproduce the electronic and magnetic properties of those hydrogen-terminated zGNRs. Remarkably, I termination can significantly lower the critical electric field required to induce the half-metallic behavior. These results open new opportunities in fabricating spintronics devices based on zGNRs.

High performance current and spin diode of atomic carbon chain between transversely symmetric ribbon electrodes.

We demonstrate that giant current and high spin rectification ratios can be achieved in atomic carbon chain devices connected between two symmetric ferromagnetic zigzag-graphene-nanoribbon electrodes. The spin dependent transport simulation is carried out by density functional theory combined with the non-equilibrium Green's function method. It is found that the transverse symmetries of the electronic wave functions in the nanoribbons and the carbon chain are critical to the spin transport modes. In the parallel magnetization configuration of two electrodes, pure spin current is observed in both linear and nonlinear regions. However, in the antiparallel configuration, the spin-up (down) current is prohibited under the positive (negative) voltage bias, which results in a spin rectification ratio of order 104. When edge carbon atoms are substituted with boron atoms to suppress the edge magnetization in one of the electrodes, we obtain a diode with current rectification ratio over 106.

Semiconductor–halfmetal–metal transition and magnetism of bilayer graphene nanoribbons/hexagonal boron nitride heterostructure

The paper presents the results of ab initio study of electronic structure modulation and edge magnetism in the antiferromagnetic (AF) bilayer zigzag graphene nanoribbons (AF-BZGNR)/hexagonal boron nitride (h-BN(0001)) semiconductor heterostructure induced with transverse external electric field (Eext) and nanomechanical compression (extension), performed within the framework of the density functional theory using Grimme׳s DFT(PBE)-D2 scheme. For the first time we established critical values of Eext and interlayer distance in the bilayer for the BZGNR/h-BN(0001) heterostructure providing for semiconductor–halfmetal–metal phase transition for one of the electron spin configurations. We discovered the effect of preserved local magnetic moment (0.3μB) of edge carbon atoms of the lower (buffer) graphene nanoribbon during nanomechanical uniaxial compression (or extension) of the BZGNR/h-BN(0001) semiconductor heterostructure. It has been demonstrated that magnetic properties of the AF-BZGNR/h-BN(0001) semiconductor heterostructure can be controlled using Eext. In particular, the local magnetic moment of edge carbon atoms decreases by 10% at a critical value of the positive potential. We have established that local magnetic moments and band gaps can be altered in a wide range using nanomechanical uniaxial compression and Eext, thus making the AF-BZGNR/h-BN(0001) semiconductor heterostructure potentially promising for nanosensors, spin filters, and spintronics applications.

Manipulation of edge magnetism in hexagonal graphene nanoflakes

We explore possible ways to manipulate the intrinsic edge magnetism in hexagonal graphene nanoflake with zigzag edges, using density functional theory supplemented with on-site Coulomb interaction. The effect of carrier doping, chemical modification at the edge, and finite temperature on the edge magnetism has been studied. The magnetic phase diagram with varied carrier doping, and on-site Coulomb interaction is found to be complex. In addition to the intrinsic antiferromag- netic solution, as predicted for charge neutral hexagonal nanoflake, fully polarized ferromagnetic, and mixed phase solutions are obtained depending on the doped carrier concentration, and on-site Coulomb interaction. The complexity arises due to the competing nature of local Coulomb in- teraction and carrier doping, favoring antiferromagnetic and ferromagnetic coupling, respectively. Chemical modification of the edge atoms by hydrogen leads to partial quenching of local moments, giving rise to a richer phase diagram consisting of antiferromagnetic, ferromagnetic, mixed, and nonmagnetic phases. We further report the influence of temperature on the long-range magnetic ordering at the edge using ab initio molecular dynamics. In agreement with the recent experimental observations, we find that temperature can also alter the magnetic state of neutral nanoflake, which is otherwise antiferromagnetic at zero temperature. These findings will have important implications in controlling magnetism in graphene based low dimensional structures for technological purpose, and in understanding varied experimental reports.

Edge magnetization and local density of states in chiral graphene nanoribbons

We study the edge magnetization and the local density of states of chiral graphene nanoribbons using a $\ensuremath{\pi}\text{-orbital}$ 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 nanoribbon 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 on 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.

Localization of metallicity and magnetic properties of graphene and of graphene nanoribbons doped with boron clusters

As a possible way of modifying the intrinsic properties of graphene, we study the doping of graphene by embedded boron clusters with density functional theory. Cluster doping is technologically relevant as the cluster implantation technique can be readily applied to graphene. We find that B clusters embedded into graphene and graphene nanoribbons are structurally stable and locally metallize the system. This is done both by the reduction of the Fermi energy and by the introduction of boron states near the Fermi level. A linear chain of boron clusters forms a metallic “wire” inside the graphene matrix. In a zigzag edge graphene nanoribbon, the cluster-related states tend to hybridize with the edge and bulk states. The magnetism in boron-doped graphene systems is generally very weak. The presence of boron clusters weakens the edge magnetism in zigzag edge graphene nanoribbon, rather than making the system appropriate for spintronics. Thus, the doping of graphene with the cluster implantation technique might be a viable technique to locally metallize graphene without destroying its attractive bulk properties.

Geometric phase at a graphene edge: Scattering phase shift of Dirac fermions

We study the scattering phase shift of Dirac fermions at graphene edge. We find that when a plane wave of a Dirac fermion is reflected at an edge of graphene, its reflection phase is shifted by the geometric phase resulting from the change of the pseudospin of the Dirac fermion in the reflection. The geometric phase is the Pancharatnam-Berry phase that equals the half of the solid angle on Bloch sphere determined by the propagation direction of the incident wave and also by the orientation angle of the graphene edge. The geometric phase is finite at the zigzag edge in general, while it always vanishes at the armchair edge because of intervalley mixing. To demonstrate its physical effects, we first connect the geometric phase with the energy band structure of graphene nanoribbon with the zigzag edge. The magnitude of the band gap of the nanoribbon, that opens in the presence of the staggered sublattice potential induced by edge magnetization, is related to the geometric phase. Second, we numerically study the effect of the geometric phase on the Veselago lens formed in a graphene nanoribbon. The interference pattern of the lens is distinguished between armchair and zigzag nanoribbons, which is useful for detecting the geometric phase.

The Edge Magnetization and Strip Phase of Graphene Quantum Dots with Long-Range Coulomb Interaction

We investigate the magnetism and optical absorption properties of charge neutral hexagonal graphene quantum dots (GQDs) terminated with zigzag edges by using a tight-binding Hubbard type model for the π electrons. Within the Hartree—Fock approximation and taking into account the long-range Coulomb interaction, our calculation yields a ferromagnetic ground state with magnetic moments localized on the edges for GQDs, and also gives an antiferromagnetism state with the energy very close to the ferromagnetism ground state. We find that both the ferromagnetic and the antiferromagnetic states have stripe patterned charge density distributions as a result of the long-range Coulomb interaction. The optical conductivity for GQDs has an energy gap in the low frequency regime in contrast to the bulk neutral graphene sheet where a universal constant is approached.

Electronic structures of reconstructed zigzag silicene nanoribbons

Edge states and magnetism are crucial for spintronic applications of nanoribbons. Here, using first-principles calculations, we explore structural stabilities and electronic properties of zigzag silicene nanoribbons (ZSiNRs) with Klein and pentagon-heptagon reconstructions. Comparing to unreconstructed zigzag edges, deformed bare pentagon-heptagon ones are favored under H-poor conditions, while H-rich surroundings stabilize di-hydrogenated Klein edges. These Klein edges have analogous magnetism to zigzag ones, which also possess the electric-field-induced half-metallicity of nanoribbons. Moreover, diverse magnetic states can be achieved by asymmetric Klein and zigzag edges into ZSiNRs, which could be transformed from antiferromagnetic-semiconductors to bipolar spin-gapless-semiconductors and ferromagnetic-metals depending on edge hydrogenations.

Hydrogenation-induced edge magnetization in armchair MoS2 nanoribbon and electric field effects

We performed density functional theory study on the electronic and magnetic properties of armchair MoS2 nanoribbons (AMoS2NR) with different edge hydrogenation. Although bare and fully passivated AMoS2NRs are nonmagnetic semiconductors, it was found that hydrogenation in certain patterns can induce localized ferromagnetic edge state in AMoS2NRs and make AMoS2NRs become antiferromagnetic semiconductors or ferromagnetic semiconductors. Electric field effects on the bandgap and magnetic moment of AMoS2NRs were investigated. Partial edge hydrogenation can change a small-sized AMoS2NR from semiconductor to metal or semimetal under a moderate transverse electric field. Since the rate of edge hydrogenation can be controlled experimentally via the temperature, pressure and concentration of H2, our results suggest edge hydrogenation is a useful method to engineer the band structure of AMoS2NRs.