Strong Spin Orbit Research Articles

Enhancement of Spin–Orbit Torque by Strain Engineering in SrRuO3 Films

AbstractComplex oxides with 4d/5d transition metal ions, e.g., SrRuO3, usually possess strong spin–orbit coupling, which potentially leads to efficient charge‐spin interconversion. As the electrical transport property of SrRuO3 can be readily tuned via structure control, it serves as a platform for studying the manipulation of charge‐spin interconversion. Here, a factor of twenty enhancement of spin–orbit torque (SOT) efficiency via strain engineering in a SrRuO3/Ni81Fe19 bilayer is reported. The results show that an orthorhombic SrRuO3 leads to a higher SOT efficiency than the tetragonal one. By changing the strain from compressive to tensile in the orthorhombic SrRuO3, the SOT efficiency can be increased from an average value of 0.04 to 0.89, corresponding to a change of spin Hall conductivity from 27 to 441 × ħ/e (S cm−1). The first‐principles calculations show that the intrinsic Berry curvature can give rise to a large spin Hall conductivity (SHC) via the strain control, which is consistent with the experimental observations. The results provide a route to further enhance the SOT efficiency in complex oxide‐based heterostructures, which will potentially promote the application of complex oxides in energy‐efficient spintronic devices.

Persistent Room‐Temperature Phosphorescence from Purely Organic Molecules and Multi‐Component Systems

AbstractRecently, luminophores showing efficient room‐temperature phosphorescence (RTP) have gained tremendous interest due to their numerous applications. However, most phosphors are derived from transition metal complexes because of their intrinsic fast intersystem crossing (ISC) induced by strong spin–orbit coupling (SOC) constants of the heavy metal. Metal‐free RTP materials are rare and have become a promising field because they are inexpensive and environmentally friendly. This review summarizes organic molecular materials with long triplet lifetimes at room temperature from the perspective of whether they stem from a molecular or multi‐component system. Among purely organic phosphors, heteroatoms are usually introduced into the backbone in order to boost the singlet–triplet ISC rate constant. In multi‐component systems, useful strategies such as host–guest, polymer matrix, copolymerization, and supramolecular assembly provide a rigid matrix to restrict nonradiative pathways thus realizing ultralong RTP.

Electrical control of Ruderman–Kittel–Kasuya–Yosida exchange interaction in zigzag edge MoS2 nanoflakes

In this study, we investigate the spin–spin correlation of two magnetic impurities in zigzag edge MoS2 nanoflakes using a three-orbital tight-binding model. In particular, we consider the dependence of the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction on the local electric field strength for providing a degree of tunability in triangular and rectangular nanoflakes. Our results indicate that manipulating the local electric field at the zigzag edges could allow the control of all the different components of the RKKY interaction in MoS2 nanoflakes. Interestingly, the Dzyaloshinskii–Moriya JDM oscillates but does not decay due to strong spin–orbit coupling. Moreover, the electric field induces a sizable JDM interaction as a function of the magnetic separation along zigzag edges for various Fermi energies. Our findings provide a method for enhancing the RKKY exchange interactions with magnetic arrays in these materials.

Dependence of the electronic structure of the EuS/InAs interface on the bonding configuration

Recently, the EuS/InAs interface has attracted attention for the possibility of inducing magnetic exchange correlations in a strong spin-orbit semiconductor, which could be useful for topological quantum devices. We use density functional theory (DFT) with a machine-learned Hubbard $U$ correction [npj Comput. Mater. 6, 180 (2020)] to elucidate the effect of the bonding configuration at the interface on the electronic structure. For all interface configurations considered here, we find that the EuS valence band maximum (VBM) lies below the InAs VBM. In addition, dispersed states emerge at the top of the InAs VBM at the interface, which do not exist in either material separately. These states are contributed mainly by the InAs layer adjacent to the interface. They are localized at the interface and may be attributed to charge transfer from the EuS to the InAs. The interface configuration affects the position of the EuS VBM with respect to the InAs VBM, as well as the dispersion of the interface state. For all interface configurations studied here, the induced magnetic moment in the InAs is small. This suggests that this interface, in its coherent form studied here, is not promising for inducing equilibrium magnetic properties in InAs.

Finite transverse conductance and anisotropic magnetoconductance under an applied in-plane magnetic field in two-dimensional electron gases with strong spin–orbit coupling

The current in response to a bias in certain two-dimensional electron gas (2DEG), can have a nonzero transverse component under a finite magnetic field applied in the plane where electrons are confined. This phenomenon known as planar Hall effect (PHE) is accompanied by dependencies of both the longitudinal and the transverse components of the current on the angle ϕ between the bias direction and the magnetic field. This effect can be observed in a variety of systems, for example in topological insulators where spin-momentum locking of the topologically protected surface states is the root cause for the effect and in magnetic systems where anisotropic magnetic ordering induces it. In 2DEG with spin orbit coupling (SOC) such as oxide interfaces, this effect has been experimentally witnessed. Further, a fourfold oscillation in longitudinal resistance as a function of ϕ has also been observed. Motivated by these, we perform scattering theory calculations on a 2DEG with SOC in presence of an in-plane magnetic field connected to two-dimensional leads on either sides to obtain longitudinal and transverse conductances. We find that the longitudinal conductance is π-periodic and the transverse conductance is 2π-periodic in ϕ. The magnitude of oscillation in transverse conductance with ϕ is enhanced in certain patches in (α, b)-plane where α is the strength of SOC and b is Zeeman energy due to magnetic field. The oscillation in transverse conductance with ϕ can be highly multi-fold for large values of α and b. The highly multi-fold oscillations of transverse conductance are due to Fabry–Pérot type interference between the modes in the central region as backed by its length dependent features. Our study establishes that SOC in a material is sufficient to observe PHE without the need for anisotropic magnetic ordering or nontrivial topology of the bandstructure.

Selective Substrate-Orbital-Filtering Effect to Realize the Large-Gap Quantum Spin Hall Effect.

Although Pb harbors a strong spin-orbit coupling effect, pristine plumbene (the last group-IV cousin of graphene) hosts topologically trivial states. Based on first-principles calculations, we demonstrate that epitaxial growth of plumbene on the BaTe(111) surface converts the trivial Pb lattice into a quantum spin Hall (QSH) phase with a large gap of ∼0.3 eV via a selective substrate-orbital-filtering effect. Tight-binding model analyses show the pz orbital in half of the Pb overlayer is selectively removed by the BaTe substrate, leaving behind a pz-px,y band inversion. Based on the same working principle, the gap can be further increased to ∼0.5-0.6 eV by surface adsorption of H or halogen atoms that filters out the other half of the Pb pz orbitals. The mechanism of selective substrate-orbital-filtering is general, opening an avenue to explore large-gap QSH insulators in heavy-metal-based materials. It is worth noting that plumbene has already been widely grown on various substrates experimentally.

Highly dose dependent damping-like spin–orbit torque efficiency in O-implanted Pt

Damping-like torque (DLT) arising from the spin Hall effect (SHE) in heavy metals and their alloys has been widely explored for applications in spin–orbit torque MRAM, auto-oscillations, spin waves, and domain wall motion. In conventional materials, the DLT efficiency is limited by intrinsic properties, while attaining strong spin–orbit coupling and higher spin-charge interconversion, with no compromise to electric properties, is the need of the hour. In this Letter, we report more than 3.5 times increase in DLT efficiency, θDL, of modified Pt-oxide by employing a better approach of low energy 20 keV O+ ion implantation. The highest fluence of O+ implantation (1 × 1017 ions cm−2) in Pt enhanced the DLT efficiency from 0.064 to 0.230 and improved the spin transmission for a smaller trade-off in the longitudinal resistivity (ρPt to ρPt−Oxide) from 55.4 to 159.5 μΩ cm, respectively. The transverse spin Hall resistivity, ρSH, is found to be proportional to the square of the longitudinal resistivity, i.e., ρSHimp∝ρimp2, implying that the enhanced SHE in O-implanted Pt is due to a side-jumping mechanism. Further, no break in the twofold as well as mirror symmetry of torques from the O-implanted Pt allows the use of spin-torque ferromagnetic resonance-based line shape analysis to quantify such torques.

Interface-driven electrical magnetochiral anisotropy in Pt/PtMnGa bilayers

Nonreciprocal charge transport, which is frequently termed as electrical magnetochiral anisotropy (EMCA) in chiral conductors, touches the most important elements of modern condensed matter physics. Here, we have investigated the large EMCA in Pt/PtMnGa (PMG) bilayers, which can be attributed to the nonreciprocal response of an interface-driven chiral transport channel. Different from the traditional linear current-dependent EMCA, for Pt/PMG bilayers, higher-order EMCA coefficients should be phenomenally added especially for the small current region. This unusual behavior has been explained based on both quantum transport and semiclassical transport models. Furthermore, a combination of asymmetrical electron scattering and spin-dependent scattering furnish the PMG thickness-dependent chiral transport behaviors in Pt/PMG bilayers. The dramatically enhanced anomalous Hall angle of PMG further demonstrates the modified surface state properties by strong spin–orbit coupling.

Observation of the Out‐of‐Plane Polarized Spin Current from CVD Grown WTe2

AbstractWeyl semimetal Td‐phase WTe2 possesses the spin‐resolved band structure with strong spin–orbit coupling, holding promises as a useful spin source material. The noncentrosymmetric crystalline structure of Td‐WTe2 endows the generation of the out‐of‐plane polarized spin, which is of great interest in magnetic memory applications. Previously, WTe2 was explored in spin devices based on mechanically exfoliated single crystal flakes with a size of micrometers. For practical spintronics applications, it is highly desirable to implement wafer‐scale thin films. In this work, centimeter‐scale chemical vapor deposition (CVD) grown Td‐WTe2 thin films are used and the spin current generation is studied by the spin torque ferromagnetic resonance (ST‐FMR) technique. The in‐plane and out‐of‐plane spin conductivities of 7.36 × 103 () (Ωm)–1 and 1.76 × 103 () (Ωm)–1, respectively, are found in CVD‐growth 5 nm‐WTe2. The current‐induced magnetization switching in WTe2/NiFe is demonstrated at room temperature in the domain wall motion regime, which may invigorate potential spintronic device innovations based on Weyl semimetals.

First Observation of Superlattice Reflections in the Hidden Order at 105 K of Spin–Orbit Coupled Iridium Oxide Ca5Ir3O12

We report the inelastic X-ray scattering (IXS) experimental results of iridium oxide Ca5Ir3O12 with a strong spin–orbit interaction, showing the hidden order at 105 K where no superlattice reflections were observed so far. We measured the IXS spectra of Ca5Ir3O12 along Γ–A, Γ–M, Γ–K–M, M–L, and K–H directions in the Brillouin zone of a hexagonal lattice down to 20 K. The obtained phonon spectra show almost no change on cooling; there are no soft phonon modes. However, the superlattice reflections specified by wavevector \(\textbf{q} = (1/3,1/3,1/3)\) are observed below 105 K. For the order parameter in the hidden order, the characteristic on intensity for observed superlattice reflections can lead to the irreducible representation A2 order parameter in the point group 31m. Furthermore, the theoretical study indicates that the hidden order at 105 K comes from an electric toroidal dipole or higher-order multipole ordering.

Topological Insulating Phase in Single-Layer Pentagonal Covalent Organic Frameworks: A Reticular Design Using Metal Phthalocyanine

Organic topological insulators have received tremendous attention lately due to their designability and highly chemical diversity. In this work, we propose an isoreticular series of single-layer π-conjugated covalent organic frameworks with the Cairo pentagonal tiling, termed mcm-ZnPc-1, mcm-ZnPc-2, mcm-ZnPc-3, and mcm-ZnPc-3N, where mcm is the crystal net’s symbol and ZnPc stands for zinc phthalocyanine. First-principles calculations using density functional theory show that mcm-ZnPc-1 and mcm-ZnPc-2 are trivial insulators with band gaps of 0.96 and 1.18 eV, respectively. Interestingly, expanding the π-conjugated system of mcm-ZnPc-2, followed by a chemical substitution, results in mcm-ZnPc-3 and mcm-ZnPc-3N with a distorted Dirac point and a quadratic band-crossing point in their band structures, respectively. The topological analyses indicate that mcm-ZnPc-3N is a Z2 topological insulator with a nontrivial gapless edge state. Our tight-binding model for the pentagonal lattice suggests that the topologically trivial-to-nontrivial transition can be attributed to the sufficient electronic coupling between two adjacent linkers and to the fact that the band-crossing point locates at the Fermi level. Remarkably, we show that replacing Zn in mcm-ZnPc-3N with Cd and Hg can substantially enlarge the nontrivial band gap from 0.3 meV up to 10 meV due to strong spin–orbit coupling strengths although a biaxial strain is necessary to tune the Fermi level of mcm-HgPc-3N in order to enter its topological insulating phase. Our proposed materials are predicted to be dynamically, thermally, and mechanically stable, thus paving the way for their experimental realization. This work not only introduces the pentagonal lattice as a building motif for framework structures but also demonstrates a strategy to engineer the exotic band structure of organic materials.

Novel group 14 element-bridged bithiophene dimers appended with terminal electron-withdrawing groups: Red-to-near infrared fluorescence and efficient photosensitized singlet oxygen generation

A series of group 14 element bridged bithiophene dimers bearing dicyanovinyl (DC) or thiobarbiturylmethylene (TB) electron-withdrawing groups at the molecular termini (series E-1a and series E-1b; E = C, Si, and Ge) were developed as red absorbing singlet oxygen (1O2) photosensitizing dyes. These dyes exhibited broad and strong absorption in the red-to-deep red regions and efficient red-to-NIR fluorescence (photoluminescence quantum yield ΦPL; 0.32–0.45 in dichloromethane). The terminal TB groups led to more red-shifted electronic absorption and fluorescence emission than the DC groups, owing to more intense intramolecular charge transfer. The 1O2 generation photosensitization abilities of series E-1a and series E-1b were evaluated by the 1,3-diphenylisobenzofuran method, and large 1O2 generation quantum yields (ΦΔ) were obtained upon irradiation of 630 nm light, ranging from 0.35 to 0.68 in dichloromethane. Therefore, these dyes show fluorescence properties as well as 1O2 generation photosensitization ability. Introduction of bromo groups at the 3,3′-positions in Si-1a and Si-1b afforded the new potential photosensitizers Si-2a and Si-2b that accompanied blue-shifted electronic absorption as well as blue-shifted fluorescence emission with the low ΦPL. Unfortunately, as Si-2a exhibited quite low absorptivity in the red region, 1O2 generation photosensitization with 630 nm light was not evaluated for Si-2a. On the other hand, Si-2b showed significantly efficient 1O2 generation photosensitization upon irradiation of 630 nm light (ΦΔ; 0.88). Considering the result of theoretical calculations showing that the bromo groups effectively contribute to the frontier orbitals, these bromo substituents should facilitate the intersystem crossing from the excited singlet state to the triplet excited state through the strong spin‒orbit coupling.

Localized Wannier function based tight-binding models for two-dimensional allotropes of bismuth

With its monoelemental composition, various crystalline forms and an inherently strong spin–orbit coupling, bismuth has been regarded as an ideal prototype material to expand our understanding of topological electronic structures. In particular, two-dimensional bismuth thin films have attracted a growing interest due to potential applications in topological transistors and spintronics. This calls for an effective physical model to give an accurate interpretation of the novel topological phenomena shown by two-dimensional bismuth. However, the conventional semi-empirical approach of adapting bulk bismuth hoppings fails to capture the topological features of two-dimensional bismuth allotropes because the electronic band topology is heavily influenced by crystalline symmetries. Here we provide a new parameterization using localized Wannier functions derived from the Bloch states in first-principles calculations. We construct new tight-binding models for three types of two-dimensional bismuth allotropes: a Bi (111) bilayer, bismuthene and a Bi (110) bilayer. We demonstrate that our tight-binding models can successfully reproduce the electronic and topological features of these two-dimensional allotropes. Moreover, these tight-binding models can be used to explain the physical origin of the occurrence of novel band topology and the perturbation effects in these bismuth allotropes. In addition, these models can serve as a starting point for investigating the electron/spin transport and electromagnetic response in low-dimensional topological devices.

Interface-enhanced helicity dependent photocurrent in metal/semimetal bilayers

One of the hallmarks of light-spin interaction in solids is the appearance of photocurrent that depends on the light helicity. Recent studies have shown that helicity dependent photocurrent (HDP) emerges due to light induced spin current and the inverse spin Hall effect of semimetal thin films. We have studied HDP in metal/semimetal bilayers. Compared to Bi single layer films, we find the HDP is enhanced in metal/Bi bilayers. For the bilayers, the sign of HDP under back illumination reverses from that of front illumination. The back illumination photocurrent is the largest for Ag/Bi bilayers among the bilayers studied. Using a diffusive spin transport model, we show that the HDP sign reversal under back illumination is caused by spin absorption and spin to charge conversion at the interface. Such interfacial effects contribute to the HDP enhancement under front illumination for the bilayers when the Bi layer thickness is small. These results show that the HDP can be used to assess interface states with strong spin orbit coupling.

Topological insulators for efficient spin–orbit torques

Current-induced magnetic switching via spin–orbit torques has been extensively pursued for memory and logic applications with promising energy efficiency. Topological insulators are a group of materials with spin-momentum locked electronic states at the surface due to spin–orbit coupling, which can be harnessed to reach strong spin–orbit torques. In this paper, we summarize and compare the methods for calibrating the charge-spin conversion efficiency in topological insulators, with which topological insulators are identified as outstanding spin–orbit torque generators compared with the well-studied heavy metals. We then review the results of magnetic switching under reduced current density in topological insulator/ferromagnet heterostructures. Finally, we provide insights on current challenges as well as possible exploration directions in the emerging field of topological spintronics.

Ordered ground states of kagome magnets with generic exchange anisotropy

There is a growing family of rare-earth kagome materials with dominant nearest-neighbor interactions and strong spin orbit coupling. The low symmetry of these materials makes theoretical description complicated, with six distinct nearest-neighbor coupling parameters allowed. In this Article, we ask what kinds of classical, ordered, ground states can be expected to occur in these materials, assuming generic (i.e. non-fine-tuned) sets of exchange parameters. We use symmetry analysis to show that there are only five distinct classical ground state phases occurring for generic parameters. The five phases are: (i) a coplanar, 2-fold degenerate, state with vanishing magnetization (${\sf A_1}$), (ii) a noncoplanar, 2-fold degenerate, state with magnetization perpendicular to the kagome plane (${\sf A_2}$), (iii) a coplanar, 6-fold degenerate, state with magnetization lying within the kagome plane (${\sf E}$-coplanar), (iv) a noncoplanar, 6-fold degenerate, state with magnetization lying within a mirror plane of the lattice (${\sf E}$-noncoplanar$_{6}$), (v) a noncoplanar, 12-fold degenerate, state with magnetization in an arbitrary direction (${\sf E}$-noncoplanar$_{12}$). All five are translation invariant (${\bf q}=0$) states. Having found the set of possible ground states, the ground state phase diagram is obtained by comparing numerically optimized energies for each possibility as a function of the coupling parameters. The state ${\sf E}$ noncoplanar$_{12}$ is extremely rare, occupying $<1\%$ of the full phase diagram, so for practical purposes there are four main ordered states likely to occur in anisotropic kagome magnets with dominant nearest neighbor interactions. These results can aid in interpreting recent experiments on ``tripod kagome'' systems R$_3$A$_2$Sb$_3$O$_{14}$, as well as materials closer to the isotropic limit such as Cr- and Fe- jarosites.

Interfacial spin-orbit torque and spin transparency in Co/Pt bilayer

We show that interfacial spin–orbit coupling is an efficient source of spin-orbit torques in a Co/Pt bilayer, a prototypical spin-orbitronic device. We find that the spin–orbit coupling at the Co/Pt interface can be effectively manipulated by inserting a thin TiN layer. We show that the strong spin–orbit coupling at the Co/Pt interface enables efficient generation of interfacial spin-orbit torques, as well as suppresses bulk spin-orbit torques, resulting in the dominance of the interfacial contribution in the generation of the spin-orbit torques. This result provides an important information for developing efficient spin-orbitronic devices based on the interfacial spin–orbit coupling.

Microstructure of bismuth centers in silicon before and after irradiation with 15 MeV protons

A decrease of two-gamma annihilation rate of a positron in a strong spin–orbit field of the annihilation site of bismuth impurity center 209Bi (J = 9/2) in silicon with natural isotope composition was revealed (J is the nuclear spin). This decrease was observed along with increasing occupancy of Bi donor states (binding energy E{Bi} ≈ 69 meV). Atoms of 29Si (J = 1/2) isotope are involved in spin interactions of positron with Bi impurity centers. The growth of occupancy of Bi donor states inhibits two-gamma annihilation rate. The estimated cross-section of positron trapping by the Bi impurity center is σ + ≈ (1.23–1.5) × 10−13 cm2. Together with this surprisingly large value, the integral rate of two-gamma annihilation in a hypothetical polyelectron system of the Bi impurity center is by a factor of just Δ ∼ 2.18 higher compared to the magnitude ≈2.09 × 109 s−1 known for elemental isolated polyelectron, (e−e+e−). Possible formation of the positron-containing exciton-like states, (e+)D 0 X (D = Bi, P) is also discussed. Irradiation of material with 15 MeV protons results in decreasing the factor Δ by ∼11% due to forming the radiation complex in which Bi atom is in an open volume ambient it. Such complex is suggested to have D 3d symmetry and be the deep donor. Low-temperature measurements of both the positron annihilation rate and Hall effect have been applied for studying the isochronal annealing of these point radiation defects which were found to be thermally stable up to ∼370 °C; they can be annealed at ∼430 °C – 470 °C. According to available data of ab initio cluster calculations, the complex of Bi atom with a simulated vacancy has D 3d symmetry with the energy gain ∼0.92 eV, thus indicating qualitative agreement between experimental and theoretical data.

Disorder upon disorder: Localization effects in the Kitaev spin liquid

In recent years, several magnetic Mott insulators with strong spin–orbit couplings were suggested to be proximate to the Kitaev quantum spin liquid (QSL) whose one of the most exciting features is the fractionalization of spin excitations into itinerant Majorana fermions and static Z2 fluxes. Motivated by the emergence of this plethora of 4d and 5d transition metal Kitaev materials and by the fact that some level of disorder is inevitable in real materials, here we study how the Kitaev QSL responds to various forms of disorder, such as vacancies, impurities, and bond randomness. First, we argue that the presence of the quenched disorder in the Kitaev QSL can lead to the Anderson localization of Majorana fermions and the appearance of Lifshitz tails. We point out that the Anderson localization of low-energy states is particularly strong in the extended Kitaev model with the time reversal symmetry breaking term. Second, we show that the disorder effects on the low-energy Majorana fermion modes can be detected in thermal transport. Third, we show that at finite temperatures the Z2 fluxes become thermally excited and give rise to an additional disorder for the Majorana fermions. This source of the disorder dominates at high temperatures. Fourth, we demonstrate that both the structure of the energy spectrum and the thermal transport properties of the disordered Kitaev QSL depend strongly on the character of disorder. While we find that both the site disorder and the bond randomness suppress the longitudinal thermal conductivity, the low-energy localization is stronger in the case of the site disorder.

Novel Spin-Orbit Torque Generation at Room Temperature in an All-Oxide Epitaxial La0.7 Sr0.3 MnO3 /SrIrO3 System.

Spin-orbit torques (SOTs) that arise from materials with large spin-orbit coupling offer a new pathway for energy-efficient and fast magnetic information storage. SOTs in conventional heavy metals and topological insulators are explored extensively, while 5d transition metal oxides, which also host ions with strong spin-orbit coupling, are a relatively new territory in the field of spintronics. An all-oxide, SrTiO3 (STO)//La0.7 Sr0.3 MnO3 (LSMO)/SrIrO3 (SIO) heterostructure with lattice-matched crystal structure is synthesized, exhibiting an epitaxial and atomically sharp interface between the ferromagnetic LSMO and the high spin-orbit-coupled metal SIO. Spin-torque ferromagnetic resonance (ST-FMR) is used to probe the effective magnetization and the SOT efficiency in LSMO/SIO heterostructures grown on STO substrates. Remarkably, epitaxial LSMO/SIO exhibits a large SOT efficiency, ξ|| =1, while retaining a reasonably low shunting factor and increasing the effective magnetization of LSMO by ≈50%. The findings highlight the significance of epitaxy as a powerful tool to achieve a high SOT efficiency, explore the rich physics at the epitaxial interface, and open up a new pathway for designing next-generation energy-efficient spintronic devices.