Giant Spin Research Articles

Multivalley Superconductivity in Monolayer Transition Metal Dichalcogenides.

In transition metal dichalcogenides (TMDs), Ising superconductivity with an antisymmetric spin texture on the Fermi surface has attracted wide interest due to the exotic pairing and topological properties. However, it is not clear whether the Q valley with a giant spin splitting is involved in the superconductivity of heavily doped semiconducting 2H-TMDs. Here by taking advantage of a high-quality monolayer WS2 on hexagonal boron nitride flakes, we report an ionic-gating induced superconducting dome with a record high critical temperature of ∼6 K, accompanied by an emergent nonlinear Hall effect. The nonlinearity indicates the development of an additional high-mobility channel, which (corroborated by first principle calculations) can be ascribed to the population of Q valleys. Thus, multivalley population at K and Q is suggested to be a prerequisite for developing superconductivity. The involvement of Q valleys also provides insights to the spin textured Fermi surface of Ising superconductivity in the large family of transition metal dichalcogenides.

Emergence of cubic ordered full-plane persistent spin textures in lead-free materials

The materials preserving a uniform spin configuration in the momentum space, known as persistent spin texture (PST), provide long carrier spin lifetimes through persistent spin-helix mechanism. However, most of the PSTs studied until now are attributed to the linear in $\mathbit{k}$ splitting and cease to exist locally around certain high-symmetry points of the first Brillouin zone (BZ). The PSTs with purely cubic spin splittings (PCS) have drawn attention owing to unique benefits in spin transport. Here, using relativistic first-principles calculations supplemented with $\mathbit{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbit{p}$ analysis, we report the emergence of PCS belonging to ${D}_{3h}$ point group, which is enforced by in-plane mirror and threefold rotation operations. In addition, the in-plane mirror symmetry operation sustains the PST in a larger region, i.e., full planes of BZ alongside giant spin splitting. The observed PSTs provide a route to nondephasing spin transport with larger spin Hall conductivity, thereby offering a promising platform for future spintronics devices.

Full-zone persistent spin textures with giant spin splitting in two-dimensional group IV–V compounds

Persistent spin texture (PST), a property of solid-state materials maintaining unidirectional spin polarization in the momentum k-space, offers a route to deliver the necessary long carrier spin lifetimes through the persistent spin helix (PSH) mechanism. However, most of the discovered PST locally occurred in the small part around certain high symmetry k-points or lines in the first Brillouin zone (FBZ), thus limiting the stability of the PSH state. Herein, by symmetry analysis and first-principles calculations, we report the emergence of full-zone PST (FZPST), a phenomenon displaying the PST in the whole FBZ, in the two-dimensional group IV–V (A = Si, Sn, Ge; B = Bi, Sb) compounds. Due to the existence of the in-plane mirror symmetry operation in the wave vector point group symmetry for the arbitrary in the whole FBZ, fully out-of-plane spin polarization is observed in the k-space, thus maintaining the FZPST. Importantly, we observed giant spin splitting in which the PST sustains, supporting large spin–orbit coupling parameters and small wavelengths of the PSH states. Our analysis demonstrated that the FZPST is robust for the non-degenerate bands, which can be effectively controlled by the application of an external electric field, thus offering a promising platform for future spintronic applications.

Wide-angle giant photonic spin Hall effect

The photonic spin Hall effect is a manifestation of the spin-orbit interaction of light and can be measured by a transverse shift $\ensuremath{\delta}$ of photons with opposite spins. The precise measurement of transverse shifts can enable many spin-related applications, such as precise metrology and optical sensing. However, this transverse shift is generally small (i.e., $\ensuremath{\delta}/\ensuremath{\lambda}<{10}^{\ensuremath{-}1}$, where $\ensuremath{\lambda}$ is the wavelength), which impedes its precise measurement. To date, proposals to generate a giant spin Hall effect (namely, with $\ensuremath{\delta}/\ensuremath{\lambda}>{10}^{2}$) have severe limitations, particularly its occurrence only over a narrow angular cone (with a width of $\mathrm{\ensuremath{\Delta}}\ensuremath{\theta}<{1}^{\ensuremath{\circ}}$). Here we propose a universal scheme to realize the wide-angle giant photonic spin Hall effect with $\mathrm{\ensuremath{\Delta}}\ensuremath{\theta}>{70}^{\ensuremath{\circ}}$ by exploiting the interface between free space and uniaxial epsilon-near-zero media. The underlying mechanism is ascribed to the almost-perfect polarization splitting between $s$ and $p$ polarized waves at the designed interface. Remarkably, this almost-perfect polarization splitting does not resort to the interference effect and is insensitive to the incident angle, which then gives rise to the wide-angle giant photonic spin Hall effect.

Polarization manipulation of giant photonic spin Hall effect using wave-guiding effect

The enhanced photonic spin Hall effect was previously possible only for the horizontal polarization (H-polarized) in plasmonic systems. The wave-guiding surface plasmonic resonance (SPR) effect is used to report a giant photonic spin Hall effect (G-PSHE) of reflected light for horizontal and vertical polarized waves. This novel work investigated the polarization-manipulated G-PSHE in the modified Kretschmann configuration with an additional glass dielectric thin wave-guiding layer. The ultrathin gold layer and an additional dielectric wave-guiding layer are responsible for achieving millimeter-scale (more than 2 mm to submillimeter) G-PSHE. With this novel approach, polarization manipulation is achieved by employing wave-guiding and the SPR effect. Using a finite element method based simulation study, the impact of an additional thin dielectric wave-guiding layer on G-PSHE is investigated. This study enables the potential application of both horizontal and vertical polarization-based quantum devices and sensors for which light spin plays a pivotal role.

Giant Orbital Anisotropy with Strong Spin-Orbit Coupling Established at the Pseudomorphic Interface of the Co/Pd Superlattice.

Orbital anisotropy at interfaces in magnetic heterostructures has been key to pioneering spin–orbit‐related phenomena. However, modulating the interface's electronic structure to make it abnormally asymmetric has been challenging because of lack of appropriate methods. Here, the authors report that low‐energy proton irradiation achieves a strong level of inversion asymmetry and unusual strain at interfaces in [Co/Pd] superlattices through nondestructive, selective removal of oxygen from Co3O4/Pd superlattices during irradiation. Structural investigations corroborate that progressive reduction of Co3O4 into Co establishes pseudomorphic growth with sharp interfaces and atypically large tensile stress. The normal component of orbital to spin magnetic moment at the interface is the largest among those observed in layered Co systems, which is associated with giant orbital anisotropy theoretically confirmed, and resulting very large interfacial magnetic anisotropy is observed. All results attribute not only to giant orbital anisotropy but to enhanced interfacial spin–orbit coupling owing to the pseudomorphic nature at the interface. They are strongly supported by the observation of reversal of polarity of temperature‐dependent Anomalous Hall signal, a signature of Berry phase. This work suggests that establishing both giant orbital anisotropy and strong spin–orbit coupling at the interface is key to exploring spintronic devices with new functionalities.

Large intrinsic spin Hall conductivity and spin Hall angle in the nodal-line semimetals ThAl2 and ThGa2

The intrinsic spin Hall effect and topological properties of ${\mathrm{ThAl}}_{2}$ and ${\mathrm{ThGa}}_{2}$ have been investigated based on first-principles electronic structure calculations. The band structures calculated without the spin-orbital coupling (SOC) show that both ${\mathrm{ThAl}}_{2}$ and ${\mathrm{ThGa}}_{2}$ host multiple nodal lines near the Fermi level. Once the SOC is taken into account, these nodal lines are gapped and huge spin Berry curvatures can be generated. As a result, giant spin Hall conductances of ${\mathrm{ThAl}}_{2}$ and ${\mathrm{ThGa}}_{2}$ are induced around the Fermi level. Moreover, large spin Hall angles can be achieved due to the giant spin Hall conductance and the small electric conductivity. Our work indicates that ${\mathrm{ThAl}}_{2}$ and ${\mathrm{ThGa}}_{2}$ not only provide an ideal platform for exploring the interplay between spin Hall effect and topological property, but also have promising applications in spin-orbitronic devices.

Spin Hall effect driven by the spin magnetic moment current in Dirac materials

The spin Hall effect of a Dirac Hamiltonian system is studied using semiclassical analyses and the Kubo formula. In this system, the spin Hall conductivity is dependent on the definition of spin current. All components of the spin Hall conductivity vanish when spin current is defined as the flow of spin angular momentum. In contrast, the off-diagonal components of the spin Hall conductivity are non-zero and scale with the carrier velocity (and the effective $g$-factor) when spin current consists of the flow of spin magnetic moment. We derive analytical formula of the conductivity, carrier mobility and the spin Hall conductivity to compare with experiments. In experiments, we use Bi as a model system that can be characterized by the Dirac Hamiltonian. Te and Sn are doped into Bi to vary the electron and hole concentration, respectively. We find the spin Hall conductivity ($\sigma_\mathrm{SH}$) takes a maximum near the Dirac point and decreases with increasing carrier density ($n$). The sign of $\sigma_\mathrm{SH}$ is the same regardless of the majority carrier type. The spin Hall mobility, proportional to $\sigma_\mathrm{SH}/n$, increases with increasing carrier mobility with a scaling coefficient of $\sim$1.4. These features can be accounted for quantitatively using the derived analytical formula. The results demonstrate that the giant spin magnetic moment, with an effective $g$-factor that approaches 100, is responsible for the spin Hall effect in Bi.

Spin Orbit Torque-Assisted Magnetic Tunnel Junction-Based Hardware Trojan

With the advancement of beyond-CMOS devices to keep Moore’s law alive, several emerging devices have found application in a wide range of applications. Spintronic devices offer low power, non-volatility, inherent spatial and temporal randomness, simplicity of integration with a silicon substrate, etc. This makes them a potential candidate for next-generation hardware options. This work explores the giant spin Hall effect (GSHE)-driven spin-orbit torque (SOT) magnetic tunnel junction (MTJ) as a potential candidate for creating an externally triggered hardware Trojan and insertion into logic-locked hardware security considering the effect of process and temperature variations.

Topological Surface-Dominated Spintronic THz Emission in Topologically Nontrivial Bi1- x Sbx Films.

Topological materials have significant potential for spintronic applications owing to their superior spin–charge interconversion. Here, the spin‐to‐charge conversion (SCC) characteristics of epitaxial Bi1− x Sb x films is investigated across the topological phase transition by spintronic terahertz (THz) spectroscopy. An unexpected, intense spintronic THz emission is observed in the topologically nontrivial semimetal Bi1− x Sb x films, significantly greater than that of Pt and Bi2Se3, which indicates the potential of Bi1− x Sb x for spintronic applications. More importantly, the topological surface state (TSS) is observed to significantly contribute to SCC, despite the coexistence of the bulk state, which is possible via a unique ultrafast SCC process, considering the decay process of the spin‐polarized hot electrons. This means that topological material‐based spintronic devices should be fabricated in a manner that fully utilizes the TSS, not the bulk state, to maximize their performance. The results not only provide a clue for identifying the source of the giant spin Hall angle of Bi1− x Sb x , but also expand the application potential of topological materials by indicating that the optically induced spin current provides a unique method for focused‐spin injection into the TSS.

Spin-polarized transport properties of the FeCl2/WSe2/FeCl2 van der Waals heterostructure

The discovery of the giant magnetoresistance effect has led to the rapid development of spintronics. Although the half-metals can provide a 100% spin polarization rate and significantly improved giant magnetoresistance, the materials with low Curie temperatures present challenges for their use at room temperature. In an attempt to identify the half-metallic material with high Curie temperatures for spintronics, this study investigates a van der Waals heterostructure with vertically integrated FeCl2/WSe2/FeCl2. The spin-polarized transport properties of the device based on the heterostructure studied by the density function theory combined with nonequilibrium Green's function reveal comprehensive spintronics functions, including giant magnetoresistance, spin filtering, and negative differential resistance effect. The mechanism of the negative differential resistance effect has further been elucidated by the band alignment of the heterostructure under different biases within the bias window.

Giant Spin Hall Effect and Spin-Orbit Torques in 5d Transition Metal-Aluminum Alloys from Extrinsic Scattering.

The generation of spin currents from charge currents via the spin Hall effect (SHE) is of fundamental and technological interest. Here, some of the largest SHEs yet observed via extrinsic scattering are found in a large class of binary compounds formed from a 5d element and aluminum, with a giant spin Hall angle (SHA) of ≈1 in the compound Os22 Al78 . A critical composition of the 5d element is found at which there is a structural phase boundary between poorly and highly textured crystalline material, where the SHA exhibits its largest value. Furthermore, a systematic increase is found in the spin Hall conductivity (SHC) and SHA at this critical composition as the atomic number of the 5d element is systematically increased. This clearly shows that the SHE and SHC are derived from extrinsic scattering mechanisms related to the potential mismatch between the 5d element and Al. These studies show the importance of extrinsic mechanisms derived from potential mismatch as a route to obtaining large spin Hall angles with high technological impact. Indeed, it is demonstrated that a state-of-the-art racetrack device has a several-fold increased current-induced domain wall efficiency using these materials as compared to prior-art materials.

Giant photoinduced inverse spin Hall effect of the surface states in three dimensional topological insulators Bi2Te3 with different thickness.

The photoinduced inverse spin Hall effect (PISHE) has been studied in three dimensional (3D) topological insulator (TI) Bi2Te3 thin films with different thicknesses (3, 5, 12 and 20 quintuple layer (QL)). The sign of the PISHE current flips only once in the 3- and 20-QL Bi2Te3 films, but it flips three times in the 5-, 7- and 12-QL samples. The three-times sign flip is due to the superposition of the PISHE current of the top and bottom surface states in Bi2Te3 films. By analyzing the x-ray photoelectron spectroscopy (XPS) of the Bi2Te3 films, we find that the top surface of the 3- and 20-QL Bi2Te3 films are severely oxidized, leading to only one sign flip in the PISHE. The PISHE contributed by the top and bottom surface states in Bi2Te3 films have been successfully separated by fitting a theoretical model to the PISHE current. The impact of the bulk states on PISHE current has been determined. The PISHE current is also measured at different light powers, and all the measurement results are in good agreement with the theoretical model. In addition, it is found that the PISHE current in Bi2Te3 films grown on Si substrate is more than two orders larger than that grown on SrTiO3 substrates, which can be attributed to the larger absorption coefficient for Bi2Te3/Si samples. It is revealed that the PISHE current in 3D TI Bi2Te3 is as large as 140 nA/W in the 3-QL Bi2Te3 film grown on Si substrate, which is more than one order larger than that reported in GaAs/AlGaAs heterojunction (about 2 nA/W) and GaN/AlGaN heterojunction (about 1.7 nA/W). The giant PISHE current demonstrates that the TIs with strong SOC may have good application prospects in spintronic devices with high spin-to-charge conversion efficiency.

Tuning of Cr-Cr Magnetic Exchange through Chalcogenide Linkers in Cr2 Molecular Dimers.

A set of three Cr-dimer compounds, Cr2Q2(en)4X2 (Q: S, Se; X: Br, Cl; en: ethylenediamine), with monoatomic chalcogenide bridges have been synthesized via a single-step solvothermal route. Chalcogenide linkers mediate magnetic exchange between Cr3+ centers, while bidentate ethylenediamine ligands complete the distorted octahedral coordination of Cr centers. Unlike the compounds previously reported, none of the chalcogenide atoms are connected to extra ligands. Magnetic susceptibility studies indicate antiferromagnetic coupling between Cr3+ centers, which are moderate in Cr2Se2(en)4X2 and stronger in Cr2S2(en)4Cl2. Fitting the magnetic data requires a biquadratic exchange term. High-frequency EPR spectra showing characteristic signals due to coupled S = 1 spin states could be interpreted in terms of the "giant spin" Hamiltonian. A fourth compound, Cr2Se8(en)4, has a single diatomic Se bridge connecting the two Cr3+ centers and shows weak ferromagnetic exchange interactions. This work demonstrates the tunability in strength and type of exchange interactions between metal centers by manipulating the interatomic distances and number of bridging chalcogenide linkers.

Nanosecond ultralow power spin orbit torque magnetization switching driven by BiSb topological insulator

Topological insulators (TIs) are promising for spin–orbit torque (SOT) switching thanks to their giant spin Hall angle. SOT switching using TIs has been studied so far in the thermal activation regime by direct currents or relatively long pulse currents (≥10 ns). In this work, we studied SOT magnetization switching of (Pt/Co) multilayers with strong perpendicular magnetic anisotropy by the BiSb topological insulator in both thermal activation and fast switching regime with pulse width down to 1 ns. We reveal that the zero-Kelvin threshold switching current density Jth0BiSb is 2.5 × 106 and 4.1 × 106 A/cm2 for the thermal activation regime and fast switching regime in a 800 nm-wide Hall bar device via domain wall depining. From time-resolved measurements using 1 ns pulses, we find that the domain wall velocity is 430–470 m/s at JBiSb = 1.6 × 107–1.7 × 107 A/cm2. Our work demonstrates the potential of the BiSb thin film for ultralow power and fast operation of SOT-based spintronic devices.

Giant bulk spin–orbit torque and efficient electrical switching in single ferrimagnetic FeTb layers with strong perpendicular magnetic anisotropy

Efficient manipulation of antiferromagnetically coupled materials that are integration-friendly and have strong perpendicular magnetic anisotropy (PMA) is of great interest for low-power, fast, dense magnetic storage and computing. Here, we report a distinct, giant bulk damping-like spin–orbit torque in strong-PMA ferrimagnetic Fe100−xTbx single layers that are integration-friendly (composition-uniform, amorphous, and sputter-deposited). For sufficiently thick layers, this bulk torque is constant in the efficiency per unit layer thickness, ξDLj/t, with a record-high value of 0.036 ± 0.008 nm−1, and the damping-like torque efficiency ξDLj achieves very large values for thick layers, up to 300% for 90 nm layers. This giant bulk torque by itself switches tens of nm thick Fe100−xTbx layers that have very strong PMA and high coercivity at current densities as low as a few MA/cm2. Surprisingly, for a given layer thickness, ξDLj shows strong composition dependence and becomes negative for composition where the total angular momentum is oriented parallel to the magnetization rather than antiparallel. Our findings of giant bulk spin torque efficiency and intriguing torque-compensation correlation will stimulate study of such unique spin–orbit phenomena in a variety of ferrimagnetic hosts. This work paves a promising avenue for developing ultralow-power, fast, dense ferrimagnetic storage and computing devices.

Giant Extrinsic Spin Hall Effect in Platinum-Titanium Oxide Nanocomposite Films.

Although the spin Hall effect provides a pathway for efficient and fast current‐induced manipulation of magnetization, application of spin–orbit torque magnetic random access memory with low power dissipation is still limited to spin Hall materials with low spin Hall angles or very high resistivities. This work reports a group of spin Hall materials, Pt1 −x (TiO2) x nanocomposites, that combines a giant spin Hall effect with a low resistivity. The spin Hall angle of Pt1 −x (TiO2) x in an yttrium iron garnet/Pt1 −x (TiO2) x double‐layer heterostructure is estimated from a combination of ferromagnetic resonance, spin pumping, and inverse spin Hall experiments. A giant spin Hall angle 1.607 ± 0.04 is obtained in a Pt0.94(TiO2)0.06 nanocomposite film, which is an increase by an order of magnitude compared with 0.051 ± 0.002 in pure Pt thin film under the same conditions. The great enhancement of spin Hall angle is attributed to strong side‐jump induced by TiO2 impurities. These findings provide a new nanocomposite spin Hall material combining a giant spin Hall angle, low resistivity and excellent process compatibility with semiconductors for developing highly efficiency current‐induced magnetization switching memory devices and logic devices.

Surface states-dependent giant quantized photonic spin Hall effect in a magnetic topological insulator thin film

We theoretically investigate the in-plane and transverse photonic spin Hall effect (PSHE) in a magnetic topological ultra-thin film (MTF) in the terahertz regime in the presence of an external magnetic field. We take into account the hybridization between the top and bottom surface states (SSs) and demonstrate that both the in-plane and transverse PSHE are quantized due to the Landau levels (LLs) quantization of the magneto-optical (MO) conductivities. The interplay between the effective energy induced by the hybridization interaction and the Zeeman energy plays an important role as it results in topological and normal insulating phases. By manipulating the interplay between these two interaction energies we drive the system through several topological quantum phase transitions (TQPTs). We show that the PSHE can be greatly enhanced and the magnitude of the giant PSHE in MTF is several hundreds times of the incident gaussian beam wavelength. Furthermore, we show that the PSHE is sensitive to the SSs of the Dirac fermions which is due to its topological character. This work sheds important light both on the PSHE and on the MO transport properties in MTFs in the presence of an external magnetic field which harbor great potentials in the development of new-generation photonics spin-Hall and optoelectronic devices.

Utilizing Valley–Spin Hall Effect in Monolayer WSe2 for Designing Low Power Nonvolatile Spintronic Devices and Flip-Flops

In this article, we propose low power nonvolatile flip-flops (NVFFs) utilizing valley–spin Hall effect (VSHE) in monolayer tungsten diselenide (WSe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ). The proposed designs are based on nonvolatile spintronic devices comprising of WSe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -based charge-to-spin converter coupled with magnetic tunnel junctions. The proposed devices have an integrated back-gate to control the flow of charge and spin currents. VSHE leads to the flow of opposite spins in divergent directions perpendicular to the WSe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layer with valley/spin Hall angle ~1. This enables encoding true and complementary bits in a single device utilizing magnets with perpendicular magnetic anisotropy (PMA). By introducing exchange coupling between PMA magnets mediated by Ta and FeCo-oxide layers, we electrically isolate but magnetically couple the PMA free layers, which enable the design of backup/restore module of NVFFs with minimal number of transistors. Using object-oriented micromagnetic framework (OOMMF) simulation, we establish the robustness of the exchange-coupled PMA systems and show that the misalignment and reduction in exchange coupling strength, induced by process variations, have a minimal impact on their functionality. Exploiting the unique features of the proposed devices, we design two flavors of VSHE-based NVFFs (VSHE-NVFFs) and carry out their energy–delay characterization, including an extensive variation analysis. The proposed VSHE-NVFFs achieve 74%–75% lower backup energy and 55%–59% lower restore energy than the existing NVFFs based on giant spin Hall effect (GSHE).

Multifunctional spintronic device based on zigzag SiC nanoribbon heterojunction via edge asymmetric dual-hydrogenation

The authors propose a method to obtain the multifunctional spintronic device by investigating the spin-resolved transport characteristics of zigzag SiC nanoribbon (zSiCNR) heterojunction via edge asymmetric dual-hydrogenation. The spin-resolved band structures show that the dual-hydrogenation on edge C or Si atoms all can change the initial metallicity of the pristine zSiCNR with the edge mono-hydrogenation to semiconductor in the presence of ferromagnetic field. The spin-resolved current-voltage characteristics of the zSiCNR heterojunction can show spin current rectification in the same rectify direction under the parallel magnetic field. The up-spin rectification ratio of our junction can be close to 10 13 at −0.5 V, which is much larger than rectification ratios of the previous junctions induced by the anti-parallel magnetic field. The zSiCNR heterojunction also exhibits a perfect spin filtering behavior with 100% spin filtering efficiency in both positive and negative bias regions under the anti-parallel magnetic field. More interestingly, the magnetoresistance is achieved by manipulating the external magnetic field in our zSiCNR heterojunction with the giant magnetoresistance ratio 5000% at 0.5 V. Therefore, the zSiCNR heterojunction via edge asymmetric dual-hydrogenation can be designed into the multifunctional spintronic devices, which has broad application prospects in the field of future spintronics. • Zigzag silicon carbon nanoribbon has potential in spin devices. • Giant spin current rectification is obtained in the presence of a ferromagnetic field. • Maximum spin current rectification ratios is closing to 10 13 . • Perfect spin filtering and magnetoresistance behaviors are also obtained.