Spin-polarized Quantum Transport Research Articles

Electronic structure, magnetoresistance and spin filtering in graphene|2 monolayer-CrI33|graphene van der Waals magnetic tunnel junctions.

In the pursuit of designing van der Waals magnetic tunneling junctions (vdW-MTJs) with two-dimensional (2D) intrinsic magnets, as well as to quantitatively reveal the microscopic nature governing the vertical tunneling pathways beyond the phenomenological descriptions on CrI3-based vdW-MTJs, we investigate the structural configuration, electronic structure and spin-polarized quantum transport of graphene|2 monolayer(2ML)-CrI3|graphene heterostructure with Ag(111) layers as the electrode, using density functional theory (DFT) and its combination of non-equilibrium Green's function (DFT-NEGF) methods. The in-plane lattice of CrI3 layers is found to be stretched when placed on the graphene (Gr) layer, and the layer-stacking does not show any site selectivity. The charge transfer between CrI3 and Gr layers make the CrI3 layer lightly electron-doped, and the Gr layer hole-doped. Excitingly, the inter-layer hybridization between graphene and CrI3 layers render the CrI3 layer metallic in the majority spin channel, giving rise to an insulator-to-half-metal transition. Due to the metallic/insulator characteristics of the spin-majority/minority channel of the 2ML-CrI3 barrier in vdW-MTJs, Gr|2ML-CrI3|Gr heterostructures exhibit an almost perfect spin filtering effect (SFE) near the zero bias in parallel magnetization, a giant tunneling magnetoresistance (TMR) ratio up to 2 × 104%, and remarkable negative differential resistance (NDR). Our results not only give an explanation for the observed giant TMR in CrI3-based MTJs but also show the direct implications of 2D magnets in vdW-heterostructures.

Atomic chain of carbon atoms: Smallest negative differential resistance device

In the present work, we report giant negative differential resistance action in probably the simplest molecular nanoelectronic device comprising of carbon chain placed between two ferromagnetic electrodes named as graphitic carbon nitride (g-C4N3). The negative differential resistance action is spin polarized and only evident at spin down channel. Spin polarized quantum transport studies using Keldysh non-equilibrium Green's function based density functional theory reports very high negative differential resistance over the bias range of ±0.1 V to ±0.3 V. This symmetric negative differential resistancec feature has been explained by an analysis of transmission spectrum across the Fermi energy level and Molecular Projected Self-Consistent Hamiltonian states (MPSH) of the system. Role of in-phase and out of phase electron waves in ensuring negative differential resistance feature has been justified through transmission pathways of the system. The simplicity of the molecular system added with robust spin polarized negative differential action added with experimental relevance certainly establishes the uniqueness of the device in respect of modern spintronic research.

Spin-polarized quantum transport in Si dangling bond wires.

We report theoretical modeling of spin-dependent quantum transport properties of dangling bond wires (DBWs) on the Si(100)-2 × 1:H surface. A single spin-polarized dangling bond center (DBC) near the DBW may strongly affect transport as characterized by anti-resonances or dips in the transmission spectra. Such spin-dependent gating can be effective up to a distance of 1.5 nanometer between the DBW and the DBC. At the energies where anti-resonances occur, the scattering states of the system are found to be "attracted" to the DBC - rather than moving forward to the existing electrode. The variety of gating effects can be well organized by a physical picture, i.e. a strong hybridization or interaction between the spin-polarized DBW and DBC occurs with the same spin polarization (at DBW and DBC) and at similar energy. The sharp spin-resolved anti-resonance in the DBW gives rise to a spin-filtering effect up to 100% efficiency.

Valley and spin polarized quantum transport in an irradiated silicene junction

Spin and valley dependent transport properties through a two-dimensional silicene junction irradiated by a monochromatic circularly polarized light are investigated within the framework of Floquet scattering matrix. The light irradiation generates gaps in dispersion and results in selective suppression in ballistic conductance. Spin and valley polarized transmission can be achieved by tuning the driving strength of irradiation. Spin and valley polarization of conductance exhibits specific symmetry with respect to site potential, which is conveniently tuned by an external field. The controllable spin and valley polarization obtained here suggests the silicene junction ideal for future spintronics and valleytronics applications.

Spin-polarized quantum transport in Fe4N based current-perpendicular-to-plane spin valve

Fe4N has been confirmed to possess high spin polarization of 81.3% and low Gilbert damping constant of 0.021 ± 0.02 in the recent experiment. To explore the potential applications of Fe4N in spintronics devices, the current-perpendicular-to-plane spin valve employing Fe4N as electrode and Ag as spacer is simulated to study the spin polarized quantum transport by utilizing the first principles calculations combined with nonequilibrium Green’s function. The project density of states (PDOS), transmission coefficient, spin-polarized current, magnetoresistance (MR) ratio and spin injection efficiency (SIE) as a function of bias voltage are studied. Our calculations reveal that spin down electron is the majority spin polarized electron and the absolute value of MR ratio of Fe4N/Ag/Fe4N at equilibrium reaches up to 174%, and it decreases with the bias increases. Besides, our results indicate that Fe4N/Ag/Fe4N device has stable SIE value of about 40% and stable MR ratios of about 150% when bias increases from 0 V to 0.1 V, and the device has a better performance within this voltage range.

Thickness Control of the Spin-Polarized Two-Dimensional Electron Gas in LaAlO3/BaTiO3 Superlattices

We explored the possibility of increasing the interfacial carrier quantum confinement, mobility and conductivity in the (LaAlO3)n/(BaTiO3)n superlattices by thickness regulation using the first-principles electronic structure calculations. Through constructing two different interfacial types of LaAlO3/BaTiO3 superlattices, we discovered that the LaO/TiO2 interface is preferred from cleavage energy consideration. We then studied the electronic characteristics of two-dimensional electron gas (2DEG) produced at the LaO/TiO2 interface in the LaAlO3/BaTiO3 superlattices via spin-polarized density functional theory calculations. The charge carrier density of 2DEG has a magnitude of 1014 cm−2 (larger than the traditional system LaAlO3/SrTiO3), which is mainly provided by the interfacial Ti 3dxy orbitals when the thicknesses of LaAlO3 and BaTiO3 layers are over 4.5 unit cells. We have also revealed the interfacial electronic characteristics of the LaAlO3/BaTiO3 system, by showing the completely spin-polarized 2DEG mostly confined at the superlattice interface. The interfacial charge carrier mobility and conductivity are found to be converged beyond the critical thickness. Therefore, we can regulate the interfacial confinement for the spin-polarized 2DEG and quantum transport properties in LaAlO3/BaTiO3 superlattice via controlling the thicknesses of the LaAlO3 and BaTiO3 layers.

H-BN/graphene van der Waals vertical heterostructure: a fully spin-polarized photocurrent generator.

By constructing transport junctions using graphene-based van der Waals (vdW) heterostructures in which a zigzag-edged graphene nanoribbon (ZGNR) is sandwiched between two hexagonal boron-nitride sheets, we computationally demonstrate a new scheme for generating perfect spin-polarized quantum transport in ZGNRs by light irradiation. The mechanism lies in the lift of spin degeneracy of ZGNR induced by the stagger potential it receives from the BN sheets and the subsequent possibility of single spin excitation of electrons from the valence band to the conduction band by properly tuning the photon energy. This scheme is rather robust in that we always achieve desirable results irrespective of whether we decrease or increase the interlayer distance by applying compressive or tensile strain vertically to the sheets or shift the BN sheets in-plane relative to the graphene nanoribbons. More importantly, this scheme overcomes the long-standing difficulties in traditional ways of using solely electrical field or chemical modification for obtaining half-metallic transport in ZGNRs and thus paves a more feasible way for their application in spintronics.

In Silico Test of Different Derivatives of Donor−σ−Acceptor System To Realize Bipolar and Unipolar Spin Rectifier

Herein, we report for the first time the underlying mechanism of realizing a bipolar rectifier from a donor−σ−acceptor system (11-mercaptoundeca-2,4,8,10-tetraenenitrile) in a configuration where the spins of the magnetic electrodes are kept parallel. To find out the role of the σ bond on the unique spin polarized quantum transport properties, we have critically examined how a shift in the position of the σ bond in the same molecular skeleton can lead to the transformation of a bipolar rectifier to a unipolar one. It has also been noticed that the addition of another σ bond in the said molecular skeleton can change the transport properties further. In this case, the new device shows bias specific rectification in both the spin channels and in particular, the rectification ratio (42) associated with the down spin channel is appreciably high. Our analyses reveal that both the position and number of the σ bond can modify the relative abundance of the 2p-density of states of the individual carbon atoms near the Fermi level of the left and right side Fe (100) electrodes which actually is accountable for the realization of such interesting device characteristics. The projected device density of states, transmission spectra and transmission eigenstates also corroborate nicely with the present in-silico observation. All the calculations are performed with density functional theory based nonequilibrium Green functions technique.

Spin-polarized quantum transport properties through flexible phosphorene

We report a first-principles study on the tunnel magnetoresistance (TMR) and spin-injection efficiency (SIE) through phosphorene with nickel electrodes under the mechanical tension and bending on the phosphorene region. Both the TMR and SIE are largely improved under these mechanical deformations. For the uniaxial tension (εy) varying from 0% to 15% applied along the armchair transport (y-)direction of the phosphorene, the TMR ratio is enhanced with a maximum of 107% at εy = 10%, while the SIE increases monotonously from 8% up to 43% with the increasing of the strain. Under the out-of-plane bending, the TMR overall increases from 7% to 50% within the bending ratio of 0%–3.9%, and meanwhile the SIE is largely improved to around 70%, as compared to that (30%) of the flat phosphorene. Such behaviors of the TMR and SIE are mainly affected by the transmission of spin-up electrons in the parallel configuration, which is highly dependent on the applied mechanical tension and bending. Our results indicate that the phosphorene based tunnel junctions have promising applications in flexible electronics.

Nonequilibrium spin injection in monolayer black phosphorus.

Monolayer black phosphorus (MBP) is an interesting emerging electronic material with a direct band gap and relatively high carrier mobility. In this work we report a theoretical investigation of nonequilibrium spin injection and spin-polarized quantum transport in MBP from ferromagnetic Ni contacts, in two-dimensional magnetic tunneling structures. We investigate physical properties such as the spin injection efficiency, the tunnel magnetoresistance ratio, spin-polarized currents, charge currents and transmission coefficients as a function of external bias voltage, for two different device contact structures where MBP is contacted by Ni(111) and by Ni(100). While both structures are predicted to give respectable spin-polarized quantum transport, the Ni(100)/MBP/Ni(100) trilayer has the superior properties where the spin injection and magnetoresistance ratio maintains almost a constant value against the bias voltage. The nonequilibrium quantum transport phenomenon is understood by analyzing the transmission spectrum at nonequilibrium.

Spin dependent transport properties of Mn-Ga/MgO/Mn-Ga magnetic tunnel junctions with metal(Mg, Co, Cr) insertion layer

We report a first principles theoretical investigation of spin polarized quantum transport in Mn2Ga/MgO/Mn2Ga and Mn3Ga/MgO/Mn3Ga magnetic tunneling junctions (MTJs) with the consideration of metal(Mg, Co, Cr) insertion layer effect. By changing the concentration of Mn, our calculation shows a considerable disparity in transport properties: A tunneling magnetoresistance (TMR) ratio of 852% was obtained for Mn2Ga-based MTJs, however, only a 5% TMR ratio for Mn3Ga-based MTJs. In addition, the influence of insertion layer has been considered in our calculation. We found the Co insertion layer can increase the TMR of Mn2Ga-based MTJ to 904%; however, the Cr insertion layer can decrease the TMR by 668%; A negative TMR ratio can be obtained with Mg insertion layer. Our work gives a comprehensive understanding of the influence of different insertion layer in Mn-Ga based MTJs. It is proved that, due to the transmission can be modulated by the interfacial electronic structure of insertion, the magnetoresistance ratio of Mn2Ga/MgO/Mn2Ga MTJ can be improved by inserting Co layer.

Tunnel Magnetoresistance of M^(+)@C(subscript 60) (M = Cs, Li, and Na) Molecular Junctions

In this research, a theoretical study of spin-polarized quantum transport has been reported through a M^(+)@C(subscript 60) (M = Cs, Li, Na) molecule attached to two square lattice FM electrodes with finite cross sections. The effects of dimerization bond, the contact types, the ion positions into cage and ion types on the transmission coefficient, the spin-injection factor, and the TMR ratio have been investigated using the Keldysh nonequilibrium Green's function formalism. The results indicate that the spin-injection factor can reach to as large as 100% with the proper selection of the contact types, the ion positions and types. When the Li atom locates at 2 A distance from the cage center, the TMR ratio in the Li^(+)@C(subscript 60) molecule varies from 100% to -80% corresponding to the contact types.

Organic magnetic tunnel junctions: The role of metal-molecule interface

We report a first-principles theoretical investigation of spin-polarized quantum transport in organic magnetic tunnel junctions (OMTJs) to provide a microscopic understanding on the sign of the tunnel magnetoresistance ratio (TMR). We consider two different OMTJs, formed by sandwiching 1-stearic acid radicals (1-SAR) or 1,18-stearic diacid radicals (1,18-SDR) between two Ni electrodes. Even though the main difference between them is only on one of the Ni/molecule contacts, such a structure difference is found to induce a significant sign change of the TMR. The TMR is negative for 1-SAR at -19.6%, but is positive for 1,18-SDR at 13.7%. By investigating the concept of scattering density of states (SDOS), we found that scattering processes of p electrons at the Ni/molecule interface determines the sign of TMR. Based on spin polarization of the SDOS, we extend the Julliere model to explain both the sign and the value of the TMR qualitatively and semiquantitatively. It is concluded that understanding spin-polarized quantum transport in organic magnetic tunnel junction requires a comprehensive knowledge of the electronic structures of the molecule, the metal electrode, and the metal-molecule contacts. DOI: 10.1103/PhysRevB.86.224419

Single-Molecule Sensing Using Carbon Nanotubes Decorated with Magnetic Clusters

First-principles and nonequilibrium Green's function techniques are used to investigate magnetism and spin-polarized quantum transport in metallic carbon nanotubes (CNT) decorated with transition metal (Ni(13), Pt(13)) magnetic nanoclusters (NC). For small cluster sizes, the strong CNT-NC interaction induces spin-polarization in the CNT. The adsorption of a benzene molecule is found to drastically modify the CNT-NC magnetization. Such a magnetization change should be large enough to be detected via magnetic-AFM or SQUID magnetometry, hence suggesting a novel approach for single-molecule gas detection.

Spin-dependent transport in graphene nanoribbons adsorbed with vanadium in different positions

The spin-polarized quantum transport properties of graphene nanoribbons (GNRs) adsorbed with vanadium atoms and terminated with hydrogen are investigated by density functional theory (DFT) associated with nonequilibrium Green's function theory (NGFT). The spin current behaves nonlinearly against bias voltage for the 4-zigzag graphene nanoribbons (ZGNRs), even changing signal at certain voltages due to specific features of the coupling between molecular states and magnetic leads; For the V-adsorbed 7-armchair graphene nanoribbons (ACGNRs), the transmission coefficients is dramatical large. All these results suggest that the present structures are suitable for nanoelectronics and spintronics applications.

Effect of doped atom magnetism on electronic transport through C59X and C69X(X = B and N) molecular junctions

In this paper, a theoretical study of spin-polarized quantum transport through a CnX molecular junction is presented applying the Keldysh non-equilibrium Green's function formalism. The effects of contacts, doped atom and cage type and the gate and bias voltages on spin-polarized quantum transport through the CnX molecular junction are considered in calculations. The calculations indicate that the spin-dependent local density of states of the CnX molecules is the cause of magnetic moment on every carbon atom in the vicinity of the doped atom. Also, the spin polarization can reach as high as about 100% with proper selection of bias and gate voltages.

Generation and detection of spin current in the three-terminal quantum dot

We propose a novel device composed of a quantum dot tunneling coupled to ferromagnetic,superconducting, and normal-metal leads. This device can generate, manipulate, and detectpure spin current through the interplay between the spin-polarized quantum transport andthe Andreev reflection. The spin current in this device is a well-defined conserved currentsince there is no need for any spin–orbit coupling. The proposed device is realizable usingpresent nanotechnology.

Magnetism and spin-polarized transport in carbon atomic wires

We report ab initio calculations of magnetic and spin-polarized quantum transport properties of pure and nitrogen-doped carbon atomic wires. For finite-sized wires with even number of carbon atoms, total magnetic moment of $2{\ensuremath{\mu}}_{B}$ is found. On the other hand, wires with odd number atoms have no net magnetic moment. Doped with one or two nitrogen atom(s), the carbon atomic wires exhibit a spin-density-wave-like state. The magnetic properties can be rationalized through bonding patterns and unpaired states. When the wire is sandwiched between Au electrodes to form a transport junction, perfect spin filtering effect can be induced by slightly straining the wire.

Spin-Polarized Current of a Transistor in Single Mn12 Molecular Magnets

Focusing on the framework of how to realize the molecular spintronics in a single molecular magnet, we present theoretical studies on the spin-polarized quantum transport behavior through a single Mn12 molecular magnet. Our theoretical results were obtained by carrying out density functional theoretical calculation within the Keldysh nonequilibrium Green function formalism. The ultimate goal of the molecular spintronics is to develop single molecule transistors which generate spin-polarized currents through the molecular magnet. We obtained the densityof states, the transmission coefficients and the characteristic features of the current–voltage (I–V) on the spin-polarized transport properties of Mn12 by the theoretical calculation. These results show the possibility for the realization of molecular spintroinics using single molecular magnets.

Spin-Polarized Current of a Transistor in Single Mn12 Molecular Magnets

Focusing on the framework of how to realize the molecular spintronics in a single molecular magnet, we present theoretical studies on the spin-polarized quantum transport behavior through a single Mn12 molecular magnet. Our theoretical results were obtained by carrying out density functional theoretical calculation within the Keldysh nonequilibrium Green function formalism. The ultimate goal of the molecular spintronics is to develop single molecule transistors which generate spin-polarized currents through the molecular magnet. We obtained the density of states, the transmission coefficients and the characteristic features of the current-voltage (I-V) on the spin-polarized transport properties of Mn12 by the theoretical calculation. These results show the possibility for the realization of molecular spintroinics using single molecular magnets.