Ferromagnetic Graphene Research Articles

Analysis of spin current in ferromagnetic graphene junctions

Magnetization transport in a ferromagnetic/normal/ferromagnetic graphene junction has been investigated. Inspired by a seminal paper [Phys. Rev. B 66, 014,407, 2002], the longitudinal and transverse components of spin current i.e. components parallel and perpendicular to the magnetization direction in the ferromagnetic layer have been studied. It has been shown that only when the reflected and transmitted waves from the interface between two normal/ferromagnetic layers have the same angle as that of the incident wave, there will be no torque associated with the transport of longitudinal spin current. In general, for an arbitrary direction of magnetization, all spin current components are coexisted and therefore the parallel and the perpendicular components of spin transfer torque are existed. Therefore it is not possible to draw a general conclusion that the torque associated with the transport of longitudinal spin current is always zero.

Room‐Temperature Macroscopic Ferromagnetism in Multilayered Graphene Oxide

AbstractGraphene has a long spin lifetime and hyperfine interactions, favoring its potential application as spintronics. Despite the recent discoveries of spin‐containing graphene materials, graphene‐based materials with room‐temperature macroscopic ferromagnetism are extremely rare. In this article, room‐temperature ferromagnetic amorphous graphene oxide (GO) is synthesized by introducing abundant oxygen‐containing functional groups and C defects into single‐layered graphene, followed by a self‐assembly process under supercritical CO2 (SC CO2). Such amorphous GO exhibits the highest saturation magnetization (1.71 emu g−1) and remanent magnetization (0.251 emu g−1) compared to the rest of metal‐free graphene‐based materials at room temperature. Experimental and theoretical investigations attribute such strong ferromagnetism to the bridging of the adjacent graphene layers though the out‐of‐plane oxygen‐containing groups, which leads to asymmetric lattices with large net magnetic moments.

Synthesis and intrinsic magnetism of high-concentration pure semi-ionic F-doped graphene

Pure semi-ionic F-doped graphene with the s-F concentration high up to 17.4 at.% (s-FG-17.4) and covalent F-doped graphene with the c-F concentration of 17.8 at.% (c-FG-17.8) were synthesized by fluorination of graphene respectively at the low temperature of 100 °C and the conventional temperature of 200 °C. The electronic and magnetic properties of the two samples were studied. The results show that s-FG-17.4 is metal and exhibits strong ferromagnetism with the Curie temperature of 229 K and high magnetization of 1.1 emu/g; by contrast, c-FG-17.8 is semiconductor and exhibits spin-half paramagnetism with low magnetization of 0.1 emu/g. Thus, this study provided a reliable method to synthesize high-concentration pure s-FG and proposed an effective sp3 dopants for obtaining metallic and ferromagnetic graphene with high magnetization.

Preparation of the graphene-based smart hydrophobic nanocomposite and its application in oil/water separation.

Designing and synthesizing materials with smart hydrophobicity against an external magnetic field for efficient oil/water separation is of great importance due to the increasing problems caused by oil pollution. Here,the nanocomposites were fabricatedbased on graphene and different iron oxides exhibit smart hydrophobicity against an external magnetic fieldand they arein powder form eliminating the requirementfor a substrate employing a facile and echofriendly method. The results prove that autoclaving of graphene leads to its ferromagnetic property; then it is attached to iron oxides by magnetic attraction and a nanocomposite is produced. The magnetic property of the resulting nanocomposite is higher than the magnetic property of its individual components. In addition, following nanocomposite formation, its hydrophobicity and surface area also change. FESEM images were taken from the nanocomposites to study their surface morphology, and EDS-MAP analysis to observe the elemental distribution uniformity of the nanocomposites. Also, to measure the surface area and pore size, BET analysis has been performed on pure materials and graphene-black iron oxide nanocomposite (graphene@black iron oxide). The results show that the specific surface area of black iron oxide increases after being composited with graphene dispersed at 5000rpm. Indeed, graphene forms a composite by binding to iron oxide, and therefore, its specific surface area increases compared to iron oxide and graphene alone. These results show an increase in oil sorption and better separation of oil from water by the prepared nanocomposite. Also, to measure the magnetic properties of pure materials, graphene@black iron oxide, and ferromagnetic graphene at 3000 and 5000rpm, the Vibrating Sample Magnetometeranalysis has been performed. The results have proven that the nanocomposite powder prepared by a simple method obtained from cost-effective and available materials is hydrophobic and becomes more hydrophobic by applying an external magnetic field. Due to the ease with which oil can be readily removed from the nanocomposite by eliminating the external magnetic field, this nanocomposite is an excellent choice for the separation of oil from water.

Rotating single molecule-based devices: Single-spin switching, negative differential electrical and thermoelectric resistance

Single molecule-based devices have been one of the most candidates to realize the miniaturization of traditional electronic devices. Here we investigate the spin-polarized transport properties of an iron-complex molecule sandwiched between two ferromagnetic zigzag-edged graphene nanoribbon electrodes based on the non-equilibrium Green’s function formalism combined with the density functional theory. When the iron-complex molecule rotates, a single-spin switching effect is observed at the Fermi level, and meanwhile a perfect spin filtering effect appears in the molecular device. As the electric bias increases, an obvious negative differential electrical resistance is observed. In addition, we also find a negative differential thermoelectric resistance in the absence of the electric bias under a small molecular rotation, and the sign and magnitude of the thermally driven current can be tuned by the temperature. These theoretical findings imply that the iron-complex molecular devices have potential applications in the next-generation spin electric and thermoelectric devices.

A potential candidate material for quantum anomalous Hall effect: Heterostructures of ferromagnetic insulator and graphene

Discovery of novel topological materials have motivated tremendous research owing to their peculiar energy bands and potential fundamental and practical prospects. In contrast to conventional nonmagnetic topological insulators (TIs), new type of magnetic TIs, such as Cr2Ge2Te6, revealed a quantum Hall effect (QHE) at zero magnetic field. The phenomenon is called anomalous QHE. In the magnetic TIs showing QAHE, the interaction of spontaneous magnetism with conducting surface states opens an insulating band gap at the Dirac points by breaking the time-reversal symmetry. In contrast to inducing the phenomenon of QHE through spontaneous magnetism by chemical doping, the alternative suitable option is the heterostructure engineering of 2D Dirac materials with semiconductor magnetic insulators. In this study, we first time investigate the phenomenon of QAHE in semiconducting magnetic insulator Cr2Ge2Te6 by proposing its bulk and monolayer heterostructure with graphene by the first-principles calculations. We employed density functional theory to compute electronic properties of Cr2Ge2Te6 and its heterostructure with graphene. The exchange-correlation in view of meta-generalized gradient approximations with tight binding method reveal the results for bulk Cr2Ge2Te6 are close to the experimental values. The maximum achieved energy gap in the heterostructure is up to 5 meV.

H-BN as a perfect spin splitter in ferromagnetic zigzag graphene nanoribbons

The manipulation of spin transport is always a central task in spintronics and two very well-known findings along this line thus far are the observations of half-metallicity and pure spin current, both very important in practical applications. In this paper, we propose another control over spin, termed as a spin splitter, with an aim to split the two spin channels from the spatially overlapped paths into two isolated branches. Based on a density functional study, we implement this idea in ferromagnetic zigzag graphene nanoribbons (ZGNRs) and find that BN nanoribbon embedded in the ZGNR acts as a perfect spin splitter. It originates from the different interfacial dipoles developed at the N–C and B–C interfaces and the subsequent different built-in transverse electric fields, which drives the two narrow side ZGNRs into half-metallicity with different spin polarity. The findings will be of great significance in spintronics.

Tunnel magnetoresistance of trilayer graphene-based spin valve

Using the non-equilibrium Green’s function method and within the framework of the tight-binding Hamiltonian model in Landauer–Büttiker formalism, the spin-dependent transport and tunnel magnetoresistance ( T M R ) of AAA- and ABC-stacked trilayer zigzag graphene nanoribbon (TLG) connected to two ferromagnetic ( F M ) semi-infinite single-layer zigzag graphene nanoribbons (FM/TLG/FM system) have been theoretically investigated. Results show that the T M R for both AAA- and ABC-stacked cases can be increased about 100% because of the band-selective rule, and the magnitude of this 100% T M R region, enhances with the increase of the magnetization strength in FM graphene nanoribbon electrodes . Also, the TLG dimension has a prominent effect on the T M R ratio of the system. It is shown that the large values of the TMR ratio can be achieved very nicely by the variation of the width and length of the FM/TLG/FM system. The present theoretical results may be useful for designing few-layer graphene-based spin valves . • The TMR of trilayer graphene-based spin valve have been investigated. • The TMR for both AAA- and ABC-stacked cases can be increased about 100%. • The magnitude of 100% TMR region enhances with the magnetization strength. • The effects of width and length on the TMR ratio have been investigated.

Vacuum barrier induced large spin polarization, giant magnetoresistance, and pure spin photocurrent in ferromagnetic zigzag graphene nanoribbons

Based on first-principle calculations combined with non-equilibrium Green’s function method, we investigate the spin-polarized transport properties of a two-probe device which consists of a vacuum barrier sandwiched between two semi-infinite ferromagnetic zigzag graphene nanoribbons (ZGNRs). It is well known that any vacuum barrier itself is not spin-polarized, and meanwhile, the infinite pristine ferromagnetic ZGNR presents no spin polarization in the transmission function around the Fermi level. However, our computational results indicate that robust large spin polarization is found in the proposed device. More interestingly, giant magnetoresistance up to can be obtained at low bias voltage. Furthermore, the photogalvanic effect-induced fully spin-polarized photocurrent or pure spin photocurrent can be generated in such device by illumination with polarized light, depending on the symmetry of structure. Consequently, our results indicate that vacuum barrier may show interesting spin-polarized transport properties in certain systems and should be taken into consideration in the construction of molecular junctions and spintronic devices.

Spin- and valley-polarized Goos–Hänchen-like shift in ferromagnetic mass graphene junction with circularly polarized light

We investigate the band structure and Goos–Hänchen-like shift in ferromagnetic mass graphene junction modulated by the circularly polarized light. It is found that both spin and valley-related energy gaps can be opened by employing the circularly polarized light and the exchange field in mass graphene. The valley-polarized Goos–Hänchen-like shift can be identified in the presence of circularly polarized light, and the spin-polarized Goos–Hänchen-like shift can be realized with introduction of exchange field in mass graphene. Furthermore, the spin and valley polarization-related Goos–Hänchen-like shift can be achieved by combination of circularly polarized light and exchange field in mass graphene. It is hopeful that our work will be more conducive for future applications in graphene polarization transport devices.

Synthesis and application of ferromagnetic graphene oxide nanocomposite as an effective adsorbent for Clonazepam: Batch experiments, modeling, regeneration, and phytotoxicity

In this work, a ferromagnetic graphene oxide nanocomposite (mGO) was produced, characterized, and tested for clonazepam removal from water. After the nanocomposite production and characterization, adsorption tests showed that the initial pH does not influence the clonazepam removal. Clonazepam adsorption onto mGO was an extremely fast process, with equilibrium reached within the first minutes. Sips isotherm showed the best fit to the experimental data, presenting a maximum adsorption capacity of 14.81 mg g−1, indicating that the clonazepam adsorption process is favorable. Pseudo-first order was the empirical kinetic model that best fit the experimental data, with R2 > 0.99; as for the phenomenological models, both the Linear and Quadratic Driving Force models fit the data with R2 > 0.97. Moreover, the regenerative and reuse potential of the nanocomposite was analyzed, exhibiting remarkable regeneration potential, even after 5 adsorption/desorption cycles. Finally, according to the phytotoxicity tests, it was observed that the effluent treated with mGO did not present toxicity to watercress seeds, as all seeds germinated. This work stands out because it proposes a nanoparticle composite material for the viable, efficient, eco-friendly, and easy removal of an emerging contaminant from water.

Ultra-Broadband Strong Electromagnetic Interference Shielding with Ferromagnetic Graphene Quartz Fabric.

Flexible electromagnetic interference (EMI) shielding materials with ultrahigh shielding effectiveness (SE) are highly desirable for high-speed electronic devices to attenuate radiated emissions. For hindering interference of their internal or external EMI fields, however, a metallic enclosure suffers from relatively low SE, band-limited anti-EMI responses, poor corrosion resistance, and non-adaptability to the complex geometry of a given circuit. Here, a broadband, strong EMI shielding response fabric is demonstrated based on a highly structured ferromagnetic graphene quartz fiber (FGQF) via a modulation-doped chemical vapor deposition (CVD) growth process. The precise control of the graphitic N-doping configuration endows graphene coatings on specifically designable quartz fabric weave with both high conductivity (3906 Scm-1 ) and high magnetic responsiveness (a saturation magnetization of ≈0.14 emug-1 under 300 K), thus attaining synergistic effect of EMI shielding and electromagnetic wave (EMW) absorption for broadband anti-EMI technology. The large-scale durable FGQF exhibits extraordinary EMI SE of ≈107dB over a broadband frequency (1-18GHz), by configuring ≈20nm-thick graphene coatings on a millimeter-thick quartz fabric. This work enables the potential for development of an industrial-scale, flexible, lightweight, durable, and ultra-broadband strong shielding material in advanced applications of flexible anti-electronic reconnaissance, antiradiation, and stealthy technologies.

Magnetothermoelectric properties of Al-Porphyrin sandwiched by graphene nanoribbon electrode based on quantum interference

Rational design of single-molecular junction for efficient thermoelectric properties is a key prerequisite for thermoelectric conversion and fundamental challenge because of the existence of restrictive relationship between Seebeck coefficient and conductance. Inspired by the promising mechanism for promoting thermoelectric properties depended on spin-filter efficiency, magnetothermoelectric properties of aluminum porphyrin (Al-porphyrin) molecular junction have been investigated in contact with ferromagnetic graphene electrodes at different temperature. It is found that spin-resolved Seebeck coefficient is larger than charge-resolved Seebeck coefficient, leading to an excellent spin-resolved power factor about 0.3 pW/K2 at 0.25 eV for Al-porphyrin molecular junction. Ferromagnetic leads effectively realize thermal spin-filtering effect (SFE), which is induced by spin-resolved quantum interference (SQI) between graphene electrode and central nonmagnetic Al-porphyrin molecule. These results indicate the prospects of Al-porphyrin molecular junction applications in spin-caloritronics.

All-carbon multifunctional molecular spintronic device: A first-principles study

Based on the first-principles density functional theory combined with the nonequilibrium Green’s function methodology, we have designed and investigated the spin-polarized transport properties of an all-carbon molecular device, in which a C60 fullerene is sandwiched between two ferromagnetic zigzag-edge graphene nanoribbon electrodes. The results show that the spin-polarized properties are strongly dependent on the external magnetic field modulation and spin-filtering, giant magnetoresistance and spin-rectifying effects are realized in this device. The mechanisms are proposed for these interesting phenomena. These results demonstrate that the designed all-carbon device holds great potentials in the development of multifunctional molecular spintronic device.

Bio-inspired ferromagnetic graphene oxide/magnetic ionic liquid membrane for highly efficient CO2 separation

The design of high-performance CO2 separation membrane with both ultrathin thickness and robust stability remains a challenge. Herein, inspired by the function of AQP1 within the biological membrane for gas selectivity, an untrathin (∼45 nm) graphene oxide/magnetic ionic liquid membrane (GO/MILM) is prepared by infusing [P6,6,6,14][FeCl4] into the 2D nano-space of stacked GO nanosheets. Due to the affinity difference of various gases in MIL, the synergy of electrostatic and magnetic interaction between GO and MIL, ordered arrangement of magnetic anions [FeCl4]− in the nanospace between GO nanosheets is achieved, resulting efficient CO2 separation. The molecular dynamics simulation manifests the separation mechanism. In consequence, the optimized GO/MILM exhibits fast CO2 permeance of 82.9 GPU with high CO2/N2 and CO2/CH4 selectivity of 180 and 76, respectively, surpassing most of reported membranes. Moreover, a stable performance more than 7 days renders the GO/MILM great potential for industrial CO2 separation. This bioinspired strategy may provide a potential to fabricate 2D membranes integrating high-performance, low cost and ultrathin thickness for CO2 capture.

Isolated flat bands in a lattice of interlocking circles

Flat-band physics has attracted much attention in recently years because of its interesting properties and important applications. Some typical lattices have been proposed to generate flat bands, such as Kagome and Lieb lattices. The flat bands in these lattices contact with other bands rather than isolated. However, an ideal flat band should be isolated, because isolation is a prerequisite for a number of important applications. Here, we propose a new lattice that can produce isolated flat bands. The lattice is named as interlocking-circles lattice because its pattern seems like interlocking circles. Moreover, the new lattice is realized in graphene by hydrogenation. In the hydrogenated graphene, there are two nontrivial isolated flat bands appearing around the Fermi level. Upon hole or electron doping, the flat bands split into spin-polarized bands and then result in a ferromagnetic graphene. Our work not only proposes a new type of lattice but also opens a way to find systems with isolated flat bands.

Facilely synthesized N-doped graphene sheets and its ferromagnetic origin

Inducing ferromagnetism into graphene is vital today because it has a wide range of applications such as spintronics devices and magnetic memory devices. In this paper, we will report a new method to synthesize ferromagnetic graphene by nitrogen doping. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy were utilized to testify the N-doped material and further discuss the N-doped process. The superconducting quantum interference device (SQUID) was put in and used to analyze the magnetic properties of the N-doped graphene sheets. It shows that the material exhibits ferromagnetism at both 3 K and 300 K and the ferromagnetic saturation moment is 0.412 emu/g and 0.051 emu/g, respectively. The mechanism of the origin of the ferromagnetism in N-doped graphene sheets will also be discussed in this paper. It shows that, when the amount graphitic N reached the threshold, the origin of the ferromagnetism will change from defects induced by nitrogen atoms to the transition in energy band caused by graphitic N.

Spin-Polarized Positronium Time-of-Flight Spectroscopy for Probing Spin-Polarized Surface Electronic States.

The energy spectrum of positronium atoms generated at a solid surface reflects the electron density of states (DOS) associated solely with the first surface layer. Using spin-polarized positrons, the spin-dependent surface DOS can be studied. For this purpose, we have developed a spin-polarized positronium time-of-flight spectroscopy apparatus based on a ^{22}Na positron source and an electrostatic beam transportation system, which enables the sampling of topmost surface electrons around the Γ point and near the Fermi level. We applied this technique to nonmagnetic Pt(111) and W(001), ferromagnetic Ni(111), Co(0001) and graphene on them, Co_{2}FeGa_{0.5}Ge_{0.5} (CFGG) and Co_{2}MnSi (CMS). The results showed that the electrons of Ni(111) and Co(0001) surfaces have characteristic negative spin polarizations, while these spin polarizations vanished upon graphene deposition, suggesting that the spin polarizations of graphene on Ni(111) and Co(0001) were mainly induced at the Dirac points that were out of range in the present measurement. The CFGG and CMS surfaces also exhibited only weak spin polarizations suggesting that the half-metallicity expected for these bulk states was not maintained at the surfaces.

Tunable lateral spin polarization and spin-dependent collimation in velocity-modulated ferromagnetic-gate graphene structures

The influence of spatial variation of Fermi velocity on the spin-polarized transport properties of massless Dirac fermions in proximity-induced ferromagnetic graphene junction was investigated. We found that the velocity ratio ξ causing the width of the spin conductance dip near the Dirac point is broadened as the Fermi velocity ratio increases. In contrast, the effect of the Fermi velocity mismatch does not affect the shifting of the Dirac point. It leads to the indirect measurement of the Fermi velocity ratio that can be tuned by the spin-dependent conductance. In addition to the presence of both the exchange field and the Fermi velocity modulation, we found high spin filtering occurs when the Fermi velocity ratio ξ ≥ 1. Due to the transmission probability of electron with spin down, T↓ is blocked by the effect of Fermi velocity modulation similar to waves travelling through a different media. Moreover, we also found that the resonant transmission of the electron with spin up can be perfectly transmitted through the Fermi velocity barrier at the limit injected angle θ → π/2 for H/EF ≈ 1. The spin transport properties can be controlled by suitably modifying the strength of the ferromagnetic insulator and varying the Fermi velocity modulation, which leads to the highest spin polarization. By manipulating the behavior of spin-dependent collimation and spin beam splitting in this structure, we proposed a spin transport device for making lateral spin polarization. These interesting features will help make the development of spintronic devices in the graphene-based nanostructure.

Bipolar Magnetic and Thermospin Transport Properties of Graphene Nanoribbons with Zigzag and Klein Edges

Magnetic nanoribbons based on one‐dimensional materials are potential candidates for spin caloritronics devices. Here, we constructed ferromagnetic graphene nanoribbons with zigzag and Klein edges (N‐ZKGNRs, N = 4–21) and found that the N‐ZKGNRs are in the indirect‐gap bipolar magnetic semiconducting state (BMS). Moreover, when a temperature difference is applied through the nanoribbons, spin‐dependent currents with opposite flow directions and opposite spin directions are generated, indicating the occurrence of the spin‐dependent Seebeck effect (SDSE). In addition, the spin‐dependent Seebeck diode effect (SDSD) also appeared in these devices. More importantly, we found that the BMS with a larger bandgap is promising for generating the SDSD, while the BMS with a smaller bandgap is promising for generating the SDSE. These findings show that ZKGNRs are promising candidates for spin caloritronics devices.