Spin-charge Conversion Research Articles

Spin-Orbit Torques and Spin Hall Magnetoresistance Generated by Twin-Free and Amorphous Bi0.9 Sb0.1 Topological Insulator Films.

Topological insulators have attracted great interest as generators of spin-orbit torques (SOTs) in spintronic devices. Bi1-x Sbx is a prominent topological insulator that has a high charge-to-spin conversion efficiency. However, the origin and magnitude of the SOTs induced by current-injection in Bi1-x Sbx remain controversial. Here, the investigation of the SOTs and spin Hall magnetoresistance resulting from charge-to-spin conversion in twin-free epitaxial layers of Bi0.9 Sb0.1 (0001) coupled to FeCo are investigated, and compared with those of amorphous Bi0.9 Sb0.1 . A large charge-to-spin conversion efficiency of 1 in the first case and less than 0.1 in the second is found, confirming crystalline Bi0.9 Sb0.1 as a strong spin-injector material. The SOTs and spin Hall magnetoresistance are independent of the direction of the electric current, indicating that charge-to-spin conversion in single-crystal Bi0.9 Sb0.1 (0001) is isotropic despite the strong anisotropy of the topological surface states. Further, it is found that the damping-like SOT has a non-monotonic temperature dependence with a minimum at 20K. By correlating the SOT with resistivity and weak antilocalization measurements, charge-spin conversion is concluded to occur via thermally excited holes from the bulk states above 20K, and conduction through the isotropic surface states with increasing spin polarization due to decreasing electron-electron scattering below20K.

Room Temperature Spin-to-Charge Conversion in Amorphous Topological Insulating Gd-Alloyed BixSe1-x/CoFeB Bilayers.

Disordered topological insulator (TI) films have gained intense interest by benefiting from both the TI's exotic transport properties and the advantage of mass production by sputtering. Here, we report on the clear evidence of spin-charge conversion (SCC) in amorphous Gd-alloyed BixSe1-x (BSG)/CoFeB bilayers fabricated by sputtering, which could be related to the amorphous TI surface states. Two methods have been employed to study SCC in BSG (tBSG = 6-16 nm)/CoFeB(5 nm) bilayers with different BSG thicknesses. First, spin pumping is used to generate a spin current in CoFeB and detect SCC by the inverse Edelstein effect (IEE). The maximum SCC efficiency (SCE) is measured to be as large as 0.035 nm (IEE length λIEE) in a 6 nm thick BSG sample, which shows a strong decay when tBSG increases due to the increase of BSG surface roughness. The second method is THz time-domain spectroscopy, which reveals a small tBSG dependence of SCE, validating the occurrence of a pure interface state-related SCC. Furthermore, our angle-resolved photoemission spectroscopy data show dispersive two-dimensional surface states that cross the bulk gap until the Fermi level, strengthening the possibility of SCC due to the amorphous TI states. Our studies provide a new experimental direction toward the search for topological systems in amorphous solids.

Proximity-Induced Tunable Magnetic Order at the Interface of All-van der Waals-Layered Heterostructures.

Spin-orbit coupling (SOC) plays a crucial role in controlling the spin-charge conversion efficiency, spin torque, and complex magnetic spin structures. In this study, we investigate the interplay between SOC and ferromagnetism in heterostructures of large-SOC and magnetic materials. We highlight the importance of the SOC-proximity effect on magnetic ordering in all-van der Waals-layered heterostructures, specifically Fe3GeTe2(FGT)/monolayer W1-xVxSe2 (x = 0 and 0.05). By increasing the SOC strength, we demonstrate various magnetic orderings induced at the interface of the heterostructure, including spin-flop, spin-flip, and inverted magnetization. Moreover, we show a sharp magnetic switching from antiferromagnetic state to ferromagnetic state in FGT/W0.95V0.05Se2, which is characteristic of the synthetic antiferromagnetic structure. This proof-of-concept result offers the possibility of interface-tailoring spintronics, including two-dimensional magnetoresistive random access memory toggle switching. Our findings provide insight into the design and development of next-generation spintronic devices by exploiting the interplay between SOC and magnetic ordering in all-van der Waals-layered heterostructures.

Growth and characterization of the sputtered type-II topological semimetal PdTe2 thin films and PdTe2/Co60Fe20B20 heterostructures

The topological phases such as Dirac/Weyl semimetals are recently observed in Layered transition metal dichalcogenides (TMDs). The family of materials XTe2 (with X = Pd, Pt and Ni) possesses a type-II topological semimetal (TSM) phase. Here, we report the synthesis of the layered XTe2 thin films using the industry-friendly sputtering technique. The structural and morphological qualities of the sputtered-grown XTe2 thin films are analysed using X-ray diffraction and Raman measurements. The crystallites are highly oriented along the c-axis of the sapphire substrate, which suggests that growth is driven by van der Waals epitaxy. The magneto-transport measurements namely temperature-dependent resistivity and magnetoresistance measurement in out-of-plane field configuration are performed on PdTe2 thin films. The semi-metallic trend and superconducting transition are observed in the temperature-dependent resistivity curve. The weak antilocalization behavior observed in the magnetoconductance curves at a low magnetic field signifies the possibility of a non-zero Berry phase, possibly arising from the Dirac fermionic nature of electronic states. The spin dynamics measurements are performed on the PdTe2/Co60Fe20B20 bilayers. The effective spin mixing conductance and spin current density are found to be ∼2×1020m-2and 2.5MAm-2, respectively in PdTe2/Co60Fe20B20 bilayer which are quite larger than those observed in heavy metal-ferromagnets bilayers, and comparable to MBE-grown topological insulator-ferromagnets heterostructures. This work paves the way to synthesize Te-based layered TMDs using the industrially viable sputtering technique, and demonstrates the prospects for exploring the topological superconductive phase in PdTe2 films and large spin-charge conversion efficiency in PdTe2/Co60Fe20B20 heterostructures.

Spin-to-Charge Conversion in All-Oxide La2/3Sr1/3MnO3/SrIrO3 Heterostructures.

Spin injection and spin-charge conversion processes in all-oxide La2/3Sr1/3MnO3/SrIrO3 (LSMO/SIO) heterostructures with different SIO layer thickness and interfacial features have been studied. Ferromagnetic resonance (FMR) technique has been used to generate pure spin currents by spin pumping (SP) in ferromagnetic (FM) half-metallic LSMO. The change of the resonance linewidth in bare LSMO layers and LSMO/SIO heterostructures suggests a successful spin injection into the SIO layers. However, low values of the spin mixing conductance, compared to more traditional permalloy (Py)/Pt or yttrium iron garnet (YIG)/Pt systems, are found. A thorough analysis of the interfaces by high-resolution scanning transmission electron microscopy (HR-STEM) imaging suggests that they are structurally clean and atomic sharp, but a compositional analysis by energy-dispersive X-ray spectroscopy (EDS) reveals the interdiffusion of La, Ir, and Mn atomic species in the first atomic layers close to the interface. Inverse spin Hall effect (ISHE) measurements evidence that interfacial features play a very relevant role in controlling the effectiveness of the spin injection process and low transversal ISHE voltage signals are detected. In addition, it is found that larger voltage signals are detected for the lowest SIO layer thickness highlighting the role of the spin diffusion length (λsd)/SIO layer thickness ratio. The values of ISHE voltage are rather low but allow us to determine the spin Hall angle of SIO (θSH ≈ 1.12% at T = 250 K), which is remarkably similar to that obtained for the well-known Py/Pt system, therefore suggesting that SIO could be a promising spin-Hall material.

Room‐Temperature Charge‐to‐Spin Conversion from Quasi‐2D Electron Gas at SrTiO3‐Based Interfaces

Interfacial two-dimensional electron gas (2DEG), especially the SrTiO3-based ones at the unexpected interface of insulators, have emerged to be a promising candidate for efficient charge-spin current interconversion. In this article, to gain insight into the mechanism of the charge-spin current interconversion at the oxide-based 2DEG, we focused on conducting interfaces between insulating SrTiO3 and two types of aluminium-based amorphous insulators, namely SrTiO3/AlN and SrTiO3/Al2O3, and estimated their charge-spin conversion efficiency, {\theta}_cs. The two types of amorphous insulators were selected to explicitly probe the overlooked contribution of oxygen vacancy to the {\theta}_cs. We proposed a mechanism to explain results of spin-torque ferromagnetic resonance (ST-FMR) measurements and developed an analysis protocol to reliably estimate the {\theta}_cs of the oxide based 2DEG. The resultant {\theta}_cs/t, where t is the thickness of the 2DEG, were estimated to be 0.244 nm-1 and 0.101 nm-1 for the SrTiO3/AlN and SrTiO3/Al2O3, respectively, and they are strikingly comparable to their crystalline counterparts. Furthermore, we also observe a large direct current modulation of resonance linewidth in SrTiO3/AlN samples, confirming its high {\theta}_cs and attesting an oxygen-vacancy-enabled charge-spin conversion. Our findings emphasize the defects' contribution to the charge-spin interconversion, especially in the oxide-based low dimensional systems, and provide a way to create and enhance charge-spin interconversion via defect engineering.

Spin–Charge Conversion in Hybrid Structure PbZrO3/Y3Fe5O12/Pt

Multifunctional oxide heterostructures exhibiting magnetoelectric properties show great potential in advanced applications which are attracting a number of recent investigations. Herein, hybrid structures are presented including the antiferroelectricity film PbZrO3 (PZO) and the ferrimagnetic insulator (FMI) yttrium iron garnet (Y3Fe5O12, (YIG)) film by means of pulse laser deposition and magnetron sputtering, respectively. A visible double ferroelectricity hysteresis loop for PZO at low electrical field and the distinct X‐ray magnetic circular dichroism (XMCD) spectra for YIG are obtained. To study the spin current transfer processes between platinum (Pt) and the PZO/YIG system, the Pt film is deposited on the PZO/YIG bilayers as the conversion detector and the spin–charge conversion voltage can be obtained via inverse spin Hall effect (ISHE) under thermal gradient excitation. These results indicate that the hybrid structure PZO/YIG/Pt provides a potential route to realize an electric field control of spin–charge conversion and is instructive for future low‐power multiferroic heterostructures‐based spintronic devices.

Spin-Momentum Locking and Ultrafast Spin-Charge Conversion in Ultrathin Epitaxial Bi1 - x Sbx Topological Insulator.

The helicity of three-dimensional (3D) topological insulator surface states has drawn significant attention in spintronics owing to spin-momentum locking where the carriers' spin is oriented perpendicular to their momentum. This property can provide an efficient method to convert charge currents into spin currents, and vice-versa, through the Rashba-Edelstein effect. However, experimental signatures of these surface states to the spin-charge conversion are extremely difficult to disentangle from bulk state contributions. Here, spin- and angle-resolved photo-emission spectroscopy, and time-resolved THz emission spectroscopy are combined to categorically demonstrate that spin-charge conversion arises mainly from the surface state in Bi1 - x Sbx ultrathin films, down to few nanometers where confinement effects emerge. This large conversion efficiency is correlated, typically at the level of the bulk spin Hall effect from heavy metals, to the complex Fermi surface obtained from theoretical calculations of the inverse Rashba-Edelstein response. Both surface state robustness and sizeable conversion efficiency in epitaxial Bi1 - x Sbx thin films bring new perspectives for ultra-low power magnetic random-access memories and broadband THzgeneration.

Different spin relaxation properties observed in linearly and circularly polarized laser induced terahertz emission from a Bi/Co bilayer

Recently, helicity-dependent photocurrent was reported in Bi single thin fi lms. It is proposed that the origin of this photocurrent is the combination of photo-spin conversion and spin-charge conversion effects in Bi and efficient spin conversion in Bi is expected. In this study, we measured two types of terahertz (THz) emissions from Bi/Co bilayer films induced by spin current generation using laser-induced demagnetization of the Co layer and photo-spin conversion effect in the Bi layer to investigate the spin current induced by the two mechanisms simultaneously. We clearly observed diff erent Bi thickness dependence of peak intensity and that of bandwidth for THz spin current in two experiments, i.e., spin current induced by demagnetization of Co and that by photo-spin conversion in Bi. The different Bi thickness dependence of spin current intensity and bandwidth in two experiments is caused by different spin relaxation properties of optically excited spin currents in Bi layers.

Enhancement of spin-charge conversion efficiency for Co3Sn2S2 across transition from paramagnetic to ferromagnetic phase

Co$_{3}$Sn$_{2}$S$_{2}$ (CSS) is one of the shandite compounds and becomes a magnetic Weyl semimetal candidate below the ferromagnetic phase transition temperature ($\textit{T}_\textrm{C}$). In this paper, we investigate the temperature ($\textit{T}$) dependence of conversion between charge current and spin current for the CSS thin film by measuring the spin-torque ferromagnetic resonance (ST-FMR) for the trilayer consisting of CSS / Cu / CoFeB. Above $\textit{T}_\textrm{C}$ ~ 170 K, the CSS / Cu / CoFeB trilayer exhibits the clear ST-FMR signal coming from the spin Hall effect in the paramagnetic CSS and the anisotropic magnetoresistance (AMR) of CoFeB. Below $\textit{T}_\textrm{C}$, on the other hand, it is found that the ST-FMR signal involves the dc voltages ($\textit{V}_\textrm{dc}$) not only through the AMR but also through the giant magnetoresistance (GMR). Thus, the resistance changes coming from both AMR and GMR should be taken into account to correctly understand the characteristic field angular dependence of $\textit{V}_\textrm{dc}$. The spin Hall torque generated from the ferromagnetic CSS, which possesses the same symmetry as that for spin Hall effect, dominantly acts on the magnetization of CoFeB. A definite increase in the spin-charge conversion efficiency ($\xi$) is observed at $\textit{T}$ < $\textit{T}_\textrm{C}$, indicating that the phase transition to the ferromagnetic CSS promotes the highly efficient spin-charge conversion. In addition, our theoretical calculation shows the increase in spin Hall conductivity with the emergence of magnetic moment at $\textit{T}$ < $\textit{T}_\textrm{C}$, which is consistent with the experimental observation.

Emergence of spin–charge conversion functionalities due to spatial and time-reversal asymmetries and chiral symmetry

Spin–charge conversion (SCC) leads to the driving principle of spintronics devices, such as non-volatile magnetic memory and energy harvesting devices from light, sound, and heat to charge current. Recently, controllable SCCs have emerged in materials with spatial- and time-reversal asymmetry as a new route for efficient manipulation and realization of novel functionalities of future spintronics devices. This study overviews the SCC from the fundamental mechanism to the recent research progress in novel materials, such as topological magnets and atomically layered materials. Additionally, we discuss the chiral organic materials from the viewpoint of a new pathway for the emergence of spin functionalities.

Controllable unidirectional magnetoresistance in ferromagnetic films with broken symmetry

Spin-related unidirectional magnetoresistance (UMR) exhibits nonreciprocal transport to either charge current or magnetization reversal, providing a potential application for direct electrical readout of in-plane magnetization using simple two-terminal geometry. To achieve such a UMR, it is usually believed that an adjacent heavy metal or other materials with strong spin-orbit coupling (SOC) as the spin polarizer is indispensable, leading to most efforts on multilayer structures rather than single-layer ferromagnetic films. Here, we report the observation of UMR in a single CoPt film by introducing a vertical composition gradient to break the structural inversion symmetry. Moreover, unlike conventional heavy-metal/ferromagnetic-metal bilayer films, the UMR in the CoPt film with a positive Co composition gradient is controllable as a function of current amplitude, which shows the decrease, sign reversal, and then enhancement with the increment of current density. These results reveal two competing UMR mechanisms with opposite signs in electrical current dependency, i.e., bulk Rashba effect induced by composition gradients and anomalous Nernst effect driven by a vertical temperature gradient. Our work provides a promising way to manipulate the UMR in a single ferromagnetic film with the combination of charge-spin conversion and magnetothermal effect.

Enhancing spin splitting by symmetry and molecular orbital hybridization in VO2

In spintronic devices, one of the concerned focuses is spin splitting, which is existing in heavy metal with strong spin–orbit coupling (SOC) and some materials with broken symmetry. Recently, non-relativistic spin splitting originated from symmetric breaking has been proved in materials with low atomic numbers. Herein, based on inherent asymmetric octahedrons in vanadium dioxide (VO2) and the adjustable charge-spin conversion efficiency, we mainly survey non-relativistic spin splitting in VO2 from a symmetric perspective of bulk, interface, and surface, and hybridization of the molecular orbital. The results demonstrate that there is a tiny spin splitting (8.2 meV) in the rutile-like monoclinic (RM) VO2 bulk. Nevertheless, a huge splitting can be realized by modulating the hybridization of molecular orbital by fabricating the interface with different materials. Moreover, when we give priority to VO2(RM)/TiO2 composite layer (164 meV), the results suggest that the splitting can be also manipulated by different interfacial types and increased the number of layers of VO2(RM). This manner of manipulating symmetries of interface and surface and changing the hybridization of molecular orbital sheds light on understanding the electronic structure and acquiring an enhanced splitting of VO2, which provides the possibility of the application of VO2 in spintronic devices.

Crystallinity Control of the Topological-Insulator Surface Bi85Sb15(012) via Interfacial Engineering for Enhanced Spin-Orbit Torque

Topological insulators demonstrate high charge-spin conversion efficiency due to their spin-momentum locking at the Dirac surface states. However, the surface states are sensitive to disruption caused by exchange coupling when interfaced with a ferromagnet. Here, we demonstrate the use of various nonmagnetic insertion layer materials, $\mathrm{Ti},\phantom{\rule{0.2em}{0ex}}\mathrm{Cu},$ and $\mathrm{Pt}$, at the $\mathrm{Co}/\mathrm{Bi}$-$\mathrm{Sb}(012)$ interface to preserve the topological surface state and promote spin-orbit-torque efficiency through the crystallinity control of $\mathrm{Bi}$-$\mathrm{Sb}(012)$. For 20-nm-thick $\mathrm{Bi}$-$\mathrm{Sb}$, a spin Hall angle of up to 8.93 is observed with the use of a $\mathrm{Pt}$ insertion layer, while it is otherwise negligible for $\mathrm{Co}/\mathrm{Bi}$-$\mathrm{Sb}(012)$ interfaces. We further explore the enhancement of $\mathrm{Bi}$-$\mathrm{Sb}(012)$ crystallinity with increasing $\mathrm{Bi}$-$\mathrm{Sb}$ thickness, revealing a rapidly increasing spin-orbit-torque efficiency that gradually saturates above 30 nm. A clear correlation between spin-orbit-torque efficiency and $\mathrm{Bi}$-$\mathrm{Sb}(012)$ crystalline size is identified using x-ray diffractometry, establishing the origin of the high spin-orbit efficiency to be the $\mathrm{Bi}$-$\mathrm{Sb}(012)$ crystalline orientation. Our work demonstrates the spin-orbit-torque origin in $\mathrm{Bi}$-$\mathrm{Sb}$ experimentally and paves the way for the adaptation of topological insulators as a class of low-energy spin source material for spintronics applications.

Ferroelectric-polarization modulation of charge-to-spin conversion in Pb(Mg1/3Nb2/3)O3–Pb0.7Ti0.3O3/La0.67Sr0.33MnO3 system

Modulation of charge-to-spin conversion through active control is supposed to manipulate efficiency in a wide range for future spintronic devices. Despite its great significance, passive control still dominates the manipulation while the active control of charge-to-spin conversion is still challenging. Here we experimentally observe spin pumping-type charge-to-spin conversion besides Rashba-type one in La0.67Sr0.33MnO3 thin films deposited on ferroelectric Pb(Mg1/3Nb2/3)O3–Pb0.7Ti0.3O3 (PMN-PT) substrate. We exhibit that both two types of charge-to-spin conversion efficiency give rise to a variety of sign under positive and negative remnant polarized states of PMN-PT, which can be ascribed to the effect of ferroelectric polarization. The possibility of modulating charge-spin conversion opens a route for designing low-energy spintronics.

Nonrelativistic Spin-Momentum Coupling in Antiferromagnetic Twisted Bilayers.

Spin-momentum coupling, which depends strongly on the relativistic effect of heavy elements in solids, is the basis of many phenomena in spintronics. In this Letter, we theoretically predict nonrelativistic spin-momentum coupling in two-dimensional materials. By proposing magnetic symmetry requirements for spin splitting in two-dimensional systems, we find that a simple twisting operation can realize nonrelativistic spin splitting in antiferromagnetic bilayers. Through first-principles calculations, we demonstrate that momentum-dependent spin splitting exists extensively in antiferromagnetic twisted bilayers with different crystal structures and twist angles. The size of the spin splitting caused by twisting is of the same order of magnitude as that arising from spin-orbit coupling. In particular, a transverse spin current with an extremely high charge-spin conversion ratio can be generated in twisted structures under an external electric field. The findings demonstrate the potential for achieving electrically controlled magnetism in materials without spin-orbit coupling.

Multiband character revealed from weak antilocalization in platinum thin films

Platinum (Pt) has been very much used for spin-charge conversion in spintronics research due to its large intrinsic spin-orbit interaction. Magnetoconductance originating from weak antilocalization in the quantum interference regime is used as a powerful tool to obtain the microscopic information of spin-orbit interaction and the coherence phase breaking scattering process among itinerant electrons. To acquire the knowledge of different types of scattering processes, we have performed a magnetoconductance study on Pt thin films which manifests multiband (multichannel) conduction. An extensive analysis of quantum-interference-originated weak antilocalization reveals the existence of strong (weak) interband scattering between two similar (different) orbitals. Coherence phase breaking lengths $({l}_{\ensuremath{\phi}})$ and their temperature dependence are found to be significantly different for these two conducting bands. The observed effects are consistent with the theoretical prediction that there exist three Fermi sheets with one $s$ and two $d$ orbital character. This study provides the evidence of two independent nonsimilar conducting channels and the presence of anisotropic spin-orbit interaction along with $e\text{\ensuremath{-}}e$ correlation in Pt thin films.

Continuous modulation of charge-spin conversion by electric field in Pt/Co2FeSi/Pb(Mg1/3Nb2/3)O3-Pb0.7Ti0.3O3 heterostructures

The half-metallic Heusler alloy Co2FeSi is an ideal material in spintronic devices due to its higher spin polarization, higher Curie temperature and lower damping parameters. In this work, the dynamic magnetism of Co2FeSi is modulated by electric field and it is demonstrated that the charge-spin conversion efficiency ξ is continuous and controllable by the electric field. We further find an extremely high ξ in ferromagnetic/ferroelectric (FM/FE) heterostructures, which could be ascribed to interfacial effect in FM/FE interface. Moreover, we investigate that the charge-spin conversion efficiency varies with the electric field in a butterfly-like behavior, which accords with the S–E curve of Pb(Mg1/3Nb2/3)O3-Pb0.7Ti0.3O3(PMN-PT) and could be attributed to strain effect. The modulation of charge-spin conversion efficiency in FM/FE heterostructures via electric field presents a profound potential for next-generation spintronic devices and applications of current-induced magnetization switching.

Research progress of novel quantum states and charge-spin interconversion in transition metal oxides

For efficient storage and processing of massive data in the information technology era, spintronic device attracts tremendous attention due to its low power consumption and non-volatile feature. Spin source material, which can efficiently generates spin current, is an important constituent of novel spin-orbit torque device. The efficiency of spin current generation in spin source material directly determines the performances of various spintronic devices. In the past two decades, great progress has been made in exploring high-efficient spin source material systems and understanding the relevant physical mechanisms. A wide variety of materials are explored, ranging from traditional heavy metals and semiconductors to topological insulators and two-dimensional (2D) materials. Recently, the material family of transition metal oxides attracts tremendous attention due to its efficient and highly tunable charge-spin conversion intimately related to its emerging novel quantum states and electronic structure. The mechanism of charge-spin conversion generally has two contributions: the bulk spin Hall effect and the spin-momentum locked interface with inversion symmetry breaking. Novel electronic structures such as topological band structures and spin-momentum locked surface states can realize efficient charge-spin conversion. For example, the Weyl points in SrRuO&lt;sub&gt;3&lt;/sub&gt; and the topological Dirac nodal line in SrIrO&lt;sub&gt;3&lt;/sub&gt; are predicted to give rise to a large Berry curvature and corresponding spin Hall conductance; the topological surface states can generate spin accumulation due to spin-momentum locking; the Rashba states at the oxide interface such as the 2D electron gas in SrTiO&lt;sub&gt;3&lt;/sub&gt; and KTaO&lt;sub&gt;3&lt;/sub&gt; can generate spin current by Rashba-Edelstein effect. Furthermore, the entanglement of various degrees of freedom, including spin, charge, lattice and orbit in transition metal oxides lead to the electronic structure being highly tunable by various methods including gate voltage, substrate constraint, thickness, interface engineering, etc. Therefore, charge-spin conversion in transition metal oxides is of great significance for both modulating of novel electronic structure in fundamental research and exploring its promising potential in future spintronic devices. In this review, we focus on introducing aspects of exotic electronic structures, spin transport mechanism, charge-spin interconversion characterization, efficiency and manipulation in transition metal oxides, and giving a prospect on the future development trend.

Spin-orbit torque efficiency improved by BiSePt alloy

In order to achieve high-efficiency spin-orbit torque devices, higher charge-spin conversion efficiency, and lower resistivity are required in the strong spin-orbit coupling layer that provides the spin current. In this work we prepare BiSePt alloy/Co heterostructures with in-plane magnetic anisotropy by magnetron sputtering deposition. The alloy layer is deposited via one of two procedures, either co-sputtering or alternative-sputtering. We study the BiSePt alloy samples and find that the spin orbit torque (SOT) efficiency decreases with the increase of Pt component, which is attributed to the change of topological order of Bi&lt;sub&gt;2&lt;/sub&gt;Se&lt;sub&gt;3&lt;/sub&gt; amorphous surface, caused by Pt doping. And the resistivity decreases with the increase of Pt component, which depends on the increase of metallic property. Due to the balance of these two competing mechanisms, the spin Hall conductivity of the alloy layer varies non-monotonically with the concentration ratio, and reach an optimal value at a ratio of 67% of Bi&lt;sub&gt;2&lt;/sub&gt;Se&lt;sub&gt;3&lt;/sub&gt; component. With the increase of the Bi&lt;sub&gt;2&lt;/sub&gt;Se&lt;sub&gt;3&lt;/sub&gt; component, the SOT efficiency, electrical resistivity and spin Hall conductance of the alloy layer show different trends. At about 20%–70%, they increase/decrease tardily. At about 70%–100%, the resistivity ascends more prominently than the SOT efficiency, which leads the spin Hall conductance to decrease. Comparing with using the co-sputtering deposition, the electrical conductivity and spin Hall angle of the alloy layer obtained using alternating sputtering deposition are small, which is attributed to the enhancing of interfacial scattering and the filter effect of Pt on the spin flow. In contrast to traditional pure heavy metal materials (such as Pt, Ta) and topological insulator materials like Bi&lt;sub&gt;2&lt;/sub&gt;Se&lt;sub&gt;3&lt;/sub&gt;, our BiSePt alloy devices obtained by co-sputtering deposition achieve industry-matched preparation conditions, greater SOT efficiency, and considerable electrical conductivity of the alloy layer, thus making further applications of SOT devices possible.