Spin Polarization In Graphene Research Articles

Electrostatically controlled spin polarization in Graphene-CrSBr magnetic proximity heterostructures

The magnetic proximity effect can induce a spin dependent exchange shift in the band structure of graphene. This produces a magnetization and a spin polarization of the electron/hole carriers in this material, paving the way for its use as an active component in spintronics devices. The electrostatic control of this spin polarization in graphene has however never been demonstrated so far. We show that interfacing graphene with the van der Waals antiferromagnet CrSBr results in an unconventional manifestation of the quantum Hall effect, which can be attributed to the presence of counterflowing spin-polarized edge channels originating from the spin-dependent exchange shift in graphene. We extract an exchange shift ranging from 27 – 32 meV, and show that it also produces an electrostatically tunable spin polarization of the electron/hole carriers in graphene ranging from − 50% to + 69% in the absence of a magnetic field. This proof of principle provides a starting point for the use of graphene as an electrostatically tunable source of spin current and could allow this system to generate a large magnetoresistance in gate tunable spin valve devices.

Tunable Spin-Polarized States in Graphene on a Ferrimagnetic Oxide Insulator.

Spin-polarized two-dimensional (2D) materials with large and tunable spin-splitting energy promise the field of 2D spintronics. While graphene has been a canonical 2D material, its spin properties and tunability are limited. Here, this work demonstrates the emergence of robust spin-polarization in graphene with large and tunable spin-splitting energy of up to 132 meV at zero applied magnetic fields. The spin polarization is induced through a magnetic exchange interaction between graphene and the underlying ferrimagnetic oxide insulating layer, Tm3 Fe5 O12 , as confirmed by its X-ray magnetic circular dichroism (XMCD). The spin-splitting energies are directly measured and visualized by the shift in their Landau-fan diagram mapped by analyzing the measured Shubnikov-de-Haas (SdH) oscillations as a function of applied electric fields, showing consistent fit with the first-principles and machine learning calculations. Further, the observed spin-splitting energies can be tuned over a broad range between 98 and 166 meV by field cooling. The methods and results are applicable to other 2D (magnetic) materials and heterostructures, and offer great potential for developing next-generation spin logic and memory devices.

First-principles studies of the mixed-dimensional van der Waals heterostructures of graphene/MnF4

Constructing a mixed-dimensional (MD) graphene-based van der Waals heterostructure (vdWH) is a viable technique for opening the bandgap and introducing spin polarization in graphene. In this work, we discovered that the adjacent MnF4 can manipulate the carrier doping, bandgap opening, and spin polarization of graphene in the MD vdWH of graphene/MnF4 comprised of two-dimensional (2D) graphene and one-dimensional atomic wire (1D AW) MnF4. By adopting first-principles calculations, we found that graphene can achieve effective p-type doping with the carrier density up to ∼8.89 × 1013–1.03 × 1014 cm−2. With a twisted angle of θ = 10.89° and the compressed distance of dMn-Gra = 2.84 Å, the opened bandgap of graphene (Eg-Gra) achieves 35 and 57 meV for spin-up and spin-down channels due to the sublattice symmetry-breaking in graphene, and the spin splitting energy (ΔES) at the Dirac point reaches 78.7 meV as a result of the graphene–MnF4 interlayer interaction. Remarkably, Eg-Gra is increased to 64 and 79 meV for spin-up and spin-down channels, and ΔES with 202.7 meV is obtained at dMn-Gra = 2.84 Å when the width of 1D MnF4 is doubled. Meanwhile, the n-type Ohmic contact is also realized. Our work underscores the rich interplay in the graphene/MnF4 MD vdWH and provides a significant route with fundamental insights to engineer the spintronic band properties of graphene.

Polarization amplification by spin-doping in nanomagnetic/graphene hybrid systems

The generation of nonequilibrium electron spin polarization, spin transport, and spin detection are fundamental in many quantum devices. We demonstrate that a lattice of magnetic nanodots enhances the electron spin polarization in monolayer graphene via carrier exchange. We probed the spin polarization through a resistively detected variant of electron spin resonance and observed resonance amplification mediated by the presence of the nanodots. Each nanodot locally injects a surplus of spin-polarized carriers into the graphene, and the ensemble of all spin hot spots generates a nonequilibrium electron spin polarization in the graphene layer at macroscopic lengths. This occurs whenever the interdot distance is comparable or smaller than the spin diffusion length.

Proximity-induced magnetization in graphene: Towards efficient spin gating

Gate-tunable spin-dependent properties could be induced in graphene at room temperature through magnetic proximity effect by placing it in contact with a metallic ferromagnet. Because strong chemical bonding with the metallic substrate makes gating ineffective, an intervening passivation layer is needed. Previously considered passivation layers result in a large shift of the Dirac point away from the Fermi level, so that unrealistically large gate fields are required to tune the spin polarization in graphene. We show that a monolayer of Au or Pt used as the passivation layer between Co and graphene brings the Dirac point closer to the Fermi level. In the \Co/\Pt/\Gr system the proximity-induced spin polarization in graphene and its gate control are strongly enhanced by the presence of a surface band near the Fermi level. Furthermore, the shift of the Dirac point could be eliminated entirely by selecting submonolayer coverage in the passivation layer. Our findings open a path towards experimental realization of an optimized two-dimensional system with gate-tunable spin-dependent properties.

Spin polarization of graphene on Co2FeGe0.5Ga0.5(001) observed by spin-polarized surface positronium spectroscopy

Following previous work on graphene on Ni(111) and Co(0001) surfaces [Phys. Rev. B 97, 195405 (2018)], the spin polarization of graphene on a ${\mathrm{Co}}_{2}{\mathrm{FeGe}}_{0.5}{\mathrm{Ga}}_{0.5}$ (CFGG) (001) surface was examined using spin-polarized surface positronium spectroscopy. The graphene was found to be magnetically insulated from the CFGG(001) even after heat treatment, which increased the spin polarizations of graphene on Ni(111) and Co(0001). First-principles calculations indicated that graphene is weakly bound to the CFGG(001) surface mainly through the van der Waals interaction, and consequently the interlayer distance between graphene and the CFGG(001) surface is greater ($\ensuremath{\sim}3$ \AA{}) than that between graphene and Ni(111) or Co(0001) ($\ensuremath{\sim}2$ \AA{}). The weak hybridization between graphene and CFGG(001) sufficiently reduces the spin polarization in graphene, which enables the magnetic insulation of graphene from CFGG.

Tunable coupling by means of oxygen intercalation and removal at the strongly interacting graphene/cobalt interface

It is well known that intercalated species can strongly affect the graphene-substrate interaction. As repeatedly shown by experiment and theory, the intercalation of atomic species may establish a free-standing character in chemisorbed graphene systems. Here, we focus on graphene grown on a strongly interacting support, cobalt, and demonstrate that the film electronic structure and doping can be tuned via the intercalation/removal of interfacial oxygen. Importantly, cathode lens microscopy reveals the main mechanism of oxygen intercalation, and in particular how microscopic openings in the mesh enable oxygen accumulation at the graphene-cobalt interface. Our experiments show that this process can be carefully controlled through temperature, without affecting the film morphology and crystalline quality. The presence of oxygen at the interface induces an upward shift of the graphene π band, moving its crossing above the Fermi level, accompanied by an increased Fermi velocity and reduced momentum width. Control on the graphene coupling to cobalt may enable one to alter the induced spin polarization in graphene’s electronic states.

Induced spin polarization in graphene via interactions with halogen doped MoS2 and MoSe2 monolayers by DFT calculations.

Magnetic halogen doped MoX2 (X = S and Se) monolayers influenced the electronic structure of graphene through a proximity effect. This process was observed using state-of-the-art calculations. It was found that the substitution of a single chalcogen atom with a halogen atom (F, Cl, Br, and I) results in n-type doping of MoX2. An additional electron from the dopant is localized on binding orbitals with the nearest Mo atoms and leads to the formation of magnetism in the dichalcogenide layer. Detailed analysis of halogen doped MoX2/graphene heterostructures demonstrated the induction of spin polarization in graphene near the Fermi energy. Significant spin polarization near the Fermi energy and n-type doping were observed in the graphene layer of MoSe2/graphene heterostructures with MoSe2 doped with iodine. At the same time, fluorine-doped MoSe2 does not cause n-doping in graphene, while spin polarization still takes place. The possibility for the detection of the arrangement of the halogen impurities at the MoX2 basal plane even with the graphene layer deposited on top was demonstrated through STM measurements which will be undoubtedly useful for the fabrication of electronic schemes and elements based on the proposed heterostructures for their further application in nanoelectronics and spintronics.

Anomalous zero bias conductance peaks in a graphene-based ferromagnet/insulator/p-wave superconductor hybrid structure

We theoretically study graphene-based ferromagnet/insulator/p-wave superconductor hybrid structure, where an anomalous zero bias conductance peak (ZBCP) is exhibited. With the exchange field h increased, the zero-bias conductance dips (ZBCDs) for both the px- and py-wave situations develop into ZBCPs at h∕EF>1, which are just contrary to the situation at h∕EF<1. Furthermore, with the enhancement of barrier strength χ, the conversion from the ZBCD to ZBCP is displayed in the region of h∕EF>1 for the px-wave, whereas the sharp ZBCP remains all the time at any h for the py-wave. More interestingly, the zero-bias conductance for the px-wave keeps unchanged upon increasing χ, while for the py-wave, exhibits periodical modulation of χ. The singular features can be used to not only identify the p-wave superconductivity and spin polarization in graphene but also pave the way to a new class of tunable superconducting spintronic devices based on large-scale graphene.

Trigonal warping induced unusual spin texture and strong spin polarization in graphene with the Rashba effect

We study the electronic and scattering properties of graphene with moderate Rashba spin-orbit coupling (SOC). The Rashba SOC in graphene tends to distort the band structure and gives rise to a trigonally warped Fermi surface. For electrons at a pronouncedly warped Fermi surface, the spin direction exhibits a staircase profile as a function of the momentum, making an unusual spin texture. We also study the spin-resolved scattering on a Rashba barrier and find that the trigonal warping is essential for producing spin polarization of the transmitted current. Particularly, both the direction and strength of the spin polarization can be controlled by kinds of electric methods. Our work unveils that not only SOC but also the geometry of the Fermi surface is important for generating spin polarization.

Effect of Ti doping on spin injection and relaxation in few-layer graphene

We demonstrate the spin filtering effect in Co|MgO|TiOx|graphene junctions and a large negative spin polarization in graphene at room temperature. By systematically introducing Ti (TiOx) clusters to the exfoliated few-layer graphene, we confirm that the Ti (TiOx) clusters will increase the charged impurity scattering and lead to a decreased momentum scattering time and an abnormal Gaussian broadening effect. However, the spin relaxation mechanism is not significantly affected, indicating that the spin relaxation mechanism of graphene will not be affected by the atomic-scale Ti (TiOx) clusters induced charged impurity scattering.

Electric-field control of magnetism in graphene on chromia

First-principle calculations are used to investigate how an external electric field controls the spin polarization in graphene on chromia, a system of interest in the area of spin field-effect transistors. Both free-standing chromia thin films and graphene-bilayers are considered. The effect of the electric field depends on the thickness of the chromia and ranges from moderately strong and linear effects to very strong nonlinear magnetoelectricity. The graphene modifies and generally enhances the nonlinear magnetoelectric effect. We also find that the external electric field drastically changes the energy-dependent spin polarization in the graphene layers, which is predicted to reach values of up to about 80%.

The influence of anisotropic Rashba spin–orbit coupling on current-induced spin polarization in graphene

We consider a disordered graphene layer with anisotropic Rashba spin–orbit coupling subjected to a longitudinal electric field. Using the linear response theory we calculate current-induced spin polarization including in-plane normal and parallel components with respect to the electric field direction. Unlike the case of isotropic Rashba spin–orbit where the normal component of spin polarization is linear in terms of Fermi energy around the Dirac point, anisotropic Rashba spin–orbit can result in non-linear dependence of this component at such energies within the Lifshitz points. Furthermore, we show that anisotropic Rashba interaction allows for tuning the direction of spin polarization from perpendicular direction to the parallel one such that for certain values of Rashba parameters the magnitudes of both components can also be quenched. The effect of carriers scattering on randomly distributed non-magnetic disorders is also taken into account by calculating vertex correction. This results in modification of spin polarization components depending on the relative strength of Rashba parameters.

Tunable gap opening and spin polarization of two dimensional graphene/hafnene van der Waals heterostructures

Spin polarization of graphene is the current subject of intense investigation efforts. Herewith, the interfacial electronic structure of graphene/hafnene two dimensional van der Waals heterostructures with different stacking configurations are investigated by first-principles calculations. It is found that the distinct electronic distribution and spin-polarized characteristic of the vdW heterostructures come from the intensity of orbital overlap induced by diverse stacking configurations. The Pauli-exclusion principle takes effect between the hafnene and graphene, and subsequently drive interfacial charges to transfer, which results in n-doped graphene without breaking the p-electron Dirac points. Nevertheless, the orbital hybridization motivates a π-band gap opening with metallic gap states and spin polarization in graphene. Particularly, the electronic structure, spin splitting and band gap depend on the interfacial interactions, which can be tailored by strain and electric field. Field-effect transistor based on graphene/hafnene heterostructures has been proposed. These results strongly revive this novel system as a candidate for future graphene-based electronics.

Direct coupling between charge current and spin polarization by extrinsic mechanisms in graphene

Spintronics---the all-electrical control of the electron spin for quantum or classical information storage and processing---is one of the most promising applications of the two-dimensional material graphene. Although pristine graphene has negligible spin-orbit coupling (SOC), both theory and experiment suggest that SOC in graphene can be substantially enhanced by extrinsic means, such as functionalization by adatom impurities. We present a theory of transport in graphene that accounts for the full spin-coherent dynamics of the carriers, including hitherto-neglected spin precession processes during resonant scattering with dilute impurities. We identify a novel "anisotropic spin precession scattering" process, specific to two dimensions and extrinsic SOC, which contributes to a large current-induced spin polarization in graphene. The theory also yields a comprehensive description of the spin relaxation mechanisms in impurity-decorated graphene: the Elliot-Yafet and Dyakonov-Perel relaxation rates are both present, and we find that the latter can dominate for some choices of carrier density and impurity strength. This provides theoretical foundations for designing future graphene-based integrated spintronic devices.

Controlling spin polarization in graphene by cloaking magnetic and spin-orbit scatterers

We consider spin-dependent scatterers with large scattering cross-sections in graphene -a Zeeman-like and an intrinsic spin-orbit coupling impurity- and show that a gated ring around them can be engineered to produce an effcient control of the spin dependent transport, like current spin polarization and spin Hall angle. Our analysis is based on a spin-dependent partial-waves expansion of the electronic wave-functions in the continuum approximation, described by the Dirac equation.

Inversion of Spin Signal and Spin Filtering in Ferromagnet|Hexagonal Boron Nitride-Graphene van der Waals Heterostructures.

Two dimensional atomically thin crystals of graphene and its insulating isomorph hexagonal boron nitride (h-BN) are promising materials for spintronic applications. While graphene is an ideal medium for long distance spin transport, h-BN is an insulating tunnel barrier that has potential for efficient spin polarized tunneling from ferromagnets. Here, we demonstrate the spin filtering effect in cobalt|few layer h-BN|graphene junctions leading to a large negative spin polarization in graphene at room temperature. Through nonlocal pure spin transport and Hanle precession measurements performed on devices with different interface barrier conditions, we associate the negative spin polarization with high resistance few layer h-BN|ferromagnet contacts. Detailed bias and gate dependent measurements reinforce the robustness of the effect in our devices. These spintronic effects in two-dimensional van der Waals heterostructures hold promise for future spin based logic and memory applications.

Vacancy-induced spin polarization in graphene and B–N nanoribbon heterojunctions

By using nonequilibrium Green's functions (NEGF) and density functional theory (DFT), we investigate the spin-separated electronic transport properties in heterojunctions constructed by zigzag graphene and boron nitride nanoribbons.

Interface-Induced Spin Polarization in Graphene on Chromia

The induced spin polarization of graphene on $\text{Cr}_{{2}}\text{O}_{{3}}$ (001) is investigated using density-functional theory (DFT) and model calculations. The magnetic moment in graphene is a proximity effect and can be regarded as a second-order Stoner scenario, and similar mechanisms are likely realized for all graphene systems with an insulating magnetic substrate. In the absence of charge transfer, the magnetic moment would be quadratic in the exchange field, as contrasted to the usually encountered approximately linear dependence. The net magnetization of the graphene is small, of the order of ${0.01} \mu_{\text{B}}$ per atom, but the energy-dependent spin polarization exhibits pronounced peaks that have a disproportionally strong effect on the spin-polarized electron transport and are therefore important for spin-electronics applications.

Another Way of Looking at Reactivity Enhancement in Large-Area Graphene: The Role of Exchange Splitting from First-Principles Methods

In spintronics field, the spin polarization of large-area graphene has been experimentally observed, yet its reaction to gaseous molecules remains elusive because of the lack of spin-resolved adsorption/mechanistic probe. In this work, we quantified the exchange splitting and magnetic stability of graphene on Ni(111) and found the relationship with its enhanced reactivity toward oxygen molecule in comparison to freestanding graphene and to nitrogen-doped graphene using spin density functional theory calculations with van der Waals corrections. The significant improvement in the reactivity is attributed to the formation of spin-down density of states at the Fermi level (LDOS at EF) because of the exchange splitting. Interestingly, the LDOS at EF property is maintained on graphene atoms not bound to oxygen. Also, the spin-polarization of graphene is found to be stable under oxygen adsorption. These findings pose a new deviation from the conventional ways of improving reactivity of large-area graphene such a...