Spin Transport Properties Research Articles

Lateral Gate and Magnetic Field Controlled Quantum Transport in a Topological Dirac Semimetal Nanowire

Both the charge- and spin-transport properties of a topological Dirac semimetal (TDSM) nanowire with two lateral gates and a perpendicular magnetic field are investigated by using the Green function method in combination with the Landauer-B\uttiker formula. A fully spin-polarized surface current can be generated in the considered system and it can be turned on or off by varying the electron energy and magnetic field strength. The underlying physics of this transport phenomenon originates from the quantum anomalous Hall-like insulator state of the TDSM nanowire caused by the magnetic field. Further studies indicate the spin-polarized current with a strong robustness against disorder, displaying the feasibility of designing a topological spin filtering and switch device based on the considered system.

Robust Spin Interconnect with Isotropic Spin Dynamics in Chemical Vapor Deposited Graphene Layers and Boundaries.

The utilization of large-area graphene grown by chemical vapor deposition (CVD) is crucial for the development of scalable spin interconnects in all-spin-based memory and logic circuits. However, the fundamental influence of the presence of multilayer graphene patches and their boundaries on spin dynamics has not been addressed yet, which is necessary for basic understanding and application of robust spin interconnects. Here, we report universal spin transport and dynamic properties in specially devised single layer, bilayer, and trilayer graphene channels and their layer boundaries and folds that are usually present in CVD graphene samples. We observe uniform spin lifetime with isotropic spin relaxation for spins with different orientations in graphene layers and their boundaries at room temperature. In all of the inhomogeneous graphene channels, the spin lifetime anisotropy ratios for spins polarized out-of-plane and in-plane are measured to be close to unity. Our analysis shows the importance of both Elliott-Yafet and D’yakonov-Perel’ mechanisms with an increasing role of the latter mechanism in multilayer channels. These results of universal and isotropic spin transport on large-area inhomogeneous CVD graphene with multilayer patches and their boundaries and folds at room temperature prove its outstanding spin interconnect functionality, which is beneficial for the development of scalable spintronic circuits.

The Application of Organic Semiconductor Materials in Spintronics.

π-Conjugated semiconductors, primarily composed of elements with low atomic number, are regarded as promising spin-transport materials due to the weak spin–orbit coupling interaction and hence long spin relaxation time. Moreover, a large number of additional functions of organic semiconductors (OSCs), such as the abundant photo-electric properties, flexibility, and tailorability, endow the organic spintronic devices more unique properties and functionalities. Particularly, the integration of the photo-electric functionality and excellent spin transport property of OSCs in a single spintronic device has even shown great potential for the realization of spin manipulation in OSCs. In this review, the application of OSCs in spintronic study will be succinctly discussed. As the most important and extensive application, the long-distance spin transport property of OSCs will be discussed first. Subsequently, several multifunctional spintronic devices based on OSCs will be summarized. After that, the organic-based magnets used for the electrodes of spintronic devices will be introduced. Finally, according to the latest progress, spin manipulation in OSCs via novel spintronic devices together with other prospects and challenges will be outlined.

Temperature dependence of spin-transport properties and spin torque in a magnetic nanostructure

The study and understanding of spin-transport mechanisms including thermal fluctuation are required for the development and design of spintronic devices. In this paper, we present an approach to investigate the temperature dependence of spin-transport behavior within the magnetic structure by using the generalized spin accumulation model. The temperature affects not only the magnetization orientation, but also the spin-transport properties. Its effect on transport parameters can be taken into account by considering the spin-dependent resistivity at any finite temperature. This leads to the calculation of temperature-dependent spin-transport parameters and eventually allows the calculation of the thermal effects on spin accumulation, spin current, and spin torque. It is observed that increasing temperature is likely to decrease the value of key transport parameters relevant to the magnitude of spin torque. This study demonstrates the importance of thermal effects on spin-transport behavior which needs to be considered for spin-transfer torque based device design with high performance.

Switch effect for spin-valley electrons in monolayer WSe2 structures subjected to optical field and Fermi velocity barrier

To investigate the effects of the optical field and the Fermi velocity on the transport properties of spin and valley electrons, we impose a normal/ferromagnetic/normal (N/F/N) quantum structure based on the monolayer WSe2. The results indicate that there is a strong switch effect for spin- and valley-related electrons. When left-handed off-resonant circularly polarized light is irradiated in the intermediate ferromagnetic region, 100% polarization for K valley electrons can be achieved in the entire effective energy spectrum of the optical field. Meanwhile, 100% polarization of the K′ valley can also be gained with the right-handed off-resonant circularly polarized light in the junction. Moreover, the perfect polarization of spin-up electrons can be obtained when the ferromagnetic exchange field is applied to the structure. Additionally, the Fermi velocity barrier also changes the energy band of the studied material, which makes the spin and valley transport increase with the increase of the velocity barrier but does not produce spin or valley polarizations. These interesting results clarify that the optical field and the Fermi velocity both make a contribution to the modulation of spin electrons for the two valleys and provide a useful method for the design of novel spintronic and valleytronic devices.

Spin transport in multilayer graphene away from the charge neutrality point

Graphene is considered as a promising material in spintronics due to its long spin relaxation time and long spin relaxation length. However, its spin transport properties have been studied at low carrier density only, beyond which much is still unknown. In this study, we explore the spin transport and spin precession properties in multilayer graphene at high carrier density using ionic liquid gating. We find that the spin relaxation time is directly proportional to the momentum relaxation time, indicating that the Elliott-Yafet mechanism still dominates the spin relaxation in multilayer graphene away from the charge neutrality point.

Controlled single-spin transport in thermally driven molecular devices containing graphene electrodes modulated by quantum resonance

Resonance effects play an important role in the transport properties of low-dimensional systems. Here, we investigate the thermally driven spin-transport properties of ruthenium-terpyridine molecular devices, where the molecule is linked to graphene electrodes by the carbon atomic chains from two sides. The results show that an odd-even oscillating behavior of the transmission spectra and a perfect spin-filtering effect at the Fermi level are observed in the ruthenium-terpyridine molecular devices. Furthermore, the negative differential thermoelectric resistance more easily appears in the long even-numbered carbon atomic chains, and the sign of the thermally driven single-spin current can be tuned via the temperature. Such a behavior is attributed to the competition between the Breit-Wigner and Fano resonances in the vicinity of the Fermi level for the contributions of the thermally driven current. In brief, here we present an alternative to control the thermally driven single-spin current in molecular devices via quantum effect.

Spin-orbit coupling and linear crossings of dipolar magnons in van der Waals antiferromagnets

A magnon spin-orbit coupling, induced by the dipole-dipole interaction, is derived in monoclinic-stacked bilayer honeycomb spin lattice with perpendicular magnetic anisotropy and antiferromagnetic interlayer coupling. Linear crossings are predicted in the magnon spectrum around the band minimum in G valley, as well as in the high frequency range around the zone boundary. The linear crossings in K and K' valleys, which connect the acoustic and optical bands, can be gapped when the intralayer dipole-dipole or Kitaev interactions exceed the interlayer dipole-dipole interaction, resulting in a phase transition from semimetal to insulator. Our results are useful for analyzing the magnon spin dynamics and transport properties in van der Waals antiferromagnet.

Magneto-electronic and spin transport properties of transition metal doped antimonene nanoribbons

Abstract Recently, how to induce magnetism for two-dimensional materials have become one of research hot topics. Here, we study geometrical stability and magneto-electronic properties for antimonene nanoribbons doped with low-concentration transition metal(TM) atom (Mn, Fe, Cr and Co). It is found that these structures are stable by the formation energy calculation and molecular dynamics simulation. In particular, results show that these doped nanoribbons possess interesting magnetic phases, such as magnetic half-metal, magnetic half-semiconductor, and bipolar magnetic semiconductor, depending on the type of TM atoms. Furthermore, Mn-doped ribbon based magnetic device is constructed and the spin transport behaviors are investigated in details. The perfect double spin-filtering effect, excellent dual spin diode performance and giant magnetoresistance up to 109% are observed in this device. These findings might make a significant contribution for designing multi-function spintronic devices.

High-resolution tunneling spin transport characteristics of topologically distinct magnetic skyrmionic textures from theoretical calculations

High-resolution tunneling electron spin transport properties (longitudinal spin current (LSC) and spin transfer torque (STT) maps) of topologically distinct real-space magnetic skyrmionic textures are reported by employing a 3D-WKB combined scalar charge and vector spin transport theory in the framework of spin-polarized scanning tunneling microscopy (SP-STM). For our theoretical investigation metastable skyrmionic spin structures with various topological charges (Q=-3,-2,-1,0,1,2) in the (Pt0.95Ir0.05)/Fe/Pd(111) ultrathin magnetic film are considered. Using an out-of-plane magnetized SP-STM tip it is found that the maps of the LSC vectors acting on the spins of the magnetic textures and all STT vector components exhibit the same topology as the skyrmionic objects. In contrast, an in-plane magnetized tip generally does not result in spin transport vector maps that are topologically equivalent to the underlying spin structure, except for the LSC vectors acting on the spins of the skyrmionic textures at a specific relation between the spin polarizations of the sample and the tip. The magnitudes of the spin transport vector quantities exhibit close relations to charge current SP-STM images irrespectively of the skyrmionic topologies. Moreover, we find that the STT efficiency (torque/current ratio) acting on the spins of the skyrmions can reach large values up to ~25 meV/μA (~0.97 h/e) above the rim of the magnetic objects, but it considerably varies between large and small values depending on the lateral position of the SP-STM tip above the topological spin textures. A simple expression for the STT efficiency is introduced to explain its variation. Our calculated spin transport vectors can be used for the investigation of spin-polarized tunneling-current-induced spin dynamics of topologically distinct surface magnetic skyrmionic textures.

Spin transport in noncollinear antiferromagnetic metals

For noncollinear antiferromagnetic metals, magnon spectra display nontrivial multiband structure in which both direction and magnitude of the angular momenta of magnons are momentum dependent. We study the roles of these magnons on the spin transport properties by taking into account the momentum and angular momentum transfer between conduction electrons and magnons. We have calculated spin conductivity tensor by using the coupled Boltzmann equations for the electron spin and magnon, and we show that the unpolarized electron current driven by an electric field can efficiently induce a magnon spin current. The temperature dependence of the magnon spin conductivity is also calculated.

Giant spin signals in chemically functionalized multiwall carbon nanotubes.

Transporting quantum information such as the spin information over micrometric or even millimetric distances is a strong requirement for the next-generation electronic circuits such as low-voltage spin-logic devices. This crucial step of transportation remains delicate in nontopologically protected systems because of the volatile nature of spin states. Here, a beneficial combination of different phenomena is used to approach this sought-after milestone for the beyond-Complementary Metal Oxide Semiconductor (CMOS) technology roadmap. First, a strongly spin-polarized charge current is injected using highly spin-polarized hybridized states emerging at the complex ferromagnetic metal/molecule interfaces. Second, the spin information is brought toward the conducting inner shells of a multiwall carbon nanotube used as a confined nanoguide benefiting from both weak spin-orbit and hyperfine interactions. The spin information is finally electrically converted because of a strong magnetoresistive effect. The experimental results are also supported by calculations qualitatively revealing exceptional spin transport properties of this system.

Quantum charge and spin pumping in monolayer phosphorene

We study quantum-charge and spin-transport properties and the effects of in-plane strain on the charge and spin currents in a phosphorene monolayer using an adiabatic pumping regime. To achieve this goal, we proposed a device with three external ac gate voltages as oscillating potential barriers that are responsible for the generation of dc pumped current. Using exchange magnetic field induced by proximity effect of a ferromagnetic insulator, we determine the conditions in which fully spin-polarized current and pure spin current (with zero charge current) can be obtained. It is shown that the pumped current in the three-barrier case is about two times greater than the pumped current in a two-barrier system. The effect of strain is investigated and it is found that the spin current increases up to two orders of magnitude by applying the uniaxial strain which shows that the proposed device has high sensitivity to strain and could be used as straintronic devices such as strain switches and strain sensors. Also, in the same conditions, the pumped current in phosphorene is two, four, and five orders of magnitude greater than the pumped current in ${\mathrm{MoS}}_{2}$, silicene, and graphene, respectively. These properties show that phosphorene can be considered as a two- dimensional (2D) semiconductor with great potential for the fabrication of novel spintronic and straintronic devices that can overcome some of the limitations exhibited by conventional 2D materials.

Concepts of Spin Seebeck Effect in Ferromagnetic Metals

AbstractSpin Seebeck effect (SSE) and related spin caloritronics have attracted great interest recently. However, the definition of the SSE coefficient remains to be established, let alone a clean experiment to measure the SSE coefficient in ferromagnetic metals. The concept through a model based on the semi‐classical Botlzmann transport equation has been clarified. The model includes the vital spin‐flip process, which is frequent in metals, and points out that the length scale of SSE is much larger than the spin diffusion length. The model reveals how the spin‐flip process influences the transport equations and provides the simple relationship between the different spin‐flip relaxation times for spin‐up and ‐down electrons, which is very useful to understand the spin transport properties. This understanding allows to redefine the expression of the spin Seebeck coefficient.

Heterogeneous magnetism and kinetic arrest in antiperovskite Mn3−xNixGaC compounds with Ni2MnGa Heusler insertions

Mn-based antiperovskite compounds in the form ${\mathrm{Mn}}_{3}AX$, where $A$ is a main group element and $X$ is C or N, undergo magnetostructural transitions with which these materials acquire magnetocaloric, giant magnetoresistance, and spin-transport properties, which can be modified or tailored by manipulating the compositions of numerous compounds. This enables closer investigations and better understandings of the underlying principles governing these properties. ${\mathrm{Mn}}_{3\ensuremath{-}x}{\mathrm{Ni}}_{x}\mathrm{GaC}$, which is a derivative of the prototype ${\mathrm{Mn}}_{3}\mathrm{GaC}$ antiperovskite, would normally be expected to form a cubic structure with a homogeneous composition. Contrary to this, we find that the addition of Ni leads to a heterogenous compound consisting of an antiperovskite part and a ${\mathrm{Ni}}_{2}\mathrm{MnGa}$ Heusler insertions. The system shows kinetic arrest features, which we study as a function of Ni composition using the techniques of x-ray diffraction, magnetization, and neutron diffraction under a magnetic field.

Theoretical Investigation of Nonequilibrium Spin Transport Through a Triple Site Quantum Wire System

A triple lattice site quantum wire (QW) system is chosen to investigate nonequilibrium spin transport properties theoretically. We calculate the initial charge of the QW from the exactly derived ground state. Based on the Keldysh formalism, we describe theoretical methods and derive analytical formulas for nonequilibrium spin transport of the QW systems within Hartree-Fock approximation when Coulomb interactions are present. We report the numerical results of the nonequilibrium differential spin conductance, spin transport current, and electronic charge distribution of the triple site QW system. When only including Coulomb repulsions between the spin-up and spin-down electrons, the series of peaks and valleys in the conductance characteristics appears due to the spin resonant tunneling and spin blockade. With the increase of Coulomb interaction energy U, the conductance peaks start to split into two corresponding to the spin-up and spin-down conductance. Near the resonant points, the spin current polarization and the in-site spin charge polarizations take place. When additionally including spin-spin interactions, the differences between the spin-up and spin-down conductance characteristics are reduced, implying that the spin split is relaxed by spin-spin interactions. The spin splits, the spin current, and spin charge polarizations are relaxed at high temperatures due to the thermal fluctuations.

Multifunctionality for the nanojunction of a rotating p-phenylene vinylene molecule between graphene leads

With the multi-functional molecular device based on graphene nanoribbon being deeply studied in experiment, the zigzag-edged graphene device is still worth to investigate. Employing the ab-initio method, the spin transport properties have been studied for the nanojunctions consisting of a p-phenylene vinylene (PPV) molecule sandwiched between two-probe leads of zigzag-edged graphene nanoribbons (ZGNRs). A series of obvious electromagnetic transmission functionalities, including spin switching, negative differential resistance (NDR), dual spin-filtering, magnetoresistance and spin-diode behaviors, are numerically referred in the proposed molecular junction within spin parallel or antiparallel configurations. The performance of switching and double spin filtering can be explained by the transport spectra or total transmission pathways. Besides, the rectification effect is due to the asymmetry spatial distribution of the local density of states as well as the corresponding coupling between the PPV molecule and leads. It is expected that the designed models can be ideal candidate for future spintronic device.

Optimization of spin Hall magnetoresistance in heavy-metal/ferromagnetic-metal bilayers

We present experimental data and their theoretical description on spin Hall magnetoresistance (SMR) in bilayers consisting of a heavy metal (H) coupled to in-plane magnetized ferromagnetic metal (F), and determine contributions to the magnetoresistance due to SMR and anisotropic magnetoresistance (AMR) in five different bilayer systems: hbox {W}/text {Co}_{20}text {Fe}_{60}text {B}_{20}, text {Co}_{20}text {Fe}_{60}text {B}_{20}/hbox {Pt}, hbox {Au}/text {Co}_{20}text {Fe}_{60}text {B}_{20}, W/Co, and Co/Pt. The devices used for experiments have different interfacial properties due to either amorphous or crystalline structures of constitutent layers. To determine magnetoresistance contributions and to allow for optimization, the AMR is explicitly included in the diffusion transport equations in the ferromagnets. The results allow determination of different contributions to the magnetoresistance, which can play an important role in optimizing prospective magnetic stray field sensors. They also may be useful in the determination of spin transport properties of metallic magnetic heterostructures in other experiments based on magnetoresistance measurements.

Effects of Cr doping in δ-MoN: structural, magnetic and spin transport properties

Structural, magnetic, and spin transport properties of δ-MoN with one Cr atom substituted at different Mo sites (2a and 6c in the International Tables) have been studied by spin-polarized first-principles calculations and nonequilibrium Green’s function method. The Cr dopants located at 2a and 6c sites [corresponding to the configurations of Cr-MoN(2a) and Cr-MoN(6c)] lead to significant spin splitting of the density of states and contribute 2.86 and 2.70 μB magnetic moments, respectively. Detailed analysis reveals that interactions between the Cr dopant and its neighboring Mo atoms play crucial roles in the magnetic properties of Cr-MoN(2a) and Cr-MoN(6c). The Cr substitution induces evident antiferromagnetic polarization to its Mo neighbors, and each Mo atom possesses − 0.02 to − 0.51 μB magnetic moment antiparallel to the magnetic moment of the Cr dopant. Unlike the pure δ-MoN, the spin-up and the spin-down currents of the Cr-doped systems exhibit obvious spin polarization, and the spin-polarized effect is more enhanced when the Cr dopant is located at the 6c site. Under the examined bias range of 0–1.0 V, Cr-MoN(2a) displays no more than 6.5% spin polarization, whereas in Cr-MoN(6c) case, up to 22.8% of the spin polarization is attained.

Magnetic damping enhancement in L12-ordered Mn3Ir/Fe20Ni80 bilayers

Electron and spin transport in chiral antiferromagnets are of great interest in both fundamental and application point of views. In this letter, we explore and compare spin transport properties of a chiral antiferromagnetic L12-ordered Mn3Ir and a disordered Mn3Ir by the spin pumping experiments. Larger Gilbert damping enhancement of the Fe20Ni80 was found in L12-ordered Mn3Ir/Fe20Ni80 and L12-ordered Mn3Ir/Ti/Fe20Ni80 multilayers comparing to those with disordered ones. Our results suggest that the chiral spin structure causes larger spin dissipation than disordered spin structure.