Spin Transport Properties Research Articles

Reconfigurable spin tunnel diodes by doping engineering VS2 monolayers.

We propose a reconfigurable spin tunnel diode based on a small spin-gapped semiconductor (non-doped VS2 monolayer) and semi-metallic magnets (doped VS2 monolayer) separated by a thin insulating tunneling barrier (h-BN). By using first-principles calculations assisted by the nonequilibrium Green's function method, we have carried out a comprehensive study of spin-dependent current and spin transport properties while varying the bias. The device exhibited a magnetization-controlled diode-like behavior with forward-allowed current under antiparallel magnetizations and reverse-forbidden current under parallel magnetizations at the two electrodes. The threshold voltage is tunable by the hole doping density of VS2 monolayers. The doping effect on VS2 monolayers indicates that the magnetic moments, the Heisenberg exchange parameters and Curie temperatures can be monotonically reduced by a larger hole doping density. Our study on VS2 heterostructures has presented a simple and practical device strategy with very promising applications in spintronics.

Magnetic and spin transport properties of a two-dimensional magnetic semiconductor kagome lattice Nb<sub>3</sub>Cl<sub>8</sub> monolayer

Two-dimensional semiconductor materials with intrinsic magnetism have great application prospects in realizing spintronic devices with low power consumption, small size and high efficiency. Some two-dimensional materials with special lattice structures, such as kagome lattice crystals, are favored by researchers because of their novel properties in magnetism and electronic properties. Recently, a new two-dimensional magnetic semiconductor material Nb<sub>3</sub>Cl<sub>8</sub> monolayer with kagome lattice structure was successfully prepared, which provides a new platform for exploring two-dimensional magnetic semiconductor devices with kagome structure. In this work, we study the electronic structure and magnetic anisotropy of Nb<sub>3</sub>Cl<sub>8</sub> monolayer. We also further construct its <em>p-n</em> junction diode and study its spin transport properties by using density functional theory combined with non-equilibrium Green's function method. The results show that the phonon spectrum of the Nb<sub>3</sub>Cl<sub>8</sub> monolayer has no negative frequency, confirming its dynamic stability. The band gap of the spin-down state (1.157 eV) is significantly larger than that of the spin-up state (0.639 eV). The magnetic moment of the Nb<sub>3</sub>Cl<sub>8</sub> monolayer is 0.997 μ<sub>B</sub>, and its easy magnetization axis is in the plane and along the <em>x</em> axis direction based on its energy of magnetic anisotropy. Nb atoms make the main contribution to the magnetic anisotropy. When the strain is applied, the band gap of the spin-down states will decrease, while the band gap of the spin-up state is monotonously decreased from the negative (compress) to positive (tensile) strain. As the strain variable goes from -6% to 6%, the contribution of Nb atoms to the total magnetic moment gradually increases. Moreover, strain causes the easy magnetization axis of the Nb<sub>3</sub>Cl<sub>8</sub> monolayer to flip vertically from in-plane to out-plane. The designed <em>p-n</em> junction diode nanodevice based on Nb<sub>3</sub>Cl<sub>8</sub> monolayer exhibits an obvious rectification effect. In addition, the current in the spin-up state is larger than that in the spin-down state, exhibiting a spin-polarized transport behavior. Moreover, a negative differential resistance (NDR) phenomenon is also observed, which could be used in the NDR devices. These results demonstrate that the Nb<sub>3</sub>Cl<sub>8</sub> monolayer material has great potential application in the next generation of high-performance spintronic devices, and further experimental verification and exploration of this material and related two-dimensional materials are needed.

High temperature stability in few atomic layer MoS2 based thin film heterostructures: structural, static and dynamic magnetization properties.

Layered transition metal dichalcogenides (TMDs) have shown commendable properties for spintronic applications. From the device perspective, the structural quality of the TMD as well as its interface with the adjacent ferromagnetic (FM) layer is of paramount importance. Here, we present the spin-dynamic behaviour in the widely studied TMDs, i.e., MoS2 using Co60Fe20B20 (CoFeB), i.e., in MoS2(1-4 layers)/CoFeB(4-15 nm) heterostructures, both in the as-grown state and in the in situ annealed state (400 °C in a vacuum). Raman spectroscopy revealed systematic variation in the separation (δ) between the characteristic Raman shifts corresponding to the E2g and A1gvis-à-vis the number of layers (nL) of MoS2. The analysis of the ferromagnetic resonance (FMR) spectroscopy measurements performed on these heterostructures revealed the spin pumping from CoFeB to the MoS2 layer as evidenced by the ∼49% (∼51%) enhancement in the effective damping parameter with respect to the damping parameter of bare as-deposited (annealed) CoFeB films. This enhancement is attributed to the spin-pumping owing to the high spin-orbit coupling of monolayer MoS2. The latter is also confirmed by density functional theory calculations. By finding the effective spin mixing conductance of the MoS2/CoFeB interface, the effective spin current density in the MoS2 layer is estimated to increase from ∼0.3 to 0.7 MA m-2 with CoFeB thickness for both the as-deposited and annealed heterostructures. Furthermore, the δ vs. nL curve of the as-deposited heterostructure did not show any significant change upon annealing, which demonstrated that the spin transport and magnetic properties of these heterostructures remained unaffected even after annealing at a high temperature of 400 °C. Hence, this establishes the high thermal stability of the sputter grown MoS2/CoFeB heterostructures. Thus, this study highlights the important role of MoS2 as an efficient spin current-generating source for spin-orbit torque based magnetic memory applications, given the high-temperature stability and high-quality monolayers of MoS2 and its excellent performance with CoFeB thin films.

Large magnetoresistance in phosphorus-sulfur compounds (TMPS4) based temperature regulated spin-caloritronic devices

Two-dimensional (2D) transition metal phosphorus-sulfur compounds (TMPS 4 ) have attracted great theoretical and experimental attention because of their broadband modulation and high hole mobility that can be used in nanodevices. Therefore, their Curie temperature and thermal properties deserve further study for applications in electronic devices. The electromagnetic properties and spin transport properties of 2D transition metal phosphorus-sulfur compounds are systematically investigated by density functional theory (DFT) and DFT combined with the nonequilibrium Green's function method. We find that TMPS 4 materials cover almost all spin electronic behaviors, including half-metals, magnetic and nonmagnetic semiconductors, and metals. The simulation results show that the magnetic tunnel junction constructed based on TMPS 4 has a perfect thermal spin filtering effect and a negative differential resistance effect. Especially, we obtain up to 10 10 % thermal colossal magnetoresistance and almost 100% spin polarization, as well as a high current on/off ratio of ∼10 7 . Based on these results, it is proved that TMPS 4 can be a substrate material for the development of high performance for spin-caloritronic and spintronic devices.

Structural, electronic phase transitions and thermal spin transport properties in 2D NbSe2 and NbS2: a first-principles study.

How to effectively tune 2D electronic and magnetic properties is key to developing novel spintronic materials and devices. Although the strain induced metal-to-half-metal electronic phase transition (EPT) has been studied in 2H NbSe2 and NbS2 monolayers, the 1T phase, the Coulomb interaction and the transport properties have not been explored. Here, using first-principles calculations in junction with nonequilibrium Green's function, we present a comprehensive and comparative study on the strain tuned structural, electronic, magnetic and thermal spin transport properties for NbSe2 and NbS2 monolayers with and without Coulomb interaction. It is found that the Coulomb interaction makes the strain induced 2H-to-1T structural phase transition easier. Similar to the 2H phase, there is also a strain induced metal-to-half-metal EPT for the 1T phase without Coulomb interaction, and the Coulomb interaction makes the ETP easier. Remarkably, the 2H-NbSe2 monolayer with Coulomb interaction is a bipolar spin gapless semiconductor (SGS), and novel Dirac half-metal and usually SGS can be obtained by the tensile strain. In addition, we predict the excellent spin Seebeck effect and thermal spin diode effect in the bipolar SGS of the 2H-NbSe2 monolayer with Coulomb interaction, and expect the spin filtering effect and high magnetoresistance in the half-metals driven by the strain. We also discuss the strength of Coulomb interaction by comparing the theoretical and available experimental electronic states, indicating the indispensability of Coulomb interactions. These results suggest that 2D NbSe2 and NbS2 are promising candidates for phase-change spintronic materials and devices, and will stimulate extensive studies on this class of 2D systems.

First-principles study of single-molecule-structure determination of dithienoborepin isomers

Previous research results show that the conductance difference in molecular junction caused by quantum interference (QI) effect is an important way to identify isomers or improve the recognition sensitivity. Recently, single-molecule conductance of two fully π-conjugated dithienoborepin (DTB) isomers (DTB-A and DTB-B) with tricoordinate boron centers has been measured by using the scanning tunneling microscopy break junction technique. The result shows that QI can enhance chemical responsivity in single-molecule DTB junction. In this work, the first-principles method based on density functional theory and non-equilibrium Green's function is used to study the influence of QI effect on spin-transport property of DTB molecular junction connected to the nickel electrode, and the purpose of distinguishing DTB isomers (DTB-A and DTB-B) is realized by using amino and nitro passivation. The results show that the pristine DTB-A molecule and DTB-B molecule both have a up-spin transmission peak dominated by HOMO and a down-spin transmission peak dominated by LUMO on both sides of the Fermi level, and the energy positions and coefficients of two transmission peaks are basically the same. Therefore, the up-spin and down-spin current curves of the two junctions basically coincide, so that it is impossible to clearly distinguish the two isomers of DTB molecule simply by spin current. The QI can enhance the spin-polarized transport capability of two orbitals of amino-passivated DTB-A molecule to varying degrees but weaken the spin-polarized transport capability of two orbitals of amino-passivated DTB-B molecule. Therefore, the current of DTB-A molecular junction passivated by amino group is always higher than that of DTB-B molecular junction passivated by amino group. However, the QI can greatly enhance the spin-polarized transport capability of two orbitals of nitro-passivated DTB-B molecule but weaken the spin-polarized transport capability of two orbitals of nitro-passivated DTB-A molecule. Therefore, the current of DTB-B molecular junction passivated by nitro is always higher than that of DTB-A molecular junction passivated by nitro. Because the QI has different effects on the spin-transport capability of DTB-A and DTB-B passivated by amino or nitro group, so the two isomers of DTB molecule can be distinguished by measuring the spin current value. The above conclusions provide more theoretical guidance for the practical preparation of spin molecular junctions and the regulation of their spin-transport performance in the future.

Spin transport properties in a naphthyl diamine derivative film investigated by the spin pumping

We report the spin transport properties in a thin film of a naphthyl diamine derivative: N,N′-Bis(naphthalen-1-yl)-N,N′-bis(phenyl)-2,2′-dimethylbenzidine (α-NPD). In a palladium (Pd)/α-NPD/Ni80Fe20 tri-layer structure sample, a pure spin current is generated in the α-NPD layer with the spin pumping driven by ferromagnetic resonance (FMR). The generated spin current is absorbed into the Pd layer, and converted into a charge current with the inverse spin-Hall effect (ISHE) in Pd. An electromotive force due to the ISHE in the Pd layer is observed under the FMR of the Ni80Fe20 layer, which is clear evidence for the spin transport in an α-NPD film. The spin diffusion length in an α-NPD film is estimated to be about 62 nm at room temperature, which is long enough as a spin transport material for spintronic devices.

Superior spin transport properties based on VS2 and VCl2 ferromagnetic monolayers

Two-dimensional ferromagnetic monolayers have attracted growing interest due to their promising applications in spintronic devices. To explore the potential application of monolayer VS2 and VCl2 in spintronic devices, previously reported ferromagnetic semiconductor and half-metal, respectively, we investigate the spin transport properties of VS2 homo-junction, VCl2 homo-junction, and lateral VS2–VCl2 heterostructure using first-principles combined with non-equilibrium Green's function. We show that monolayer VS2 exhibit superior spin Seebeck effect along an armchair direction, monolayer VCl2 is an excellent platform to realize a spin valve, and the magnetoresistance ratio is up to 1.3 × 104. Moreover, the VS2–VCl2 heterostructure exhibits an excellent spin diode effect. We explain these effects from the calculated spin-dependent band structure and transmission spectrum. The superior spin transport properties make monolayer VS2 and VCl2 promising candidates for spintronic applications.

Tuning the Thermal Spin Transport Properties via Redox Reactions and Force

Spin caloritronic devices, multifunctional devices combining spintronics and caloritronic, are crucial for the sustainable development of humans. A novel spin caloritronic device is presented using anthraquinone-based molecular sandwiched among two semi-infinite zigzag graphene nanoribbons containing asymmetrical edge hydrogenation electrodes. We demonstrate that the temperature gradient between the right and the left electrodes can generate spin-up and spin-down currents flowing in opposite orientations. Besides, the mentioned currents possess approximately the same magnitudes, indicating a nearly nondissipative spin Seebeck effect. We also find that these currents are significantly dissimilar for the redox reactions and force at different temperature gradients, demonstrating that the developed system can provide acceptable switching action. The obtained results reflect that the redox reactions and force can control the spin Seebeck effect.

Comprehensive evaluation of torques in ultra-scaled MRAM devices

We present a generalization of the coupled spin-charge drift-diffusion formalism capable of accurately describing the spin and charge transport properties through magnetic tunnel junctions. Correction terms enable reproducing oscillations of the spin current in ferromagnets typical for quasi-ballistic transport. Our approach proves necessary to accurately capture an interplay between the interfacial Slonczewski and bulk-like Zhang-Li contributions to the torque in ultra-scaled MRAM devices.

Spin Transport Properties of zCrXY (X, Y = S,Se) Nanoribbons: Implications for Spintronics

Spin Transport Properties of zCrXY (X, Y = S,Se) Nanoribbons: Implications for Spintronics

Spin Hall magnetoresistance of CoFe2O4/Pt heterostructures with interface non-collinear magnetic configurations

We have investigated the spin Hall magnetoresistance (SMR) in CoFe2O4/Pt heterostructures deposited on (001) MgO and (001) MgAl2O4 substrates, respectively. It is found that the interface non-collinear magnetic configurations should exist in both of the two films in the whole temperature range from 5 to 300 K but mainly dominate the spin transport properties at low temperature. Furthermore, the spin transport has featured epitaxial-strain-modified characteristics induced by different substrates, which has been investigated by magnetic curves, SMR, anisotropic magnetoresistance, and Hall effect measurements.

Thermal Spin Transport Properties in Diarylethene-Based Molecule Devices

Spin caloritronic devices, as multifunctional devices, combining spintronics, and caloritronics, are essential for the sustainable development of humans. Here, a novel spin caloritronic device is presented using a diarylethene molecule photoswitch sandwiched among two semi-infinite zigzag graphene nanoribbons containing asymmetrical edge hydrogenation electrodes. We demonstrate that the temperature gradient between the right and the left electrodes can generate spin-up (SU) and spin-down (SD) currents moving in opposite orientations. Moreover, the mentioned currents possess approximately the same magnitudes, indicating a nearly nondissipative spin Seebeck effect. We also find that these currents are significantly dissimilar for the two photochromic isomers at different temperature gradients, demonstrating the excellent system’s switching nature. The obtained results reveal that the light can control the thermal spin transport properties.

Structural, Electronic, and Magnetic Characteristics of Graphitic Carbon Nitride Nanoribbons and Their Applications in Spintronics.

The development of quantum information and quantum computing technology requires special materials to design and manufacture nanosized spintronic devices. Possessing remarkable structural, electronic, and magnetic characteristics, graphitic carbon nitride (g-C3N4) can be a promising candidate as a building block of futuristic nanoelectronics and spintronic systems. Here, using first-principles calculations, we perform a comprehensive study on the structural stability as well as electronic and magnetic properties of triazine-based g-C3N4 nanoribbons (gt-CNRs). Our calculations show that gt-CNRs with different edge conformation exhibit distinct electronic and magnetic characteristics, which can be tuned by the edge H-passivation rate. By investigating gt-CNRs with various possible edge configurations and H-termination rates, we show that while the ferromagnetic (FM) ordering of gt-CNRs stays preserved for all of the studied configurations, half metallicity can only be achieved in nanoribbons with specific edge structure under full H-passivation rate. For spintronic application purposes, we also study spin-transport properties of half-metal gt-CNRs. By determining the suitable gt-CNR configuration, we show the possibility of developing a perfect gt-CNR-based spin filter with a spin filter efficiency (SFE) of 100%. Considering the above-mentioned notable electronic and magnetic characteristics as well as its high thermal stability, we show that gt-CNR would be a remarkable material to fabricate multifunctional spintronic devices.

Electric-Field Control of Spin Diffusion Length and Electric-Assisted D'yakonov-Perel' Mechanism in Ultrathin Heavy Metal and Ferromagnetic Insulator Heterostructure.

Electric-field control of spin dynamics is significant for spintronic device applications. Thus far, effectively electric-field control of magnetic order, magnetic damping factor and spin–orbit torque (SOT) has been studied in magnetic materials, but the electric field control of spin relaxation still remains unexplored. Here, we use ionic liquid gating to control spin-related property in the ultra-thin (4 nm) heavy metal (HM) platinum (Pt) and ferromagnetic insulator (FMI) yttrium iron garnet (Y3Fe5O12, YIG) heterostructure. It is found that the anomalous Hall effect (AHE), spin relaxation time and spin diffusion length can be effectively controlled by the electric field. The anomalous Hall resistance is almost twice as large as at 0 voltage after applying a small voltage of 5.5 V. The spin relaxation time can vary by more than 50 percent with the electric field, from 41.6 to 64.5 fs. In addition, spin relaxation time at different gate voltage follows the reciprocal law of the electron momentum scattering time, which indicates that the D’yakonov–Perel’ mechanism is dominant in the Pt/YIG system. Furthermore, the spin diffusion length can be effectively controlled by an ionic gate, which can be well explained by voltage-modulated interfacial spin scattering. These results help us to improve the interface spin transport properties in magnetic materials, with great contributions to the exploration of new physical mechanisms and spintronics device.

Two-dimensional spin-gapless semiconductors: A mini-review.

In the past decade, two-dimensional (2D) materials and spintronic materials have been rapidly developing in recent years. 2D spin-gapless semiconductors (SGSs) are a novel class of ferromagnetic 2D spintronic materials with possible high Curie temperature, 100% spin-polarization, possible one-dimensional or zero-dimensional topological signatures, and other exciting spin transport properties. In this mini-review, we summarize a series of ideal 2D SGSs in the last 3 years, including 2D oxalate-based metal-organic frameworks, 2D single-layer Fe2I2, 2D Cr2X3 (X = S, Se, and Te) monolayer with the honeycomb kagome (HK) lattice, 2D CrGa2Se4 monolayer, 2D HK Mn–cyanogen lattice, 2D MnNF monolayer, and 2D Fe4N2 pentagon crystal. The mini-review also discusses the unique magnetic, electronic, topological, and spin-transport properties and the possible application of these 2D SGSs. The mini-review can be regarded as an improved understanding of the current state of 2D SGSs in recent 3 years.

Disorder- and Topology-Enhanced Fully Spin-Polarized Currents in Nodal Chain Spin-Gapless Semimetals.

Recently discovered high-quality nodal chain spin-gapless semimetals MF_{3} (M=Pd, Mn) feature an ultraclean nodal chain in the spin up channel residing right at the Fermi level and displaying a large spin gap leading to a 100% spin polarization of transport properties. Here, we investigate both intrinsic and extrinsic contributions to anomalous and spin transport in this class of materials. The dominant intrinsic origin is found to originate entirely from the gapped nodal chains without the entanglement of any other trivial bands. The side-jump mechanism is predicted to be negligibly small, but intrinsic skew scattering enhances the intrinsic Hall and Nernst signals significantly, leading to large values of respective conductivities. Our findings open a new material platform for exploring strong anomalous and spin transport properties in magnetic topological semimetals.

Strong spin filtering by silicon nanoparticles with adsorbed bismuth atom clusters: Role of symmetry and electrostatic control of the direction of spin polarization

We present a theoretical study, based on density functional theory and tight-binding modeling, of the electronic structure and spin transport properties of silicon nanoparticles with adsorbed bismuth atoms. We find the bismuth atoms to form clusters separated by quantum tunnel barriers. We predict strong spin filtering by these nanostructures in the high conductance regime when the source and drain leads are connected to the same bismuth cluster and in the low conductance regime when the source and drain leads are connected to different bismuth clusters. We relate the spin filtering to a symmetry obeyed by the spin transmission probability matrix of the system. We also predict that for such systems the direction of the spin polarization in the drain lead can be tuned through large angles and even reversed electrostatically simply by varying the voltage applied to a gate. Realization of these silicon-bismuth nanostructures in the laboratory is feasible. We expect the predicted spin filtering to be experimentally accessible and potentially relevant for device applications.

Effect of the Rashba spin-orbit coupling on spin transport properties in WSe2 superlattice

We report a theoretical study of the spin and valley dependent transport properties in a WSe2 superlattice with external Rashba spin-orbit interaction (ERSOI). It is shown that the conductance with and without spin flip strongly depends on the ERSOI and has an oscillatory behavior with respect to ERSOI strength, thus WSe2 superlattice can act as a spin-converter. Also, the magnitude of spin-dependent conductance can be efficiently controlled by potential barrier height and number of superlattice barriers. Results show that the valley and spin polarization depend on the number of superlattice barriers and exhibit an oscillatory evolution versus ERSOI strength and potential barrier height. These results clarify that a WSe2 superlattice can be useful for the design of novel valleytronic and spintronic devices.

Spin transport properties and nanodevice simulations of NiI2 monolayer

Two-dimensitional (2D) magnetic semiconducting materials attract extensive interest in recent years. A new type of magnetic semiconducting nickel-based transition metal diiodide-NiI2 monolayer (NiI2-ML) was successfully prepared in recent experiments. Some nanodevices based on the NiI2-ML are designed and their spin-resolved transport and photoelectric properties are studied. Strong rectification and spin filtering effects are observed in the NiI2-ML pn-junction. The NiI2-ML pin-junction shows a field-effect property. Moreover, it is found that NiI2-ML has a strong response to green light that promotes its phototransistor to generate exploitable photocurrent. The results of this paper show that NiI2-ML is a multifunctional magnetic semiconducting material with potential applications in the fields of spintronic and optoelectronic devices.