Interfacial Spin-orbit Coupling Research Articles

Superconductivity assisted change of the perpendicular magnetic anisotropy in V/MgO/Fe junctions

Controlling the perpendicular magnetic anisotropy (PMA) in thin films has received considerable attention in recent years due to its technological importance. PMA based devices usually involve heavy-metal (oxide)/ferromagnetic-metal bilayers, where, thanks to interfacial spin-orbit coupling (SOC), the in-plane (IP) stability of the magnetisation is broken. Here we show that in V/MgO/Fe(001) epitaxial junctions with competing in-plane and out-of-plane (OOP) magnetic anisotropies, the SOC mediated interaction between a ferromagnet (FM) and a superconductor (SC) enhances the effective PMA below the superconducting transition. This produces a partial magnetisation reorientation without any applied field for all but the largest junctions, where the IP anisotropy is more robust; for the smallest junctions there is a reduction of the field required to induce a complete OOP transition (H_text {OOP}) due to the stronger competition between the IP and OOP anisotropies. Our results suggest that the degree of effective PMA could be controlled by the junction lateral size in the presence of superconductivity and an applied electric field. We also discuss how the H_text {OOP} field could be affected by the interaction between magnetic stray fields and superconducting vortices. Our experimental findings, supported by numerical modelling of the ferromagnet-superconductor interaction, open pathways to active control of magnetic anisotropy in the emerging dissipation-free superconducting spin electronics.

Dependence of the interfacial Dzyaloshinskii-Moriya interaction, perpendicular magnetic anisotropy, and damping in Co-based systems on the thickness of Pt and Ir layers

Ir and Pt thickness dependences of the interfacial Dzyaloshinskii-Moriya interaction (iDMI), the perpendicular magnetic anisotropy (PMA), and the damping in Co-based systems grown by sputtering on Si substrates, using Ir or Pt under or overlayer of variable thickness ($0\ensuremath{\le}{t}_{\mathrm{Pt}}\ensuremath{\le}3.5\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ and $0\ensuremath{\le}{t}_{\mathrm{Ir}}\ensuremath{\le}1.5\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$), were investigated using Brillouin light scattering. Vibrating sample magnetometry measurements revealed a significant decrease of the areal magnetic moment at saturation as the thickness ${t}_{\mathrm{Pt}}$ or ${t}_{\mathrm{Ir}}$ of the overlayer increases, most likely due to the increasing intermixing. The intermixing, found to be negligible for Ir and Pt underlayers, is stronger for Ir than Pt overlayers. Damping measurements for Pt-based systems allowed us to conclude with the strong spin-pumping contribution. The analysis of the ${t}_{\mathrm{Pt}}$ dependence of damping, within the spin-pumping model, allowed us to determine the spin-diffusion length of Pt, found to be greater when Pt is used as an underlayer. The ${t}_{\mathrm{Pt}}$ dependence of the effective PMA and iDMI constants, for which similar trends to that of damping were observed, were qualitatively analyzed to separate the contribution of each interface with Co. It was found that these constants are stronger when Co is grown on Pt, most probably due to weaker intermixing at the Pt/Co interface. Linear dependence of damping vs the PMA effective constant was evidenced, confirming their relationship with spin-orbit coupling (SOC). A linear variation of the PMA effective constant vs the iDMI constant was observed in systems using Pt underlayer, whereas quadratic or linear correlations are possible when Pt is used as an overlayer, probably due to change of the interface SOC by the strong intermixing at the Co/Pt interface. The qualitative analysis of the Ir thickness dependence of iDMI and PMA constants revealed a significant (weak) contribution of the Ir/Co interface to PMA (iDMI). The surface iDMI constant of the Co/Ir interface was estimated to be 0.4 \ifmmode\pm\else\textpm\fi{} 0.1 pJ/m, which confirms the similar iDMI sign of Co/Ir and Co/Pt interfaces.

Van der Waals heterostructure Pt2HgSe3/CrI3 for topological valleytronics

We identify a valley-polarized Chern insulator in a van der Waals heterostructure, monolayer ${\mathrm{Pt}}_{2}{\mathrm{HgSe}}_{3}$/monolayer ${\mathrm{CrI}}_{3}$, for potential applications with interplay between electric, magnetic, optical, and mechanical effects. The interlayer proximity magnetic coupling nearly closes the band gap of monolayer ${\mathrm{Pt}}_{2}{\mathrm{HgSe}}_{3}$, and the strong intralayer spin-orbit coupling further lifts the valley degeneracy by over 100 meV, leading to positive and negative band gaps at opposite valleys. In the valley with negative gap, the interfacial Rashba spin-orbit coupling opens a topological band gap of 17.8 meV, which is enlarged to 30.8 meV by adding a hexagonal boron nitride ($h$-BN) layer. We find large orbital magnetization in the ${\mathrm{Pt}}_{2}{\mathrm{HgSe}}_{3}$ layer that is much larger than spin, which can induce a measurable optical Kerr effect. The valley polarization and Chern number are coupled to the magnetic order of the nearest-neighbor ${\mathrm{CrI}}_{3}$ layer, which is switchable by electric, magnetic, and mechanical means in experiments. The presence of $h$-BN protects the topological phase, allowing the construction of superlattices with valley, spin, and layer degrees of freedom.

Spin Hall and inverse spin galvanic effects in graphene with strong interfacial spin-orbit coupling: A quasi-classical Green's function approach

van der Waals heterostructures assembled from atomically thin crystals are ideal model systems to study spin-orbital coupled transport because they exhibit a strong interplay between spin, lattice and valley degrees of freedom that can be manipulated by strain, electric bias and proximity effects. The recently predicted spin-helical regime in graphene on transition metal dichalcogenides, in which spin and pseudospin degrees of freedom are locked together [M. Offidani et al. Phys. Rev. Lett. 119, 196801 (2017)], suggests their potential application in spintronics. Here, by deriving an Eilenberger equation for the quasiclassical Green's function of two-dimensional Dirac fermions in the presence of} spin-orbit coupling\textcolor{black}{{} (SOC) and scalar disorder, we obtain analytical expressions for the dc spin galvanic susceptibility and spin Hall conductivity in the spin-helical regime. Our results disclose a sign change in the spin Hall angle (SHA) when the Fermi energy relative to the Dirac point matches the Bychkov-Rashba energy scale, irrespective of the magnitude of the spin-valley interaction imprinted on the graphene layer. The behavior of the SHA is connected to a reversal of the total internal angular momentum of Bloch electrons that reflects the spin-pseudospin entanglement induced by SOC. We also show that the charge-spin conversion reaches a maximum when the Fermi level lies at the edge of the spin-minority band in agreement with previous findings. Both features are fingerprints of spin-helical Dirac fermions and suggest a direct way to estimate the strength of proximity-induced SOC from transport data. The relevance of these findings for interpreting recent spin-charge conversion measurements in nonlocal spin-valve geometry is also discussed.

Non-Abelian effective mechanism from layer pseudospin and lattice pseudospin in twisted bilayer graphene

We identify a valley-polarized Chern insulator in the van der Waals heterostructure, Pt$_{2}$HgSe$_{3}$/CrI$_3$, for potential applications with interplay between electric, magnetic, optical, and mechanical effects. The interlayer proximity magnetic coupling nearly closes the band gap of Pt$_{2}$HgSe$_{3}$ and the strong intra-layer spin-orbit coupling further lifts the valley degeneracy by over 100 meV leading to positive and negative band gaps at opposite valleys. In the valley with negative gap, the interfacial Rashba spin-orbit coupling opens a topological band gap of 17.8 meV, which is enlarged to 30.8 meV by adding an $h$-BN layer. We find large orbital magnetization in Pt$_{2}$HgSe$_{3}$ layer that is much larger than spin, which can induce measurable optical Kerr effect. The valley polarization and Chern number are coupled to the magnetic order of the nearest neighboring CrI$_3$ layer, which is switchable by electric, magnetic, and mechanical means in experiments. The presence of $h$-BN protects the topological phase allowing the construction of superlattices with valley, spin, and layer degrees of freedoms.

Enhancement of spin-to-charge conversion efficiency in topological insulators by interface engineering

Topological insulator (TI) based heterostructure is a prospective candidate for ultrahigh spin-to-charge conversion efficiency due to its unique surface states. We investigate the spin-to-charge conversion in (Bi,Sb)2Te3 (BST)/CoFeB, BST/Ru/CoFeB, and BST/Ti/CoFeB by spin pumping measurement. We find that the inverse Edelstein effect length (λIEE) increases by 60% with a Ru insertion while remains constant with a Ti insertion. This can be potentially explained by the protection of BST surface states due to the high electronegativity of Ru. Such enhancement is independent of the insertion layer thickness once the thickness of Ru is larger than 0.5 nm, and this result suggests that λIEE is very sensitive to the TI interface. In addition, an effectively perpendicular magnetic anisotropy field and additional magnetic damping are observed in the BST/CoFeB sample, which comes from the interfacial spin–orbit coupling between the BST and the CoFeB. Our work provides a method to enhance λIEE and is useful for the understanding of charge-to-spin conversion in TI-based systems.

Large spin-to-charge conversion in ultrathin gold-silicon multilayers

Investigation of the spin Hall effect in gold has triggered increasing interest over the past decade, since gold combines the properties of a large bulk spin diffusion length and strong interfacial spin-orbit coupling. However, discrepancies between the values of the spin Hall angle of gold reported in the literature have brought into question the microscopic origin of the spin Hall effect in Au. Here, we investigate the thickness dependence of the spin-charge conversion efficiency in single Au films and ultrathin Au/Si multilayers by non-local transport and spin-torque ferromagnetic resonance measurements. We show that the spin-charge conversion efficiency is strongly enhanced in ultrathin Au/Si multilayers, reaching exceedingly large values of 0.99 +/- 0.34 when the thickness of the individual Au layers is scaled down to 2 nm. These findings reveal the coexistence of a strong interfacial spin-orbit coupling effect which becomes dominant in ultrathin Au, and bulk spin Hall effect with a relatively low bulk spin Hall angle of 0.012 +/- 0.005. Our experimental results suggest the key role of the Rashba-Edelstein effect in the spin-to-charge conversion in ultrathin Au.

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.

Effect of the spin-orbit interaction at insulator/ferromagnet interfaces on spin-orbit torques

We theoretically investigate spin-orbit torques in insulator/ferromagnet/normal-metal structures with a focus on interfacial spin-orbit coupling effect at an insulator/ferromagnet interface. Based on the spin drift-diffusion formalism generalized to consider transverse spin currents in a ferromagnet and the boundary condition to consider transverse spin currents leaving from a ferromagnet, we find that interfacial spin-orbit coupling at the insulator/ferromagnet interface contributes to dampinglike spin-orbit torque, which is important for current-driven magnetization dynamics, even when the interfacial spin-orbit coupling generates the only fieldlike component. We also calculate spin-orbit torques in a single ferromagnet sandwiched by two dissimilar insulators, which provides additional information about interfacial spin-orbit interaction at insulator/ferromagnet interfaces.

Interfacial symmetry breaking induced spin-orbit coupling in wurtzite GaN nanowires

The interfacial asymmetry-induced spin–orbit coupling (SOC) has a distinct effect on the spin relaxation and gate manipulation of spintronic devices. However, it is hard to separate the contributions from the interface and bulk by commonly used techniques, such as weak antilocalization and time-resolved Kerr rotation. Owing to the different distribution of the effective magnetic field in the momentum space, the SOC originated from the interfacial effect and the wurtzite structure inversion asymmetry (WSIA) could be distinguished by circular photo-galvanic effect (CPGE) measurements. In this work, the angle and external gate-voltage dependences of the interfacial SOC in GaN nanowires (NWs) were investigated using the CPGE. The interfacial SOC-induced CPGE shows a twofold degeneracy, while the removal of the sixfold degeneracy is attributed to the asymmetrical optical absorption. Furthermore, the interfacial SOC-induced CPGE shows a weak dependence on the external gate voltage, which is totally different from the WSIA-related SOC. This result is consistent with the clarification that the interfacial asymmetry originates from the Fermi level pinning at the GaN NW sidewall surfaces, thus clearly illustrating the special distribution and gate dependences of the interfacial SOC, which is critical for the design of spintronic devices based on GaN NWs.

Inversion of angular-dependent planar magnetoresistance in epitaxial Pt/γ′-Fe4N bilayers

The conversion between charge and spin through spin–orbit coupling (SOC) is critical in heavy nonmagnetic metal/ferromagnetic metal systems. Here, both the single γ′-Fe4N films and the epitaxial Pt/γ′-Fe4N bilayers were fabricated by facing-target sputtering. In the Pt(3 nm)/γ′-Fe4N(tFe4N ≤ 6 nm) bilayers, the anisotropy magnetoresistance (AMR) exhibits an “M” shape, which is opposite to that of the single γ′-Fe4N film with a “W” shape. Meanwhile, the planar Hall resistivity (PHR) reversal also appears. The inversion of AMR and PHR after capping a 3-nm-thick Pt layer on the γ′-Fe4N layer is mainly determined by the interfacial effect, in which the magnetic-proximity-effect induced the interface Pt local moments and the inverse-spin-Hall-effect caused the reflected spin-current to charge-current conversion. Our work helps to understand the interfacial SOC effects and has potential application in the field of magnetic sensors.

Absence of Significant Spin-Current Generation in Ti/Fe−Co−B Bilayers with Strong Interfacial Spin-Orbit Coupling

After one decade of the intensive theoretical and experimental explorations, whether interfacial spin-orbit coupling (ISOC) at metallic magnetic interfaces can effectively generate a spin current has remained in dispute. Here, utilizing the Ti/FeCoB bilayers that are unique for the negligible bulk spin Hall effect and the strong tunable ISOC, we establish that there is no significant charge-to-spin conversion at magnetic interfaces via spin-orbit filtering effect or Rashba-Edelstein(-like) effect even when the ISOC is stronger than that of a typical Pt/ferromagnet interface and can promote strong perpendicular magnetic anisotropy. We also find a minimal orbital Hall effect in 3d Ti.

Spin-memory loss induced by bulk spin–orbit coupling at ferromagnet/heavy-metal interfaces

A spin current through a ferromagnet/heavy-metal interface may shrink due to the spin-flip at the interface, resulting in the spin-memory loss. Here, we propose a mechanism of the spin-memory loss. In contrast to other mechanisms based on interfacial spin–orbit coupling, our mechanism is based on the bulk spin–orbit coupling in a heavy metal. We demonstrate that the bulk spin–orbit coupling induces the entanglement between the spin and orbital degrees of freedom and this spin-orbital entanglement can give rise to sizable spin-flip at the interface even when the interfacial spin–orbit coupling is weak. Our mechanism emphasizes crucial roles of the atomic orbital degree of freedom and induces the strong spin-memory loss near band crossing points between bands of different orbital characters.

Spin-orbit torques originating from the bulk and interface in Pt-based structures

We investigated spin-orbit torques in prototypical Pt-based spintronic devices. We found that, in Pt/Ni and Pt/Fe bilayers, the damping-like torque efficiency depends on the thickness of the Pt layer. We also found that the damping-like torque efficiency is almost identical in the Pt/Ni and Pt/Fe bilayers despite the stronger spin memory loss at the Pt/Fe interface. These results suggest that although the dominant source of the damping-like torque is the bulk spin Hall effect in the Pt layer, a sizable damping-like torque is generated by the interface in the Pt/Fe bilayer due to the stronger interfacial spin-orbit coupling. In contrast to the damping-like torque, whose magnitude and sign are almost identical in the Pt/Ni and Pt/Fe bilayers, the field-like torque strongly depends on the choice of the ferromagnetic layer. The sign of the field-like torque originating from the bulk spin Hall effect in the Pt layer is opposite between the Pt/Ni and Pt/Fe bilayers, which can be attributed to the opposite sign of the imaginary part of the spin-mixing conductance. These results demonstrate that the spin-orbit torques are quite sensitive to the electronic structure of the FM layer.

Modulation of thermal stability and spin–orbit torque in IrMn/CoFeB/MgO structures through atom thick W insertion

Antiferromagnet (AFM)/ferromagnet (FM) systems such as IrMn/CoFeB/MgO enable spin–orbit-torque- (SOT-) induced switching of perpendicular magnetization in the absence of an external magnetic field. However, the low thermal stability, weak perpendicular magnetic anisotropy (PMA), and indistinctive SOT of these AFM/FM heterostructures pose challenges to the practical application. Here, through the insertion of a thin W layer between the IrMn and CoFeB layers, we show that much larger effective PMA fields are obtained with annealing stability to 300 °C, which is guaranteed by the prevention of Mn diffusion via W insertion as shown in spherical aberration corrected transmission electron microscopy and atomic-resolution electron energy-loss spectroscopy measurement results. Furthermore, the spin–orbit torque is effectively tuned by changing the W layer thickness via modulation of the interfacial spin–orbit coupling at IrMn/W/CoFeB interfaces, which was reported to degrade the interface spin transparency for the spin currents. Finally, field-free magnetization switching was achieved with comparable exchange bias fields to samples without W insertion. This work demonstrates an effective strategy for improving the performance of the thermally robust AFM-based SOT device.

Electrical and thermal transport in antiferromagnet-superconductor junctions

We demonstrate that antiferromagnet-superconductor (AF-S) junctions show qualitatively different transport properties than normal metal-superconductor (N--S) and ferromagnet-superconductor (F-S) junctions. We attribute these transport features to presence of two new scattering processes in AF--S junctions, i.e., specular reflection of holes and retroreflection of electrons. Using the Blonder-Tinkham-Klapwijk formalism, we find that the electrical and thermal conductance depend nontrivially on antiferromagnetic exchange strength. Furthermore, we show that the interplay between the N\'eel vector direction and the interfacial Rashba spin-orbit coupling leads to a large anisotropic magnetoresistance. The unusual transport properties make AF--S interfaces unique among the traditional condensed-matter-system-based superconducting junctions.

Interfacial spin-orbit torques.

Spin-orbit torques offer a promising mechanism for electrically controlling magnetization dynamics in nanoscale heterostructures. While spin-orbit torques occur predominately at interfaces, the physical mechanisms underlying these torques can originate in both the bulk layers and at interfaces. Classifying spin-orbit torques based on the region that they originate in provides clues as to how to optimize the effect. While most bulk spin-orbit torque contributions are well studied, many of the interfacial contributions allowed by symmetry have yet to be fully explored theoretically and experimentally. To facilitate progress, we review interfacial spin-orbit torques from a semiclassical viewpoint and relate these contributions to recent experimental results. Within the same model, we show the relationship between different interface transport parameters. For charges and spins flowing perpendicular to the interface, interfacial spin-orbit coupling both modifies the mixing conductance of magnetoelectronic circuit theory and gives rise to spin memory loss. For in-plane electric fields, interfacial spin-orbit coupling gives rise to torques described by spin-orbit filtering, spin swapping and precession. In addition, these same interfacial processes generate spin currents that flow into the non-magnetic layer. For in-plane electric fields in trilayer structures, the spin currents generated at the interface between one ferromagnetic layer and the non-magnetic spacer layer can propagate through the non-magnetic layer to produce novel torques on the other ferromagnetic layer.

Strong In-Plane Magnetic Field-Induced Reemergent Superconductivity in the van der Waals Heterointerface of NbSe2 and CrCl3.

A magnetic field is generally considered to be incompatible with superconductivity as it tends to spin-polarize electrons and breaks apart the opposite-spin singlet superconducting Cooper pairs. Here, an experimental phenomenon is observed that an intriguing reemergent superconductivity evolves from a conventional superconductivity undergoing a hump-like intermediate phase with a finite electric resistance in the van der Waals heterointerface of layered NbSe2 and CrCl3 flakes. This phenomenon merely occurred when the applied magnetic field is parallel to the sample plane and perpendicular to the electric current direction as compared to the reference sample of a NbSe2 thin flake. The strong anisotropy of the reemergent superconducting phase is pointed to the nature of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state driven by the strong interfacial spin-orbit coupling between NbSe2 and CrCl3 layers. The theoretical picture of FFLO state nodes induced by Josephson vortices collectively pinning is presented for well understanding the experimental observation of the reemergent superconductivity. This finding sheds light on an opportunity to search for the exotic FFLO state in the van der Waals heterostructures with strong interfacial spin-orbit coupling.

Quantification of interfacial spin-charge conversion in hybrid devices with a metal/insulator interface

We present and experimentally verify a universal theoretical framework for the description of spin-charge interconversion in non-magnetic metal/insulator structures with interfacial spin–orbit coupling (ISOC). Our formulation is based on drift-diffusion equations supplemented with generalized boundary conditions. The latter encode the effects of ISOC and relate the electronic transport in such systems to spin loss and spin-charge interconversion at the interface. We demonstrate that the conversion efficiency depends solely on these interfacial parameters. We apply our formalism to two typical spintronic devices that exploit ISOC: a lateral spin valve and a multilayer Hall bar, for which we calculate the non-local resistance and the spin Hall magnetoresistance, respectively. Finally, we perform measurements on these two devices with a BiOx/Cu interface and verify that transport properties related to the ISOC are quantified by the same set of interfacial parameters.

Spin texture driven spintronic enhancement at the LaAlO3/SrTiO3 interface

Recent experiments have shown that transition metal oxide heterostructures such as SrTiO$_3$-based interfaces, exhibit large, gate tunable, spintronic responses. Our theoretical study showcases key factors controlling the magnitude of the conversion, measured by the inverse Edelstein and Spin Hall effects, and their evolution with respect to an electrostatic doping. The origin of the response can be linked to spin-orbital textures. These stem from the broken inversion symmetry at the interface which produces an unusual form of the interfacial spin-orbit coupling, provided a bulk atomic spin-orbit contribution is present. The amplitudes and variations of these observables are direct consequences of the multi-orbital subband structure of these materials, featuring avoided and topological crossings. Interband contributions to the coefficients lead to enhanced responses and non-monotonic evolution with doping. We highlight these effects using analytical approaches and low energy modeling.