Spin Transport Properties Research Articles

Spin Hall effect in a spin-1 chiral semimetal

The spin-1 chiral semimetal is a state of quantum matter hosting unconventional chiral fermions that extend beyond the common Dirac and Weyl fermions. B20-type CoSi is a prototypal material that accommodates such an exotic quasiparticle. To date, the spin-transport properties in the spin-1 chiral semimetals have not been thoroughly explored. In this work, we fabricated B20-CoSi thin films on sapphire $c$-plane substrates by magnetron sputtering and studied the spin Hall effect (SHE) by combining experiments and first-principles calculations. The SHE of CoSi was investigated using CoSi/CoFeB/MgO heterostructures via spin Hall magnetoresistance and harmonic Hall measurements. First-principles calculations yield an intrinsic spin Hall conductivity (SHC) at the Fermi level that is consistent with the experiments and reveal its unique Fermi-energy dependence. Unlike the Dirac and Weyl fermion-mediated Hall conductivities that exhibit a peaklike structure centering around the topological node, SHC of B20-CoSi is odd and crosses zero at the node with two antisymmetric local extrema of opposite sign situated below and above in energy. Hybridization between Co $d$-Si $p$ orbitals and spin-orbit coupling are essential for the SHC, despite the small (\ensuremath{\sim}1%) weight of the Si $p$ orbital near the Fermi level. This work expands the horizon of topological spintronics and highlights the importance of Fermi-level tuning in order to fully exploit the topology of spin-1 chiral fermions for spin-current generation.

Thermal induced spin-polarized current protected by spin-momentum locking in ZrTe5 nanowires

Spin-momentum locking arising from strong spin-orbit coupling is one of the key natures of topological materials. Since charge can induce a spin polarization due to spin-momentum locking, the search for materials that exhibit this feature has become one of the top priorities in the field of spintronics. In this paper, we report the electrical detection of the spin-transport properties of ${\mathrm{ZrTe}}_{5}$ nanowires, using a nonlocal geometry measurement. A clear hysteresis voltage signal, which depends on the relative orientations between the magnetization of the ferromagnetic electrodes and the carrier spin polarization, has been observed. The hysteresis voltage states can be reversed by altering the electron movement direction, providing direct evidence of the spin-momentum locking feature of ${\mathrm{ZrTe}}_{5}$ nanowires and revealing its topological nature. Furthermore, the current-dependent measurement suggests that the charge (spin) current is induced by thermal effect, which utilizes the thermoelectric properties of ${\mathrm{ZrTe}}_{5}$. Using the thermal effect to control the spin-polarized current protected by spin-momentum locking offers possibilities for small-sized devices based on the topological materials.

Effect of Contact Geometry on Spin Transport in Amine-Ended Single-Molecule Magnetic Junctions.

We employ the first-principles calculation with non-equilibrium Green’s function method to comprehensively investigate the crucial role of interfacial geometry in spin transport properties of Co/1,4-benzenediamine (BDA)/Co single-molecule magnetic junctions (SMMJs). Two bonding mechanisms are proposed for the hard–hard Co–N coupling: (1) the covalent bonding between the H-dissociated amine linker and spin-polarized Co apex atoms and (2) the dative interaction between the H-non-dissociated (denoted by +H) amine linker and Co apex atoms. The former covalent contact dominates the π-resonance interfacial spin selection that can be well preserved in H-dissociated cases regardless of the choice of top, bridge, and hollow contact sites. From our detailed analyses of spin-polarized transmission spectra, local density of states, and molecular density of states, the underlying mechanism is that the strong hybridization between Co-d, N-py, and the π-orbital of the phenyl ring in dissociated cases renders the 2-fold HOMO (4-fold LUMO) of the central molecule closer to the Fermi energy. In contrast, the enlarged Co–N bond length of the latter dative contact in the H-non-dissociated case not only destroys the spinterface coupling but also blocks the spin injection. This theoretical work may provide vital and practical insights to illustrate the spin transport property in real amine-ended SMMJs since the contact geometries and interfacial bond mechanisms remain unclear during the breaking junction technique.

Room-Temperature Spin-Transport Properties in an In0.5Ga0.5As Quantum Dot Spin-Polarized Light-Emitting Diode

An understanding of the spin-transport properties in semiconductor barriers is essential to improve the performance of spin-polarized light-emitting diodes (spin LEDs) for future optospintronics integration in information processing. Here, we report on the temperature and bias-voltage dependence of spin-transport properties in an ${\mathrm{In}}_{0.5}{\mathrm{Ga}}_{0.5}\mathrm{As}$ quantum dot (QD) spin LED using a combination of spin-dependent electroluminescence (EL) and time-resolved photoluminescence. The QD EL spin polarization increases with an increase in temperature above 125 K; this is attributed to the improved conversion efficiency from spin polarization of electrons to circular polarization of photons of the QDs. We find that both the electric field and temperature can enhance spin relaxation in the undoped $\mathrm{Ga}\mathrm{As}$ barrier above 200 K. At 298 K, the QD EL spin polarization decreases beyond 2.5 V; this is attributed to the enhanced D'yakonov Perel' spin relaxation in the undoped $\mathrm{Ga}\mathrm{As}$ barrier caused by the increase in electron temperature. This study provides valuable insights into the spin-relaxation mechanism in the semiconductor barrier during the room-temperature operation of the QD spin LED.

Electronic structures of various (001) and (111) surfaces, interfaces and spin transport properties of half-metallic fully compensated ferrimagnet Cr2Se

Half-metallic fully compensated ferrimagnets have advantages of extremely low net magnetic moments and large spin polarization. By employing the first-principles calculations within density functional theory combined with the nonequilibrium Green’s function, we investigated the electronic structures of Cr2Se (001), (111) surfaces and Cr2Se/CdS interface, as well as the transport properties of Cr2Se/CdS/Cr2Se magnetic tunnel junction (MTJ). Our calculations show that bulk half-Heusler alloy Cr2Se is a half-metallic fully compensated ferrimagnet with a direct spin-down bandgap. Although the half-metallic character of bulk is destroyed in all surface terminations owing to the emergence of surface states, CrBSe-(001) termination holds the lowest surface free energy and possesses the highest surface spin polarization (SP) of 83.6%. Calculations on electronic structures of Cr2Se/CdS heterostructures show that CrBSe-S-top type interface structure holds the lowest interface free energy and high interface SP of 71.6%. Moreover, the spin transport properties of Cr2Se/CdS/Cr2Se MTJ device are studied and the calculated tunnel magnetoresistance (TMR) ratio reaches up to a high value of 4.98×103, which shows that the device has great application potential in spintronic devices.

Antiferromagnetic Oxide Thin Films for Spintronic Applications

Antiferromagnetic oxides have recently gained much attention because of the possibility to manipulate electrically and optically the Néel vectors in these materials. Their ultrafast spin dynamics, long spin diffusion length and immunity to large magnetic fields make them attractive candidates for spintronic applications. Additionally, there have been many studies on spin wave and magnon transport in single crystals of these oxides. However, the successful applications of the antiferromagnetic oxides will require similar spin transport properties in thin films. In this work, we systematically show the sputtering deposition method for two uniaxial antiferromagnetic oxides, namely Cr2O3 and α-Fe2O3, on A-plane sapphire substrates, and identify the optimized deposition conditions for epitaxial films with low surface roughness. We also confirm the antiferromagnetic properties of the thin films. The deposition method developed in this article will be important for studying the magnon transport in these epitaxial antiferromagnetic thin films.

Spin- and valley-polarized transport and magnetoresistance in asymmetric ferromagnetic WSe2 tunnel junctions

Transition metal dichalcogenides (TMDs) offer a new platform for theoretical study of two-dimensional materials and their applications, beyond graphene. Previous studies indicate that monolayers of TMDs become ferromagnetic when put in proximity to conventional ferromagnetic, leading to remarkable spin and valley transport properties when combined with the intrinsic spin-orbit coupling (SOC) inherent in these materials. In this work, we show that a magnetic tunnel junction setup consisting of an asymmetric ferromagnetic/ferromagnetic/normal ${\mathrm{WSe}}_{2}$ junction gives rise to a fully spin- and valley-polarized current for both parallel and antiparallel magnetization configurations in the presence of an off-resonance light. Due to the strong SOC of ${\mathrm{WSe}}_{2}$ together with off-resonance light, both the spin and valley polarizations are tunable and switchable. We also find that the tunneling magnetoresistance (TMR) could be tuned to 1 by off-resonance light. The relatively much stronger SOC in the conduction and valence bands of ${\mathrm{WSe}}_{2}$ compared to other compounds of the dichalcogenides family in collaboration with an off-resonance light leads to significant negative TMR at weak enough exchange fields. In appropriate parameter regimes, the TMR oscillations from negative to positive values can be tuned by the off-resonance light. Our results suggest that magnetic tunnel junctions involving ${\mathrm{WSe}}_{2}$ have very promising potential for applications in magnetic memory and spin and valleytronics devices.

Comment on “Nonequilibrium steady state phases of the interacting Aubry-André-Harper model”

There has been interest in the spin transport properties of the Aubry-Andre-Harper model at high temperatures under weak integrability breaking, in particular for small interactions or small fields. We present old unpublished and new results that show that the model is diffusive, meaning that the claimed superdiffusion [1,2] is a finite-size effect.

Observation of Room‐Temperature Low‐Field Magnetoresistance in Reduced Graphene Oxide/CoFe2O4 Nanocomposites

The considerable spin diffusion length and high electron mobility of graphene at room temperature makes it a suitable candidate to improve the spin transport properties of magnetic semiconductors when incorporated in a suitable stoichiometry. Herein, an observation of room‐temperature magnetoresistance (MR) properties of reduced graphene oxide (rGO)/cobalt ferrite (CFO) nanocomposites, which are synthesized by chemical methods followed by mixing in different mass ratios, is reported. The magnetic field‐dependent electrical resistivity shows a decrease in resistivity with the increase in magnetic field from 0 to 0.5 T, representing a negative MR in our samples. The highest MR of around −1.4% is observed in the 1:1 ratio CFO and rGO nanocomposite, whereas the samples with 2:1 and 3:1 ratio show the MR of ≈−1%. In the CFO–rGO nanocomposite, the charge transport takes place mainly through the rGO layers due to the insulating nature of the CFO nanoparticles. However, the ferromagnetic moments of the CFO nanoparticles induce electron‐spin scattering during the charge transport. The reduction in electron‐spin scattering results due to the parallel alignment of the magnetic moments. The alignment of ferromagnetic moments under the external magnetic field has been further confirmed from the magnetocapacitance effect of CFO nanoparticles.

Spin-filtering effects and negative differential resistance in N/B-doped zigzag silicon carbide nanoribbons with asymmetric edge hydrogenation

Spin transport properties of the N/B-doping zigzag-edged silicon carbide nanoribbons (ZSiCNRs) are studied by using density functional theory and the non-equilibrium Green's function method. The ZSiCNR has Si-H2 bonded at one edge while CH bonded at the other to constructed asymmetric edge hydrogenation. Our findings show, the N atom substitutes with C site while the B atom prefers to be localized at the Si site. Also, the B and N doped ZSiCNRs exhibit half-metallic, metallic, and bipolar semiconductor behavior at ferromagnetic or antiferromagnetic states, which the possibility of designing of spintronic nanodevices would be provided. The obtained I-V characteristic shows a perfect spin filtering effect (100%) in such N/B doping ZSiCNRs with an asymmetric edge. Moreover, the spin-polarized negative differential resistance (NDR) effect is also observed in the spin-polarized currents. Our suggested devices will be promised for multifunctional spintronic devices.

Spin transport properties for B-doped zigzag silicene nanoribbons with different edge hydrogenations

Exploring silicon-based spin modulating junction is one of the most promising areas of spintronics. Using nonequilibrium Green’s function combined with density functional theory, a set of spin filters of hydrogenated zigzag silicene nanoribbons is designed by substituting a silicon atom with a boron one and the spin-correlated transport properties are studied. The results show that the spin polarization can be realized by structural symmetry breaking induced by boron doping. Remarkably, by tuning the edge hydrogenation, it is found that the spin filter efficiency can be varied from 30% to 58%. Moreover, it is also found and explained that the asymmetric hydrogenation can give rise to an obvious negative differential resistance which usually appears at weakly coupled junction. These findings indicate that the boron-doped ZSiNR is a promising material for spintronic applications.

Engineering the spin conversion in graphene monolayer epitaxial structures

Spin Hall and Rashba–Edelstein effects, which are spin-to-charge conversion phenomena due to spin–orbit coupling (SOC), are attracting increasing interest as pathways to manage rapidly and at low consumption cost the storage and processing of a large amount of data in spintronic devices as well as more efficient energy harvesting by spin-caloritronics devices. Materials with large SOC, such as heavy metals (HMs), are traditionally employed to get large spin-to-charge conversion. More recently, the use of graphene (gr) in proximity with large SOC layers has been proposed as an efficient and tunable spin transport channel. Here, we explore the role of a graphene monolayer between Co and a HM and its interfacial spin transport properties by means of thermo-spin measurements. The gr/HM (Pt and Ta) stacks have been prepared on epitaxial Ir(111)/Co(111) structures grown on sapphire crystals, in which the spin detector (i.e., top HM) and the spin injector (i.e., Co) are all grown in situ under controlled conditions and present clean and sharp interfaces. We find that a gr monolayer retains the spin current injected into the HM from the bottom Co layer. This has been observed by detecting a net reduction in the sum of the spin Seebeck and interfacial contributions due to the presence of gr and independent from the spin Hall angle sign of the HM used.

Geometric size dependence of spin-mixing conductance at Pt/YIG interface

The spin-mixing conductance (SMC) is an essential parameter for ferromagnetic-insulators-based spintronics devices. Here, we study the influence of device boundary on local spin transport properties in platinum (Pt)/yttrium iron garnet (YIG) films by reducing the lateral size of the devices. An obvious fluctuation of spin Hall magnetoresistance, Gilbert damping coefficient, and effective spin Hall angle with restricting the size of YIG films has been found at room temperature. By employing both harmonic and current-induced spin-torque ferromagnetic resonance measurements, we have found a negligible fluctuation of both the imaginary part of SMC and effective magnetization with varying the geometric size of devices. In contrast, the real part of SMC at the Pt/YIG interface has been rigidly altered. Our results show that the SMC dramatically depends on the boundary effect from Ar+-ion milling.

Spin injection and transport in single-crystalline organic spin valves based on TIPS-pentacene

Single crystals of organic semiconductors with perfect crystal structure and minimal density of defects can exhibit high mobility and low spin scattering compared with their amorphous or polycrystalline counterparts. Therefore, these materials are promising candidates as the spin transport media to obtain long spin relaxation times and spin diffusion lengths in spintronic devices. However, the investigation of spin injection and transport properties in organic single crystals is hindered by the inability to construct devices such as single-crystalline organic spin valves (OSVs). Herein, thin and large organic single crystals of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) were grown on a liquid substrate and transferred to a target substrate carrying ferromagnetic electrodes to construct single-crystalline OSVs. The magnetoresistance (MR) responses of the single crystals were investigated to study their spin injection and transport properties. MR value as high as 17% was probed with an intermediate layer thickness of 269 nm. More importantly, spin transport was still observed in a single crystal of a thickness up to 457 nm, which was much larger than that of polycrystalline thin film. Our research provides a general methodology for constructing single-crystalline OSVs and paves the way to probe the intrinsic spin transport properties of organic semiconductors based on single crystals.

Modulation of hybrid interface states and magnetoresistance in quantum interference systems via functional groups

Exploring the law of hybrid interface states (HIS) to the spin-dependent transport properties in magnetic single-molecule junctions is extremely significant for developing organic spintronic devices. Utilizing the first-principles method, the quantum interference systems consisting of a class of anthraquinone-based molecules sandwiched between two ferromagnetic electrodes are constructed and related spin transport properties are theoretically investigated. By substituting neutral, electron-withdrawing, and electron-donating functional groups on the cross-conjugated sites, valid modulation of the HIS and their spin polarization (SP) are demonstrated, which are caused by the distinct shift of molecular orbitals and the induced overlap with HIS. Accordingly, the spin-dependent transport calculation shows that an enhanced tunneling magnetoresistance (TMR) at low bias voltage and a reversed TMR at high bias voltage are realized by using the electron-withdrawing groups. We provide direct evidence of the distinctive TMR effect as the change of molecular SP around the Fermi energy combined with the shift of destructive quantum interference (DQI) feature. This work supplies a novel way to modulate the spin-dependent transport in molecular junctions by manipulating the HIS and the DQI feature simultaneously.

Perfect in-plane CrI3 spin-valve driven by photogalvanic effect

Out-of-plane spin tunneling through the two-dimensional (2D) van der Waals ${\mathrm{CrI}}_{3}$ multilayer has recently been deeply explored, and giant magnetoresistance has been achieved in various ${\mathrm{CrI}}_{3}$ magnetic tunneling junctions (MTJs) via the control of interlayer antiferromagnetic coupling by both magnetic and electric fields. In contrast, knowledge of the in-plane spin-transport properties of 2D ${\mathrm{CrI}}_{3}$ is currently very limited. Here, based on quantum transport simulations, we study the in-plane transport properties of the photocurrent in a ${\mathrm{CrI}}_{3}$ MTJ with a bilayer/monolayer/bilayer configuration. The photogalvanic effect (PGE) is induced under vertical illumination of elliptically polarized light, giving rise to a robust photocurrent in a broad visible range at zero bias. A perfect spin-valve effect can be achieved with a magnetoresistance of 100% for some photon energies with an appropriate light helicity. Moreover, the PGE photocurrent for the antiparallel configuration is enhanced, as compared to the parallel configuration due to the increased device asymmetry. Our results show that a PGE-driven ${\mathrm{CrI}}_{3}$ photodetector is a promising candidate for low-power 2D spintronic devices.

Detection of spin current through a quantum dot with Majorana bound states* *Project supported by Natural Science Fund for Colleges and Universities in Hebei Province, China (Grant No. ZD2017031) and the Doctoral Initial Funding of Hebei University of Science and Technology (Grant No. 1181291).

The spin transport properties are theoretically investigated when a quantum dot (QD) is side-coupled to Majorana bound states (MBSs) driven by a symmetric dipolar spin battery. It is found that MBSs have a great effect on spin transport properties. The peak-to-valley ratio of the spin current decreases as the coupling strength between the MBS and the QD increases. Moreover, a non-zero charge current with two resonance peaks appears in the system. In the extreme case where the dot–MBS coupling strength is strong enough, the spin current and the charge current are both constants in the non-resonance peak range. When considering the effect of the Zeeman energy, it is interesting that the resonance peak at the higher energy appears one shoulder. And the shoulder turns into a peak when the Zeeman energy is big enough. In addition, the coupling strength between the two MBSs weakens their effects on the currents of the system. These results are helpful for understanding the MBSs signature in the transport spectra.

Electrical detection of spin-microwave coupling in a cavity

Via exploring the spin dynamics and transport properties at interfaces of magnetic material/non-magnetic metal bilayers, spintronics advanced many techniques on generation, detection, and manipulation of spin currents, which laid down a foundation for designing spintronic devices. New ideas and methods for manipulating spin are required in the spintronics community. Merging with cavity quantum electrodynamics, magnetic resonance in ferromagnets inside of a microwave cavity, realizes a strong magnon-microwave coupling at room temperature, opening an avenue to manipulate spin and spin current for the spintronics community. Spin-microwave coupling strength was significantly improved by replacing a few spins in a paramagnetic spin system with magnetization in ferromagnetic materials. Thus, a strongly coupled magnon-microwave system offers a hybrid quantum platform with many measures to tune the coupled system, such as an exchange magnetic field, an anisotropy field, controlled coupling strength, controlled damping parameters, and so on. Many devices were proposed and demonstrated with potential applications. In this review, we briefly introduce the concept and mechanism of magnon-microwave coupling, a classical electrodynamical coupling model for the coupling, spin currents produced by the coupled magnon, and manipulation of the spin current via the strong coupling. In ferromagnets, magnons can couple to a microwave mode as a magnon-polariton propagating in the materials. The strongly coupled magnon-microwave system is a magnon-polariton in a cavity where the microwave was confined as a resonance mode with a high quality factor. Using the Landau-Lifshitz-Gilbert equation and Maxwell equations, we introduce the classical electrodynamical coupling model and reveal that the magnon is driven by a torque produced by the microwave magnetic field on the magnetization and the microwave is feedback via Faraday’s law. Therefore, the coupling strength of the magnon-microwave system can be controlled by changing the torque. Magnon-microwave coupling is experimentally studied using many techniques, such as microwave transmission, Brillouin light scattering, Faraday rotation measurement, and spin pumping electrical detection. Spin pumping is well explored in the spintronics community as a spin current produced by magnon and detected as a voltage via inverse spin Hall effect in a novel metal. Combining with microwave transmission of the coupled microwave subsystem, spin pumping electrical detection by directly detecting the spin current produced by the magnon subsystem in the coupled system, illustrates a more complete profile of the coupled magnon-microwave system. In addition, the strongly coupled magnon-microwave system offers a new technique to manipulate spin currents. We experimentally demonstrate that spin currents produced by the strongly coupled system are correlated with each other mediated by the microwave cavity, and can be manipulated remotely. Two almost identical magnet samples are used to couple to a cavity and the coupling strengths between each magnet and the cavity are tunable individually. By using electrical detection to locally detect each magnet sample respectively, we find that the spin currents produced by two magnet samples are coherently correlated. By tuning the coupling strength of one magnet sample to the cavity, the spin current produced by the second magnet sample is manipulated distantly over a few centimeters, which is far longer than the distance of spin-orbit interaction or exchange interaction. This distant control is only limited by the coherence length of the microwave and the dimension of the cavity. This flexibility opens the door to improve spin current generation and manipulation for cavity spintronic devices.

Linear and nonlinear thermal spin transport properties of zigzag α-graphyne nanoribbons with sp2–sp3 edges

In this paper, we investigated the electronic structures and thermal spin transport properties of zigzag α-graphyne nanoribbons (N-ZαGYNRs) with sp2–sp3 edges. As N increased, the results showed that ZαGYNRs undergo a transformation from indirect-band-gap bipolar magnetic semiconductors into half-metallic materials. We then designed two spin caloritronics devices based on two representative ZαGYNRs with N = 4 and N = 12. Both linear and nonlinear thermal spin transport properties were found in the 4-ZαGYNRs device, but only linear thermal spin transport properties could be found in the 12-ZαGYNRs device. These discoveries indicate that ZαGYNRs are potential candidate materials for spin caloritronics device applications.

Surface oxidation in a van der Waals ferromagnet Fe3-xGeTe2

We investigate the surface oxidation in a van der Waals ferromagnet Fe3-xGeTe2, a material widely utilized for spintronic applications. While known for its relative air-insensitivity, exposure to air during the handling process (e.g. device or heterostructure fabrication) can lead to reduction or disappearance of its magnetic signal. Comparison of x-ray absorption and x-ray magnetic circular dichroism spectra between pristine and air-exposed Fe3-xGeTe2 confirm a naturally oxidized surface layer on the material. The surface oxide layer has predominantly Fe3+ content. X-ray absorption spectroscopy done on micron-sized exfoliated Fe3-xGeTe2 flakes reveal that the change in the surface chemical properties can be quite significant for thin flakes. The surface modulation of Fe3-xGeTe2 can lead to inaccuracies in characterizing its interfacial magnetic and spin transport properties, and complicate device and heterostructure fabrication processes.