Graphene Spintronics Research Articles

Magnetic heterostructure of graphene with a submonolayer magnet on silicon

Graphene provides a wealth of advantageous functionalities but lags behind in spintronic applications due to lack of magnetism. A solution currently explored is proximity coupling of graphene with a 2D magnet. Recently, a class of submonolayer 2D magnets – superstructures of metal atoms on semiconductor surfaces – has been discovered. In these materials, magnetic metal atoms are not screened by anions; therefore, such 2D magnets can be employed in formation of magnetic heterostructures. Here, we report synthesis of a submonolayer 2D magnet – a superstructure of Gd on silicon – and its integration with graphene to induce spin-related phenomena. Magnetic properties of the 2D magnet and its heterostructure with graphene are rather similar with the exception that the heterostructure acquires significant magnetic coercivity. Proximity to the 2D magnet strongly affects the transport properties of graphene; in particular, the anomalous Hall effect emerges signifying spin-polarization of the carriers. Quantum oscillations are tracked across the magnetic transition; their evolution can be rationalized as an order of magnitude increase in the effective mass in the magnetic state. Being seamlessly integrated with the silicon technological platform, the heterostructure makes a prospective material for graphene spintronics.

Design of graphene spin beam splitter based on Brewster’s law

Spin beam splitter is one of the building blocks of graphene spintronics. Here, we adopt the concept of electron optics and design a new type of spin beam splitter by analogy with Brewster’s law. The device is a pristine/ferromagnetic/pristine (P/M/P) graphene junction, where the M region is formed by a proximity effect of ferromagnetic insulators, such as EuO, and acts as an “optically thinner medium” relative to the P region. It is found that, when standing waves are formed in the M region with a length of integral multiple of the half longitudinal wavelength of electrons, electrons with the corresponding spin can pass completely through the junction and only electrons with the other spin are reflected by the P/M interface. This manifests Brewster’s law and a spin beam splitter. It is also demonstrated that, due to the strong electric field effect of graphene, the Brewster angles for both spins can be monotonically modulated by a gate voltage in the M region in the whole range of − π / 2 to π / 2. Thus, our proposed spin beam splitter is not only an easily implemented and widely tunable build block for spintronics but also an interesting demonstration of electron optics.

Proximity Coupling of Graphene to a Submonolayer 2D Magnet.

Imprinting magnetism into graphene may lead to unconventional electron states and enable the design of spin logic devices with low power consumption. The ongoing active development of 2D magnets suggests their coupling with graphene to induce spin-dependent properties via proximity effects. In particular, the recent discovery of submonolayer 2D magnets on surfaces of industrial semiconductors provides an opportunity to magnetize graphene coupled with silicon. Here, synthesis and characterization of large-area graphene/Eu/Si(001) heterostructures combining graphene with a submonolayer magnetic superstructure of Eu on silicon are reported. Eu intercalation at the interface of the graphene/Si(001) system results in a Eu superstructure different from those formed on pristine Si in terms of symmetry. The resulting system graphene/Eu/Si(001) exhibits 2D magnetism with the transition temperature controlled by low magnetic fields. Negative magnetoresistance and the anomalous Hall effect in the graphene layer provide evidence for spin polarization of the carriers. Most importantly, the graphene/Eu/Si system seeds a class of graphene heterostructures based on submonolayer magnets aiming at applications in graphene spintronics.

Toward Nonvolatile Spin–Orbit Devices: Deposition of Ferroelectric Hafnia on Monolayer Graphene/Co/HM Stacks

While technologically challenging, the integration of ferroelectric thin films with graphene spintronics potentially allows the realization of highly efficient, electrically tunable, nonvolatile memories through control of the interfacial spin-orbit driven interaction occurring at graphene/Co interfaces deposited on heavy metal supports. Here, the integration of ferroelectric Hf0.5Zr0.5O2 on graphene/Co/heavy metal epitaxial stacks is investigated via the implementation of several nucleation methods in atomic layer deposition. By employing in situ Al2O3 as a nucleation layer sandwiched between Hf0.5Zr0.5O2 and graphene, the Hf0.5Zr0.5O2 demonstrates a remanent polarization (2Pr) of 19.2 μC/cm2. Using an ex situ, naturally oxidized sputtered Ta layer for nucleation, we could control 2Pr via the interlayer thickness, reaching maximum values of 28 μC/cm2 with low coercive fields. Magnetic hysteresis measurements taken before and after atomic layer deposition show strong perpendicular magnetic anisotropy, with minimal deviations in the magnetization reversal pathways due to the Hf0.5Zr0.5O2 deposition process, thus pointing to a good preservation of the magnetic stack including single-layer graphene. X-ray diffraction measurements further confirm that the high-quality interfaces demonstrated in the stack remain unperturbed by the ferroelectric deposition and anneal. The proposed graphene-based ferroelectric/magnetic structures offer the strong advantages of ferroelectricity and ferromagnetism at room temperature, enabling the development of novel magneto-electric and nonvolatile in-memory spin-orbit logic architectures with low power switching.

Topologically Frustrated Graphene Antidot Lattice Semiconductors with Room-Temperature Magnetism

Realizing graphene spintronics is intriguing due to the weak spin–orbital coupling; however, developing intrinsic room-temperature magnetic semiconductors in graphene is still a great challenge. Graphene antidot lattices (GALs), as a type of regular vacancy graphene, exhibit topology-dependent magnetism and offer an ideal platform to achieve room-temperature magnetic semiconductors. Recently, on-surface-synthesized open-shell [n]triangulene polymers as topologically frustrated graphene nanoflakes (GNFs) are the new building blocks to construct topologically frustrated GALs with robust magnetism. Herein, on the basis of the density functional theory calculations, we report seven magnetic GAL semiconductors by assembling two types of open-shell GNFs with topological frustration, that is, isomeric π-extended heptauthrene (cis triangulene dimer) and heptazethrene (trans triangulene dimer). Our results demonstrate that topologically frustrated GALs are semiconductors with either bipolar ferromagnetism or antiferromagnetism, inheriting the topologically frustrated magnetism from their building blocks. In particular, three ferromagnetic and two antiferromagnetic GALs exhibit above room-temperature magnetic order with their Curie or Néel temperatures varying from 507 to 527 or 366 to 391 K, respectively. This study provides a feasible route to obtain topologically frustrated GAL semiconductors with the above room-temperature magnetism from open-shell GNFs for graphene spintronics applications.

Magnetic proximity effect at the interface of two-dimensional materials and magnetic oxide insulators

Two-dimensional (2D) materials provide a platform for developing novel spintronic devices and circuits for low-power electronics. In particular, inducing magnetism and injecting spins in graphene have promised the emerging field of graphene spintronics. This review focuses on the magnetic proximity effect at the interface of 2D materials and magnetic oxide insulators. We highlight the unique spin-related phenomena arising from magnetic exchange interaction and spin-orbital coupling in 2D materials coupled with magnetic oxides. We also describe the fabrication of multifunctional hybrid devices based on spin transport. We conclude with a perspective of the field and highlight challenges for the design and fabrication of 2D spintronic devices and their potential applications in information storage and logic devices.

Edge Doping Engineering of High-Performance Graphene Nanoribbon Molecular Spintronic Devices.

We study the quantum transport properties of graphene nanoribbons (GNRs) with a different edge doping strategy using density functional theory combined with nonequilibrium Green’s function transport simulations. We show that boron and nitrogen edge doping on the electrodes region can substantially modify the electronic band structures and transport properties of the system. Remarkably, such an edge engineering strategy effectively transforms GNR into a molecular spintronic nanodevice with multiple exceptional transport properties, namely: (i) a dual spin filtering effect (SFE) with 100% filtering efficiency; (ii) a spin rectifier with a large rectification ratio (RR) of 1.9 ×; and (iii) negative differential resistance with a peak-to-valley ratio (PVR) of 7.1 ×. Our findings reveal a route towards the development of high-performance graphene spintronics technology using an electrodes edge engineering strategy.

Quasi-freestanding graphene on SiC(0001) via cobalt intercalation of zero-layer graphene

Modification of the electronic and crystal structure of zero-layer graphene grown on $6H$-SiC(0001) after Co intercalation is reported. Using a wide range of techniques including angle-resolved photoelectron spectroscopy, x-ray photoelectron spectroscopy, Raman spectroscopy, low-energy electron diffraction, we found that zero-layer graphene on SiC transforms into graphene monolayer as a result of cobalt intercalation. The Dirac cone of $\ensuremath{\pi}$ band characteristic of quasi-freestanding graphene is observed. In combination with high-resolution transmission electron microscopy and atomic force microscopy data, we conclude that ultrathin silicide $\mathrm{CoSi}\text{/}{\mathrm{CoSi}}_{2}$ structure is formed between graphene and SiC substrate. Investigation of magnetic properties reveals ferromagnetic behavior with open hysteresis loop. The results of this work are the basis for further implementation of magneto-spin-orbit graphene on a semiconducting substrate and are important for the future application of such graphene in spintronics.

Theory of spin–charge-coupled transport in proximitized graphene: an SO(5) algebraic approach

Establishing the conditions under which orbital, spin and lattice-pseudospin degrees of freedom are mutually coupled in realistic nonequilibrium conditions is a major goal in the emergent field of graphene spintronics. Here, we use linear-response theory to obtain a unified microscopic description of spin dynamics and coupled spin–charge transport in graphene with an interface-induced Bychkov–Rashba effect. Our method makes use of an SO(5) extension of the familiar inverse-diffuson approach to obtain a quantum kinetic equation for the single-particle density matrix that treats spin and pseudospin on equal footing and is valid for arbitrary external perturbations. As an application of the formalism, we derive a complete set of drift–diffusion equations for proximitized graphene with scalar impurities in the presence of electric and spin-injection fields which vary slowly in space and time. Our approach is amenable to a wide variety of generalizations, including the study of coupled spin–charge dynamics in layered materials with strong spin–valley coupling and spin–orbit torques in van der Waals heterostructures.

Chiral Bloch states in single-layer graphene with Rashba spin-orbit coupling: Equilibrium spin current

We study the Bloch spectrum and spin physics of 2D massless Dirac electrons in single layer graphene subject to a one dimensional periodic Kronig-Penney potential and Rashba spin-orbit coupling. The Klein paradox exposes novel features in the band dispersion and in graphene spintronics. In particular it is shown that: (1) The Bloch energy dispersion $\veps(p)$ has unusual structure: There are {\it two Dirac points} at Bloch momenta $\pm p \ne 0$ and a narrow band emerges between the wide valence and conduction bands. (2) The charge current and the spin density vector vanish. (3) Yet, all the non-diagonal elements of the spin current density tensor are finite and their magnitude increases linearly with the spin-orbit strength. In particular, there is a spin density current whose polarization is perpendicular to the graphene plane. (4) The spin density currents are space-dependent, hence their continuity equation includes a finite spin torque density.

Chiral tunneling in single-layer graphene with Rashba spin-orbit coupling: Spin currents

We study forward scattering of 2D massless Dirac electrons at Fermi energy {\varepsilon} > 0 in single layer graphene through a 1D rectangular barrier of height {u_0} in the presence of uniform Rashba spin-orbit coupling (of strength {\lambda}). The role of the Klein paradox in graphene spintronics is thereby exposed. It is shown that (1) For {\varepsilon} - 2{\lambda} < {u_0}< {\varepsilon} + 2{\lambda} there is partial Klein tunneling, wherein the transmission is bounded by 1 and, quite remarkably, for small {\lambda} > {\lambda_0} {\approx} 0.1 meV, the transmission nearly vanishes when the scattering energy equals the barrier height, {\varepsilon}={u_0}. (2) Spin density and spin-current density are shown to be remarkably different than these observables predicted in bulk single layer graphene. In particular, they are sensitive to {\lambda} and {u_0}. (3) Spin current densities are space dependent, implying the occurrence of non-zero spin torque density. Such a system may serve as a graphene based spintronic device without the use of an external magnetic field or magnetic materials.

Introducing ferromagnetism and anisotropic magnetoresistance in monolayer CVD graphene by nitrogen doping

We demonstrate a method to dope monolayer chemical vapor deposited (CVD) graphene with nitrogen and make it ferromagnetic. CVD graphene was first functionalized with hydroxyl groups by treating with H2O2 in the presence of UV light and then annealed in ammonia gas to dope it with nitrogen. Magnetization measurements showed a ferromagnetic hysteresis loop at low temperatures with a coercivity of 222 Oe at 2 K. We also investigated the effect of a change in the angle of the applied magnetic field on the anisotropic magnetoresistance effect (AMR) in the doped CVD graphene devices. Graphene shows positive AMR for temperatures from 2 K to 50 K, negative AMR at 100 K and 150 K, and no AMR for temperatures higher than 150 K. A maximum AMR of 0.92% was observed at 2 K for an in-plane magnetic field of 30 kOe. Magnetic force microscopy also confirms the introduction of magnetism in CVD graphene after doping, and electron spin resonance spectroscopy shows resonance when scanned in a magnetic field, which confirms the presence of unpaired electrons in doped graphene. The process introduced in this paper for nitrogen doping of graphene with attendant magnetism could pave the way for the applications of graphene in spintronics and other devices.

Wafer-scale epitaxial single-crystalline Ni(111) films on sapphires for graphene growth

The growth of graphene on Ni magnetic films is of great significance for graphene spintronics, whereas the existence of grain boundaries and twin crystal structures in Ni films is an obstacle for obtaining the large-scale and uniform graphene. In this paper, an epitaxial wafer-scale single-crystalline Ni(111) film with the flat and clean surface was successfully prepared on the commercial α-Al2O3(0001) substrate by a two-step method, which was demonstrated with several characterization methods. According to the abnormal grain growth mechanism, the clean and uniform sapphire surface plays a key role for the single crystallization of Ni films as it induces a weak interface energy difference between two atomic stacking structures (ABC and ACB), thus stimulating the evolution of Ni films from (111) out-of-plane textures to single crystals. Furthermore, an ultra-flat and wrinkle-free graphene monolayer was synthesized on the prepared Ni film, which further verified its high quality and effectiveness.

Colloquium : Spintronics in graphene and other two-dimensional materials

After the first unequivocal demonstration of spin transport in graphene (Tombros et al., 2007), surprisingly at room temperature, it was quickly realized that this novel material was relevant for both fundamental spintronics and future applications. Over the decade since, exciting results have made the field of graphene spintronics blossom, and a second generation of studies has extended to new two-dimensional (2D) compounds. This Colloquium reviews recent theoretical and experimental advances on electronic spin transport in graphene and related 2D materials, focusing on emergent phenomena in van der Waals heterostructures and the new perspectives provided by them. These phenomena include proximity-enabled spin-orbit effects, the coupling of electronic spin to light, electrical tunability, and 2D magnetism.

Thermally Activated Processes for Ferromagnet Intercalation in Graphene-Heavy Metal Interfaces.

The development of graphene (Gr) spintronics requires the ability to engineer epitaxial Gr heterostructures with interfaces of high quality, in which the intrinsic properties of Gr are modified through proximity with a ferromagnet to allow for efficient room temperature spin manipulation or the stabilization of new magnetic textures. These heterostructures can be prepared in a controlled way by intercalation through graphene of different metals. Using photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM), we achieve a nanoscale control of thermally activated intercalation of a homogeneous ferromagnetic (FM) layer underneath epitaxial Gr grown onto (111)-oriented heavy metal (HM) buffers deposited, in turn, onto insulating oxide surfaces. XPS and STM demonstrate that Co atoms evaporated on top of Gr arrange in 3D clusters and, upon thermal annealing, penetrate through and diffuse below Gr in a 2D fashion. The complete intercalation of the metal occurs at specific temperatures, depending on the type of metallic buffer. The activation energy and the optimum temperature for the intercalation processes are determined. We describe a reliable method to fabricate and characterize in situ high-quality Gr-FM/HM heterostructures, enabling the realization of novel spin-orbitronic devices that exploit the extraordinary properties of Gr.

Making graphene useful for spintronics

Spintronics Thanks to graphene's weak spin-orbit coupling, spin currents flow through it unimpeded. However, this also means that the spin currents are hard to manipulate—a drawback for using graphene in spintronics. Two groups now show that this needn't be the case. Leutenantsmeyer et al. and Xu et al. studied spin transport in heterostructures of bilayer graphene with hexagonal boron nitride. In the presence of an electric field, the spin lifetimes in the directions parallel to the heterostructural layers and perpendicular to them were markedly different. The long and anisotropic lifetimes may lead to useful spintronics applications. Phys. Rev. Lett. 121 , 127702, 127703 (2018).

Spintronics: Realistic Large-Scale Modeling of Rashba and Induced Spin-Orbit Effects in Graphene/High-Z-Metal Systems (Adv. Theory Simul. 10/2018)

The correct modeling of graphene-metal interfaces is crucial for the description of different properties of these systems. In article number 1800063, Elena Voloshina and Yuriy Dedkov present large-scale super-cell calculations used for the description of electronic and spin structures of different graphene/high-Z-metal systems. This study critically analyzes many recent discrepancies in the description of graphene/metal systems and paves the way for the use of graphene in spintronics.

Bias-dependent spin injection into graphene on YIG through bilayer hBN tunnel barriers

We study the spin injection efficiency into single and bilayer graphene on the ferrimagnetic insulator Yttrium-Iron-Garnet (YIG) through an exfoliated tunnel barrier of bilayer hexagonal boron nitride (hBN). The contacts of two samples yield a resistance-area product between 5 and 30 k$\Omega\mu$m$^2$. Depending on an applied DC bias current, the magnitude of the non-local spin signal can be increased or suppressed below the noise level. The spin injection efficiency reaches values from -60% to +25%. The results are confirmed with both spin valve and spin precession measurements. The proximity induced exchange field is found in sample A to be (85 $\pm$ 30) mT and in sample B close to the detection limit. Our results show that the exceptional spin injection properties of bilayer hBN tunnel barriers reported by Gurram et al. are not limited to fully encapsulated graphene systems but are also valid in graphene/YIG devices. This further emphasizes the versatility of bilayer hBN as an efficient and reliable tunnel barrier for graphene spintronics.

Recovery of edge states of graphene nanoislands on an iridium substrate by silicon intercalation

Finite-sized graphene sheets, such as graphene nanoislands (GNIs), are promising candidates for practical applications in graphene-based nanoelectronics. GNIs with well-defined zigzag edges are predicted to have spin-polarized edge-states similar to those of zigzag-edged graphene nanoribbons, which can achieve graphene spintronics. However, it has been reported that GNIs on metal substrates have no edge states because of interactions with the substrate.We used a combination of scanning tunneling microscopy, spectroscopy, and density functional theory calculations to demonstrate that the edge states of GNIs on an Ir substrate can be recovered by intercalating a layer of Si atoms between the GNIs and the substrate. We also found that the edge states gradually shift to the Fermi level with increasing island size. This work provides a method to investigate spin-polarized edge states in high-quality graphene nanostructures on a metal substrate.

Overhauser shift and dynamic nuclear polarization on carbon fibers

We report on the first experimental magnetic resonance determination of the coupling between electrons and nuclear spins (1H, 13C) in carbon fibers. Our results strongly support the assumption that the electronic spins are delocalized on graphene like structures in the fiber. The coupling between these electrons and the nuclei of the lattice results in dynamic nuclear polarization of the nuclei (DNP), enabling very sensitive NMR experiments on these nuclear spins. For possible applications of graphene in spintronics devices the coupling between nuclei and electrons is essential. We were able to determine the interactions down to 30×10−9(30ppb). We were even able to detect the coupling of the electrons to 13C (in natural abundance). These experiments open the way for a range of new double resonance investigations with possible applications in the field of material science.