Spin-dependent Transport Properties Research Articles

Lead position and lead-ring coupling effects on the spin-dependent transport properties in a two-dimensional network of quantum nanorings in the presence of Rashba spin–orbit interaction

The effects of lead positions and lead-ring coupling regimes are investigated for spin-related transport through a two-dimensional network of quantum nanorings (2DNQRs) considering Rashba spin–orbit interaction (RSOI) and magnetic flux. A matrix representation of the transmission and reflection coefficients through a single ring connected to the arbitrary number of leads has been introduced. As a specific example of 2DNQRs, the conductance, spin polarization and system efficiency are obtained via a triangular network of quantum rings (TNQRs). TNQRs are completely opaque or transparent versus RSOI strength and wave vector (k) of the incident electron. The periodicity of these functions along the k axis depends on the lead positions and is independent of lead-ring coupling. Also, the symmetric geometry and strong lead-ring coupling regime significantly improve the performance of the system as a multipurpose spintronic device (i.e., a perfect spin filter, spin splitter, spin switching, Stern–Gerlach device and an electronic switching device).

Manipulating Current Spin Polarization in Magnetic Single-Molecule Junctions via Destructive Quantum Interference

By means of the first-principles method, the effects of destructive quantum interference (DQI) on spin-dependent transport properties in meta-phenylene ethylene-type oligomer (OPE)-based magnetic single-molecule junctions are investigated. We explore one possible strategy to manipulate the spin polarization (SP) of current based on controllable DQI features using chemical substituents. It is found that the position of DQI features can obviously be tuned by inserting one nitrogen atom into the meta-OPE core at different positions, which leads to selected suppression of the transmission peaks contributed by the spin-resolved hybrid interface state (HIS). A resulting enhancement or reduction of the SP of current is achieved by manipulating the position of DQI dips. The nature of shift of the DQI feature is analyzed in terms of Green’s function and frontier molecular orbitals. In addition, such a phenomenon is further verified in the molecular junctions with side group substituents. This work demonstrates the role of controllable DQI in spin-dependent transport properties, which provides a feasible way to manipulate the spin functionality of organic spintronic devices.

First principles investigation of the spin transport properties in graphene-porphine-graphene nanojunctions

The spin-dependent transport properties of a porphine molecule linking with two zigzag-edged graphene nanoribbon (ZGNR) electrodes have been investigated by using the density of functional theory (DFT) combined with the non-equilibrium Green’s function (NEGF) method. The device shows clearly negative differential resistance (NDR), large tunnelling magnetoresistance (TMR) of 103 magnitude and nearly 100% perfect spin filter efficiency (SFE) properties, respectively. What’s more, the projected density of states (PDOS), molecular projected self-consistent Hamiltonian (MPSH) eigenvalues and transmission eigenstates were carried out to discuss the transport properties of the ZGNR/Porphine/ZGNR nanojunction. These results suggest that the ZGNR/Porphine/ZGNR device is a promising candidate for multi-functional spintronic devices.

Multi-resistance state tuned by interfacial active Pt layer in a perpendicular Hall balance

The effect of interfacial Pt layer on multi-resistance states in perpendicular Hall balance with core structure of [Co/Pt]n/NiO/[Co/Pt]n multilayers was systematically investigated. By inserting an ultrathin Pt layer between [Co/Pt]n and NiO spacer, resistance states and the Hall resistance ratio (HRR) were successfully tuned. Our results indicate that the inserted ultrathin Pt layer plays a crucial role on spin dependent transport properties, achieving a HRR of up to 30,900%. Furthermore, the resistance in Hall balance can be smoothly altered among 2-, 3-, and 4-states by optimizing the ultrathin active Pt layer. Pt catalyst with high electronic activity, as measured by X-ray photoelectron spectroscopy (XPS), provides a novel approach for modifying the interfacial orbital hybridization, thus leading to a tunable resistance states and HRR value.

Magnetic Nanowires

Magnetic nanowires are attractive materials because of their morphology-dependent remarkable properties suitable for various advanced technologies in sensing, data storage, spintronics, biomedicine and microwave devices, etc. The recent advances in synthetic strategies and approaches for the fabrication of complex structures, such as parallel arrays and 3D networks of one-dimensional nanostructures, including nanowires, nanotubes, and multilayers, are presented. The simple template-assisted electrodeposition method enables the fabrication of different nanowire-based architectures with excellent control over geometrical features, morphology and chemical composition, leading to tunable magnetic, magneto-transport and thermoelectric properties. This review article summarizing the work carried out at UCLouvain focuses on the magnetic and spin-dependent transport properties linked to the material and geometrical characteristics.

Highly spin polarized transport in γ-zigzag graphyne nanoribbon junctions

Using density functional calculations combined with non-equilibrium Green’s functions, we investigate the BN co-doping effects on the electronic structure of zigzag γ -graphyne nanoribbons and spin-dependent transport properties of related transport heterojunctions. The results show that, when properly selecting the doping sites of the B and N atoms at the two edges, half-metallicity is obtained due to the opposite energy shift in the two spin channels caused by the potential generated by the BN pairs. In transport junctions constructed by connecting two ribbons by only one edge of each ribbon to form a ‘Z’-shape structure, highly spin polarized transport is achievable, depending on the relative positions of the B and N dopants in the two ribbons. Our finding opens a new possibility of graphyne in spintronic device applications.

Large magnetoresistance and spin-dependent output voltage in a lateral MnGa/GaAs/MnGa spin-valve device

We investigated the spin-dependent transport properties of a lateral spin-valve device with a 600 nm long GaAs channel and perpendicularly magnetized MnGa electrodes. Its current–voltage characteristics show nonlinear behavior below 50 K, indicating that tunnel transport through the MnGa/GaAs Schottky barrier is dominant at low temperatures. We observed large magnetoresistance (MR) ratios up to 12% at 4 K when applying a magnetic field perpendicular to the film plane. Furthermore, a large spin-dependent output voltage of 33 mV is obtained. These values are the highest in lateral ferromagnetic metal/semiconductor/ferromagnetic metal spin-valve devices.

Tunneling magnetoresistance and light modulation in Fe4N(La2/3Sr1/3MnO3)/C60/Fe4N single molecule magnetic tunnel junctions

Spin-dependent transport properties and light modulation of Fe4N/C60/Fe4N and LSMO/C60/Fe4N single molecule magnetic tunnel junctions.

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.

Spin dependent electronic transport properties of zigzag black phosphorene nanojunctions induced by H, Li, O, Co asymmetric edge saturations

Two-dimensional (2D) material of few-layer black phosphorus (BP) has recently attracted extensive interest owing to its tunable band gap and high carrier mobility. We investigate the electronic transport properties of zigzag black phosphorene nanoribbons (ZBPNRs) with asymmetric H, Li, O and Co edge saturations by employing the density functional theory in combination with the non-equilibrium Green's function. The computational results forecast that different types of saturated atoms at both edge of ribbons mainly contribute to the electronic transport properties of molecular junctions. The metal edge saturation of Co atom is used to the one edge of ZBPNR which can induce an identical electronic transport property. Interestingly, the negative differential resistance (NDR) phenomena can be observed in our proposed ZBPNR junctions with an analysis of internal physical mechanism. Our theoretical results could support the possibility of potential applications to design 2D electronic devices based on the material of BP in future.

Effect of the linkage modes of thiolated ethynyl groups on the spin-dependent electronic transport properties in transition metal porphyrin molecular junctions

Using density functional theory and nonequilibrium Green’s function method, the spin-dependent electronic transport properties of six transition metal porphyrin molecules (VP, CrP, MnP, FeP, CoP, and NiP), which are linked to gold electrodes through the thiolated ethynyl groups, are investigated. Two different linkage modes (beta linkage and meso linkage) of the substituted ethynyl groups on the porphyrin macrocycle are considered. The results show that the linkage mode of ethynyl groups plays an important role on the spin transport properties of the molecular junctions and the beta linkage is more favorable for the spin filtering efficiency of current than the meso linkages. The spin-up and spin-down energy levels show the different evolutions which is responsible for the difference of spin filtering efficiency between the two linkage modes. The computational results of total current show that the meso-linked molecular junctions have the better conductive performances than the beta-linked ones which may be caused by the different electronic transport paths.

Spin-Dependent Electronic and Thermoelectric Transport Properties for a Sawtoothlike Graphene Nanoribbon Coupled to Two Ferromagnetic Leads

The spin-dependent electronic transport and thermoelectric properties for a sawtoothlike graphene nanoribbon between two ferromagnetic leads with parallel or antiparallel magnetic moments are investigated theoretically. Both Christmas-tree graphene nanoribbon and tree-saw graphene nanoribbon have been considered to exemplify the effect of the chirality. By using ab initio calculations combined with a non-equilibrium Green’s function method, we obtain the important result that these sawtoothlike graphene nanoribbon junctions, regardless of their chirality, can exhibit giant charge and spin Seebeck coefficients as well as high thermoelectric efficiency at room temperature by putting ferromagnetic stripes or magnetic fields on the nanoribbons. Specifically, large $$|S_\mathrm{C}|\,(\sim 5.8~\hbox {mV/K})$$, $$|S_\mathrm{S}|(\sim 1.8~\hbox {mV/K})$$, charge figure of merit $$ZT_\mathrm{C}$$ and spin figure of merit $$ZT_\mathrm{S}(\sim 20)$$ were found in these sawtoothlike graphene nanoribbon junctions. In addition, negative differential resistance and spin filtering were also realized in these sawtoothlike graphene nanoribbon junctions. The maximum of spin filtering efficiency can be up to 100%. Our studies suggest that sawtoothlike graphene nanoribbon junctions are suitable for use as highly efficient spin caloritronics at room temperature.

Spin transport in proximity-induced ferromagnetic phosphorene nanoribbons

Abstract We investigate the spin-dependent transport properties in normal/ferromagnetic/normal phosphorene nanoribbon (PNR) junctions where an external gate electrode is attached to the ferromagnetic region. The exchange splitting in ferromagnetic phosphorene is induced by ferromagnetic proximity effect. We found that the current through this junction is spin polarized due to the exchange field induced in phosphorene. The spin polarization oscillates with the external gate voltage, and the spin polarization can be reversed from 100% to −100% by a slight change of the gate voltage. The controllable spin transport in phosphorene nanoribbons can have useful applications in the development of spintronic devices.

Spintronic transport in zigzag silicon carbide nanoribbons with edge hydrogenation

Spin transport properties of zigzag silicon carbide nanoribbon (ZSiCNR) which includes four edge contacts are studied by the first principle method. All possible configurations are: (a) each edge carbon (C) and silicon (Si) atoms passivated with one hydrogen (H) atom (1H-1H), (b) each edge C atoms and Si atoms passivated with two H atoms (2H-2H), (c) each edge Si atoms bonded with one H atom and each edge C atoms bonded with two H atoms (1H-2H), and (d) edge C atoms saturated with one H atom and edge Si atoms saturated with two H atoms (2H-1H). Based on our results, three of these structures show the perfect spin filtering effect (100%) in the applied bias region (0.0 V to 0.5 V), which can be used to design spin polarizer devices. Besides, the current-limited effect is observed in the 1H-1H configuration under the bias voltage. Moreover, the negative differential resistance phenomenon is specified in the spin-polarized current for 2H-2H structure. Our findings show that suggested structures exhibit some interesting spin-dependent transport properties.

Designing of spin filter devices based on zigzag zinc oxide nanoribbon modified by edge defect**Project supported by the National Natural Science Foundation of China (Grant Nos. 61664008 and 11604286) and the Scientific Technological Innovation Team of Yan’an City, China (Grant No. 2017CXTD-01).

The spin-dependent electronic transport properties of a zigzag zinc oxide (ZnO) nanoribbon are studied by using density functional theory with non-equilibrium Green’s functions. We calculate the spin-polarized band structure, projected density of states, Bloch states, and transmission spectrum of the ZnO nanoribbon. It is determined that all Bloch states are located at the edge of the ZnO nanoribbon. The spin-up transmission eigenchannels are contributed from Zn 4s orbital, whereas the spin-down transmission eigenchannels are contributed from Zn 4s and O 2p orbitals. By analyzing the current–voltage curves for the opposite spins of the ZnO nanoribbon device, negative differential resistance (NDR) and spin filter effect are observed. Moreover, by constructing the ZnO nanoribbon modified by the Zn-edge defect, the spin-up current is severely suppressed because of the destruction of the spin-up transmission eigenchannels. However, the spin-down current is preserved, thus resulting in the perfect spin filter effect. Our results indicate that the ZnO nanoribbon modulated by the edge defect is a practical design for a spin filter.

Density functional theory study on the effect of Cu- and Na-substituted layers on spin-dependent transport and TMR in the Fe/ZnO/Fe MTJ

Using density functional theory, effects of Na- and Cu-substituted layers on the spin-dependent electronic transport properties of Fe/ZnO/Fe magnetic tunnel junction based on zinc oxide barrier tunnel, with rock-salt crystalline structure, have been studied. In zero-bias voltage, conductance and tunneling magneto-resistance (TMR) ratio of structures are calculated. It is showed that substituted layers in the pristine junction greatly affect conductance and TMR ratio of this junction. The results indicated that Cu-substituted layer with reducing conductance of pristine structure in the antiparallel alignment configuration, and increasing its conductance in the parallel alignment, leads to a large TMR ratio, up to 1800%. Due to the large conductance of pristine and Cu-substituted devices in the parallel alignment, these structures would be very beneficial for experimental applications that require the spin-polarized current.

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.

Serial and parallel spin circuits at the molecular scale with two atomic-vacancies in graphene: Amplification of spin-filtering effect

Our previous work demonstrated that graphene devices with monovacancy defects possess spin filtering effect which offers potential applications in spintronics. Here, using first-principles calculations, we further study the spin-dependent electron transport properties and spin filtering efficiency of graphene devices with double vacancies that are arrayed in parallel and serial connections. It is found that devices with vacancies in parallel connection follow classical Kirchhoff circuit law. Both spin-up and spin-down currents are amplified while spin filtering efficiency remains the same as compared to the monovacancy devices. In contrast, amplification of spin filtering efficiency is realized in devices with serially connected double vacancies. In addition, it is shown that the spin current can be flipped reversibly by nanomechanical deformation as we observed in devices with monovacancy. Our findings demonstrate the possibility to engineer the spin filtering effect for vacancy based spintronic devices.

Magnetic field effect on strained graphene junctions

We investigate the spin-dependent transport properties of a ferromagnetic/strained/normal graphene junctions with central region subjected to a magnetic field $B$. An analytical approach, based on Dirac equation, is implemented to obtain the eigenstates and eigenvalues of the charge carrier in three regions. Using the transfer matrix method, we determine the spin-dependent transmission in the presence of an applied strain along the armchair and zigzag directions of the graphene sample. We find that the strain remarkably modifies the Landau levels (LLs) originating from the applied $B$. It is shown that the spin up/down energy bands, in the first region, are shifted by the exchange $H_{ex}$ and left the whole spectrum linear as in the case of pristine graphene. In the central region, the position of the Dirac point changes due to the uniaxial strain and $B$. It is also found that the uniaxial strain in graphene induces a contraction of the LLs spectra. Moreover, the strain and $B$ modify the shape and position of some peaks in the transmission probabilities.

Aharonov-Bohm Interferometer in a T-Shaped Quantum Dot Embedded in Majorana Bound States**Supported by the National Natural Science Foundation of China under Grant No. 11547203 and the Science Foundation of Civil Aviation Flight University of China under Grant Nos. J2015-35 and JG2019-19

We theoretically study the spin-dependent transport properties of an Aharonov-Bohm (AB) interferometer composed by a T-shaped quantum dot (QD) embedded in Majorana bound states (MBS). We use the equation of motion method to calculate the conductance across the interferometer. We note that the conductance exhibits sensitive dependence on the MBS-QD coupling strength as well as the polarization strength of the leads when the phase factor of AB ring changes periodically. The conductance shows a transition from resonance to anti-resonance when the MBS-QD coupling strength changes from small to large. Also, there is different p-dependence conductance when the leads alignment changes from parallel to anti-parallel. These findings suggest that such a model could be used for a sensitive detection of MBS interactions, exploiting the high sensitivity of conductance to the AB phase in the interferometer.