Ferromagnetic Graphene Research Articles

Perfect Spin-Filtering in 4H-TAHDI-Based Molecular Devices: the Effect of N-Substitution

Based on the non-equilibrium Green's function formalism and spin-polarized density functional theory calculations, we investigate the spin transport properties of HDI and terahydrotetraazahexacene diimide (4H-TAHDI) with two ferromagnetic zigzag-edge graphene nanoribbon electrodes. Compared with HDI, four carbon atoms in the hexacene part of 4H-TAHDI are substituted by nitrogen atoms. The results show that the nitrogen substitution can improve significantly the spin-filtering performance and 4H-TAHDI can be used as a perfect spin filter. Our study indicates that suitable chemical substitution is a possible way to realize high-efficiency spin filters.

Preferential antiferromagnetic coupling of vacancies in graphene onSiO2: Electron spin resonance and scanning tunneling spectroscopy

Monolayer graphene grown by chemical vapor deposition and transferred to ${\mathrm{SiO}}_{2}$ is used to introduce vacancies by ${\mathrm{Ar}}^{+}$ ion bombardment at a kinetic energy of 50 eV. The density of defects visible in scanning tunneling microscopy is considerably lower than the ion fluence, implying that most of the defects are single vacancies as expected from the low ion energy. The vacancies are characterized by scanning tunneling spectroscopy on graphene and highly oriented pyrolytic graphite (HOPG). A peak close to the Dirac point is found within the local density of states of the vacancies similar to the peak found previously for vacancies on HOPG. The peak persists after air exposure up to 180 min, such that electron spin resonance (ESR) at 9.6 GHz can probe the vacancies exhibiting such a peak. After an ion flux of $10/{\mathrm{nm}}^{2}$, we find an ESR signal corresponding to a $g$ factor of 2.001--2.003 and a spin density of $1--2\phantom{\rule{0.28em}{0ex}}\text{spins}/{\mathrm{nm}}^{2}$. The peak width is as small as 0.17 mT indicating exchange narrowing. Consistently, the temperature-dependent measurements reveal antiferromagnetic correlations with a Curie-Weiss temperature of $\ensuremath{-}10$ K. Thus, the vacancies preferentially couple antiferromagnetically, ruling out a ferromagnetic graphene monolayer at ion induced spin densities of $1--2\phantom{\rule{0.28em}{0ex}}{\mathrm{nm}}^{2}$.

Spin-dependent dwell time through ferromagnetic graphene barrier

We investigated the dwell time of electrons tunneling through a ferromagnetic (FM) graphene barrier. The results show that the spin polarization can be efficiently controlled by the barrier width, barrier height, and the incident electron energy. Furthermore, it is found that electrons with different spin orientations will spend different times through the barrier. The difference of the dwell time between spin-up and spin-down electrons arises from the exchange splitting, which is induced by the FM strip. Study results indicate that a ferromagnetic graphene barrier can cause a nature spin filter mechanism in the time domain.

Strain-induced phase transition and electron spin-polarization in graphene spirals.

Spin-polarized triangular graphene nanoflakes (t-GNFs) serve as ideal building blocks for the long-desired ferromagnetic graphene superlattices, but they are always assembled to planar structures which reduce its mechanical properties. Here, by joining t-GNFs in a spiral way, we propose one-dimensional graphene spirals (GSs) with superior mechanical properties and tunable electronic structures. We demonstrate theoretically the unique features of electron motion in the spiral lattice by means of first-principles calculations combined with a simple Hubbard model. Within a linear elastic deformation range, the GSs are nonmagnetic metals. When the axial tensile strain exceeds an ultimate strain, however, they convert to magnetic semiconductors with stable ferromagnetic ordering along the edges. Such strain-induced phase transition and tunable electron spin-polarization revealed in the GSs open a new avenue for spintronics devices.

Effect of contacts on spin lifetime measurements in graphene

Injection, transmission, and detection of spins in a conducting channel are the basic ingredients of spintronic devices. Long spin lifetimes during transit are an important ingredient in realizing this technology. An attractive platform for this purpose is graphene, which has high mobilities and low spin-orbit coupling. Unfortunately, measured spin lifetimes are orders of magnitude smaller than theoretically expected. A source of spin loss is the resistance mismatch between the ferromagnetic electrodes and graphene. While this has been studied numerically, here we provide a closed form expression for Hanle spin precession which is the standard method of measuring spin lifetimes. This allows for a detailed characterization of the nonlocal spin valve device.

Spin-resolved Fano resonances induced large spin Seebeck effects in graphene-carbon-chain junctions

We propose a high-efficiency thermospin device constructed by a carbon atomic chain sandwiched between two ferromagnetic (FM) zigzag graphene nanoribbon electrodes. In the low-temperature regime, the magnitude of the spin figure of merit is nearly equal to that of the corresponding charge figure of merit. This is attributed to the appearances of spin-resolved Fano resonances in the linear conductance spectrum resulting from the quantum interference effects between the localized states and the expanded states. The spin-dependent Seebeck effect is obviously enhanced near these Fano resonances with the same spin index; meanwhile, the Seebeck effect of the other spin component has a smaller value due to the smooth changing of the linear conductance with the spin index. Thus, a large spin Seebeck effect is achieved, and the magnitude of the spin figure of merit can reach 1.2 at T = 25 K. Our results indicate that the FM graphene-carbon-chain junctions can be used to design the high-efficiency thermospin devices.

Quantum transport in ferromagnetic graphene super lattice in the presence of Rashba spin–orbit coupling

Abstract Using the transfer matrix method, we study the electron transport through a single-layer graphene superlattice with alternating layers of ferromagnetic and normal regions with Rashba spin–orbit coupling. We show that the transport properties of the system depend strongly on the superlattice parameters. As another result, Rashba spin–orbit coupling manifests to be of crucial importance in controlling the transmission probabilities and Giant Magneto Resistance (GMR).

Spin conductance and tunnelling magnetoresistance in a fractal graphene superlattice with two ferromagnetic graphene electrodes

We investigate the spin-polarized transport properties of relativistic electrons tunnelling through a fractal graphene superlattice (FGSL) sandwiched between two ferromagnetic graphene (FG) electrodes, where the FGSL can be regarded as a symmetric periodic graphene superlattice (SPGSL) with defects. The spin conductance and the tunnelling magnetoresistance of the FG/FGSL/FG junction are numerically calculated and compared with those of the FG/SPGSL/FG junction. We find that the spin conductance and the tunnelling magnetoresistance reach a minimum value in the vicinity of the new Dirac point and can be greatly modulated not only by the Fermi energy and the exchange splitting energy, but also by the structural configuration. Compared to those of the FG/SPGSL/FG junction, the spin conductance and the tunnelling magnetoresistance of the FG/FGSL/FG junction can exhibit a stronger and more irregular oscillation with increasing Fermi energy, along with typical high-frequency quasiperiodic oscillations as the barrier height increases. We also find that the maximum value of the tunnelling magnetoresistance is robust against change of the lattice parameters and the profile of the graphene superlattice, but dependent on the Fermi energy and the exchange splitting energy.

Long-range ferromagnetic graphene via compensated Fe/NO2 co-doping

Abstract We performed first-principles density functional theory calculation to investigate the electronic structures and magnetic properties of Fe/NO 2 co-doped graphene. First, our study shows that the Fe/NO 2 co-doped graphene preserves the unique linear dispersion of the charge carriers. More importantly, NO 2 co-doping is a much effective way to compensate the shifting of E F , which can facilitate the Fe adsorbed graphene systems to retain the unique properties of graphene. Second, magnetic exchange energy calculation finds that long-range ferromagnetic coupling exists between two n-p pairs and the coupling strength can be obviously improved after NO 2 co-doping.

Magnetic polaron formation in graphene-based single-electron transistor

We theoretically study the magnetic polaron (MP) formation in a ferromagnetic graphene single-electron transistor (SET), which consists of a graphene quantum dot (QD) between two insulating layers, i.e., a ferromagnetic insulator EuO and a nonmagnetic substrate such as SiC. In the lateral direction, the QD is electrically coupled to source, drain and gate electrodes, so that the number of electrons can be controlled by the gate voltage. Due to a proximity effect at the EuO/graphene interface, i.e., the exchange interaction between the charge carriers in graphene and the localized magnetic electrons in EuO, there is a magnetic coupling between the graphene QD and EuO. Using Green's function technique an expression for the total free energy of the SET is derived, which is then minimized at the given temperature and gate voltage. This leads to the MP formation and a consequent local enhancement of the ferromagnetic properties of EuO at the EuO/graphene interface. The MP formation is enforced by the large Coulomb interaction between the carriers in graphene. The spin polarization at the graphene/EuO interface in the QD area can be controlled by the gate voltage of the SET.

Electronic Properties of Nanopore Edges of Ferromagnetic Graphene Nanomeshes at High Carrier Densities under Ionic-Liquid Gating

Graphene edges with a zigzag-type atomic structure can theoretically produce spontaneous spin polarization despite being a critical-metal-free material. We have demonstrated this in graphene nanomeshes (GNMs) with honeycomb-like arrays of low-defect hexagonal nanopores by observing room-temperature ferromagnetism and spin-based phenomena arising from the zigzag-pore edges. Here, we apply extremely high electric fields to the ferromagnetic (FM) GNMs using an ionic-liquid gate. A large on/off-ratio for hole current is observed for even small applied ionic-liquid gate voltages (Vig). Observations of the magnetoresistance behavior reveal high carrier densities of ~1013 cm-2 at large Vig values. We find a maximum conductance peak in the high -Vig region and its separation into two peaks upon applying a side-gate (in-plane external) voltage (Vex). It is discussed that localized edge-π band with excess-density electrons induced by Vig and its spin splitting for majority and minority of spins by Vex (half-metallicity model) lead to these phenomena. The results must realize critical-element-free novel spintronic devices.

Spin-dependent Goos–Hänchen shift and spin beam splitter in gate-controllable ferromagnetic graphene

The transmission and Goos–Hänchen (GH) shift for charge carriers in gate-controllable ferromagnetic graphene induced by ferromagnetic insulator are investigated theoretically. Numerical results demonstrate that spin-up and spin-down electrons exhibit remarkably different transmission and GH shifts. The spin-dependent GH shifts directly demonstrate the spin beam splitting effect, which can be controlled by the voltage of gate. We attribute the spin beam splitting effect to the combination of tunneling through potential barrier and Zeeman interaction from the magnetic field and the exchange proximity interaction between the ferromagnetic insulator and graphene. In view of the spin beam splitting effect and the spin-dependent GH shifts, the gate-controllable ferromagnetic graphene might be utilized to design spin beam splitter.

Giant spin thermoelectric efficiency in ferromagnetic graphene nanoribbons with antidots

Thermoelectric effects in zigzag graphene nanoribbons with parallel alignment of the edge spin polarizations are investigated theoretically. Spin and charge thermopower, electrical and heat conductance, and charge and spin thermoelectric efficiency are calculated numerically for pristine nanoribbons as well as for nanoribbons with periodic one-dimensional lattice of structural defects in the form of antidots. It is shown that structural defects reduce thermal conductance due to phonons and open gaps in the corresponding electronic spectrum. This, in turn, leads to a significant enhancement of the Seebeck and spin Seebeck coefficients as well as of the thermoelectric efficiency. A giant enhancement appears in certain regions of chemical potential (controlled by doping or external gate) and survives at room temperatures.

Boltzmann conductivity of ferromagnetic graphene with magnetic impurities

We investigate the electrical conductivity of spin-polarized graphene in the presence of short-ranged magnetic scatterers within the relaxation time approximation and the semi-classical Boltzmann approach. Spin-flip scattering of the itinerant electrons from the majority spin sub-band into the minority one results in a minimum in the electrical resistivity at a finite temperature. While this behavior is reminiscent of the renowned Kondo effect, it has an entirely different origin and differs from the Kondo effect in several aspects. In particular, unlike the Kondo effect, this is a single particle phenomena, and it does not require antiferromagnetic coupling between the magnetic moments of impurities and spins of the itinerant electrons.

Spin transport and magnetoresistance in Thue-Morse graphene superlattice with two ferromagnetic graphene electrodes

We investigate theoretically the spin-dependent transport and tunnel magnetoresistance in a system consisting of two ferromagnetic graphene (FG) electrodes separated by the Thue-Morse graphene superlattice (TMGSL) or periodic graphene superlattice (PGSL). Using transfer matrix methods, the spin-dependent conductance, tunnel magnetoresistance, and spin polarization of current of the FG/TMGSL/FG junctions are numerically calculated and compared with those of the FG/PGSL/FG junctions. The results indicate that all the physical quantities considered here show typical quasiperiodic oscillations as the electrostatic barrier height increases. Furthermore, the conductances, spin polarization of the current for the parallel configuration of magnetization, and tunnel magnetoresistance monotonically increase or decrease with increasing exchange splitting energy. Interestingly, compared to the FG/PGSL/FG junctions, all the physical quantities, considered here, of the FG/TMGSL/FG junctions can exhibit a more marked growth stage effect, along with stronger and more irregular oscillations with increasing electrostatic barrier height.

Anisotropic spin-dependent thermopower and current in ferromagnetic graphene junctions

We study the spin-dependent thermoelectric transport through two-dimensional normal/ferromagnetic/normal/ferromagnetic/normal graphene (NG/FG/NG/FG/NG) junctions. It is found that both charge and spin thermopowers depend on the FGʼs magnetization direction and exhibit an anisotropic behavior. Interestingly, the spin thermopower can be as large as the charge thermopower and even can exceed the latter in magnitude. Moreover, the pure spin thermopower and spin current emerge in this device. The results obtained here suggest a feasible way of enhancing thermospin effects and generating the pure spin current in two-dimensional graphene. • Spin-dependent thermopower in ferromagnetic graphene junctions is studied. • Spin and charge thermopowers are anisotropic. • Spin thermopowers can be as large as the charge thermopower. • A pure spin thermopower (spin current) is obtained.

Charge and spin Hall effect in spin chiral ferromagnetic graphene

We predict a specific type of charge Hall effect in undoped ferromagnetic graphene that is generated by the spin Hall mechanism in the absence of an external magnetic field. The essential feature is the so-called spin chiral configuration of the spin subbands in such a magnetic material where carriers with opposite spin direction are of different type of electron-like or hole-like. Within the semiclassical theory of spin-orbital dynamics of electrons, we obtain that a longitudinal electric field can produce a spin-orbit transverse current of pure charge with no polarization of the spin and the valley.

Graphene nanoribbon intercalated with hexagonal boron nitride: Electronic transport properties from ab initio calculations

A magnetic tunnel junction consisting of a boron nitride nanoribbon contacted by two semi-infinite electrodes composed of (3,0) ferromagnetic zig-zag graphene nanoribbons was investigated. Performing spin-polarized ab initio transport calculations based on a scheme that combines non-equilibrium Green's function with density functional theory (NEGF+DFT) we predict that such system could act as spin-filter (the efficiency reaches 50%) and present a magnetoristance of 105%, depending on the length of the boron nitride region.

Realization of ferromagnetic graphene oxide with high magnetization by doping graphene oxide with nitrogen

The long spin diffusion length makes graphene very attractive for novel spintronic devices, and thus has triggered a quest for integrating the charge and spin degrees of freedom. However, ideal graphene is intrinsic non-magnetic, due to a delocalized π bonding network. Therefore, synthesis of ferromagnetic graphene or its derivatives with high magnetization is urgent due to both fundamental and technological importance. Here we report that N-doping can be an effective route to obtain a very high magnetization of ca. 1.66 emu/g, and can make graphene oxide (GO) to be ferromagnetism with a Curie-temperature of 100.2 K. Clearly, our findings can offer the easy realization of ferromagnetic GO with high magnetization, therefore, push the way for potential applications in spintronic devices.

Diluted ferromagnetic graphene by compensated n–p codoping

In this study, we propose a new approach, based on compensated n–p codoping, that can simultaneously address all the main shortcomings associated with single-element doping, effectively resulting in “diluted ferromagnetic graphene”. Our proposal is to deposit magnetic transitional metal (TM) atoms (such as Fe, Co, and Ni), acting as n-type dopants, onto already p-doped (e.g., by B) graphene. Through systematic first-principles calculations within density functional theory, we found that: (1) the electrostatic attraction between the n- and p-type dopants effectively enhances the adsorption of the TM adatoms and suppress their undesirable clustering, (2) the p-doping by B significantly enhances the magnetic moments of the TM adatoms, and (3) the compensated nature of the n–p codoping helps to preserve the Dirac nature of the charge carriers. Furthermore, through Monte Carlo simulations of a Heisenberg model with the essential magnetic coupling parameters obtained from the first-principles calculations, we found that Ni–B codoped graphene has a robust ferromagnetic order at experimentally feasible doping concentrations, with a Curie temperature as high as 212K.