Damping-like Spin-orbit Torque Efficiency Research Articles

Ti-alloyed β-W heterojunctions exhibiting spin-orbit torque switching at a wide operating temperature range

Spin-orbit torque (SOT) switching has gained significant interest, particularly in the context of non-volatile embedded memory for advanced automotive vehicles. The study of temperature-dependent SOT switching, a less explored area, is crucial for developing automotive electronics Grade-0, which requires an ambient operating temperature range from −40 to 150 °C. These SOT devices demand substantial SOT efficiency to ensure a low operating current. Additionally, materials employed in these devices must be compatible with semiconductor fabrication. This study presents the first principles calculations to estimate the spin Hall conductivity (σSH) change of Ti-alloyed β-phase W structures depending on the Ti composition. The calculation predicts the highest σSH value of −1461 (ℏ/e) S/cm at Ti 12.5 at%. In addition, we fabricate β-W-Ti (x at%) 5/Co40Fe40B20 0.9/MgO 1 (in nm) heterojunctions with various Ti compositions to confirm the above result experimentally. Under the controllable experimental condition, we observe the heterojunction with β-W-Ti 11.5 at% exhibits an enhanced damping-like SOT efficiency of 0.54 (compared to 0.30 of pure β-W) with longitudinal resistivity of 149.7 μΩ·cm. The critical current density (Jc) reaches as low as 15.5 cm2. We also demonstrate that SOT switching is possible at ambient temperatures ranging from −55 to +150 °C.

Electric field control of spin-orbit torque in annealed Ta/CoFeB/HfO x heterostructures via interfacial oxidation modulation

Electric field control of spin-orbit torque (SOT) exhibits promising potential in advanced spintronic devices through interfacial modulation. In this work, we investigate the influence of electric field and interfacial oxidation on SOT efficiency in annealed Ta/CoFeB/HfO x heterostructures. By varying annealing temperatures, the damping-like SOT efficiency reaches its peak at the annealing temperature of 320 °C, with an 80% field-free magnetization switching ratio induced by SOT having been demonstrated. This enhancement is ascribed to the annealing-induced modulation of oxygen ion migration at the CoFeB/HfO x interface. By applying voltages across the Ta/CoFeB/HfO x heterostructures, which drives the O2‒ migration across the interface, a reversible, bipolar, and non-volatile modulation of SOT efficiency was observed. The collective influence of annealing temperature and electric field effects on SOT carried out in this work provides an effective approach into facilitating the optimization and control of SOT in spintronic devices.

Enhanced Spin-Orbit Torque Efficiency in Platinum-Gadolinium Oxide Nanocomposite Films.

Spin-orbit torque (SOT) has emerged as an effective means of manipulating magnetization. However, the current energy efficiency of SOT operation is inefficient due to low damping-like SOT efficiency per unit current bias. In this work, we dope conventional rare earth oxides, GdOy, into highly conductive platinum by magnetron sputtering to form a new group of spin Hall materials. A large damping-like spin-orbit torque (DL-SOT) efficiency of about 0.35 ± 0.013 is obtained in Pt0.70(GdOy)0.30 measured by the spin-torque ferromagnetic resonance (ST-FMR) technique, which is about five times that of pure Pt under the same conditions. The substantial enhancement of the spin Hall effect is revealed by theoretical analysis to be attributed to the strong side jump induced by the rare earth oxide GdOy impurities. Moreover, this large DL-SOT efficiency contributes to a low critical switching current density (8.0 × 106 A·cm-2 in the Pt0.70(GdOy)0.30 layer) in current-induced magnetization switching measurements. This systematic study on SOT switching properties suggests that Pt1-x(GdOy)x is an attractive spin current source with large DL-SOT efficiency for future SOT applications and provides another idea to regulate the spin Hall angle.

Large out-of-plane spin–orbit torque in topological Weyl semimetal TaIrTe4

The unique electronic properties of topological quantum materials, such as protected surface states and exotic quasiparticles, can provide an out-of-plane spin-polarized current needed for external field-free magnetization switching of magnets with perpendicular magnetic anisotropy. Conventional spin–orbit torque (SOT) materials provide only an in-plane spin-polarized current, and recently explored materials with lower crystal symmetries provide very low out-of-plane spin-polarized current components, which are not suitable for energy-efficient SOT applications. Here, we demonstrate a large out-of-plane damping-like SOT at room temperature using the topological Weyl semimetal candidate TaIrTe4 with a lower crystal symmetry. We performed spin–torque ferromagnetic resonance (STFMR) and second harmonic Hall measurements on devices based on TaIrTe4/Ni80Fe20 heterostructures and observed a large out-of-plane damping-like SOT efficiency. The out-of-plane spin Hall conductivity is estimated to be (4.05 ± 0.23)×104 (ℏ ⁄ 2e) (Ωm)−1, which is an order of magnitude higher than the reported values in other materials.

Giant Spin-Orbit Torque in Antiferromagnetic-Coupled Pt/[Co/Gd]N Multilayers with Suppressed Spin Dephasing and Robust Thermal Stability.

Manipulating magnetization via power-efficient spin-orbit torque (SOT) has garnered significant attention in the field of spin-based memory and logic devices. However, the damping-like SOT efficiency (ξDL) in heavy metal (HM)/ferromagnetic metal (FM) bilayers is relatively small due to the strong spin dephasing accompanied by additional spin polarization decay. Furthermore, the perpendicular magnetic anisotropy (PMA) originating from the HM/FM interface is constrained by the thickness of FM, which is unfavorable for thermal stability in practical applications. Consequently, it is valuable to develop systems that not only exhibit large ξDL but also balance thermal stability. In this work, we designed antiferromagnetic-coupled [Co/Gd]N multilayers, where staggered Co and Gd magnetic moments effectively suppress the spin dephasing and additional spin polarization decay. The ordered Co-Gd arrangements along the out-of-plane direction provide bulk PMA, endowing Pt/[Co/Gd]N high thermal stability. The SOT of Pt/[Co/Gd]N was systematically studied with N, demonstrating a significantly large ξDL of up to 0.66. The ξDL of Pt/[Co/Gd]N is greater than those of Pt/Co and Pt/ferrimagnetic alloys. This significant enhancement relies on the effective suppression of spin dephasing in [Co/Gd]N. Our work highlights that the antiferromagnetic-coupled [Co/Gd]N multilayer is a promising candidate for low-consumption and high-density spintronic devices.

Temperature-dependent magnon torque in SrIrO3/NiO/ferromagnetic multilayers

Magnetization switching driven by magnons is a promising technology capable of substantially decreasing energy dissipation and potential damage to spintronic devices. In this study, we investigated the temperature-dependent magnon torque effect in SrIrO3/NiO/ferromagnetic multilayers. It is found that the magnon-mediated damping-like spin–orbit torque (SOT) efficiency decreases with increasing temperature. Enhanced magnon transmission was observed in the vicinity of the blocking temperature of the NiO layer, which can be ascribed to the amplification of damping-like SOT efficiency by the spin fluctuations. More importantly, we have demonstrated that the magnon-mediated SOT is an effective method to manipulate a perpendicular magnetization, particularly using a critical switching current density that is pretty low, as evidenced by ∼ 4 × 105 A/cm2 for SrRuO3/NiO/SrIrO3 trilayers in this study. These findings suggest a promising avenue for the development of highly efficient spintronic devices operable through magnon currents.

Significant enhancement of spin-orbit torque efficiency by optimizing the interlayer thickness in [Pt/Ru]n/Pt multilayers

Spin–orbit torque (SOT) is a promising strategy for switching magnetization for magnetic random access memory and maintaining magnetization coherent precession for magnetic nano-oscillator or magnon-based logic device applications. Thus, the enhancement of SOT efficiency is the crucial point for the implementation of high-performance SOT devices. Here, we demonstrate that the effective damping-like SOT efficiency ξDL in the periodic [Pt(2 nm)/Ru(1 nm)]nPt(2 nm) multilayers with a low resistivity of ∼40 μΩ cm (comparable to 23 μΩ cm of the pure Pt film) exhibits an over 100% enhancement compared to that of the pure 12 nm thick Pt (ξDL = 0.055) at the periodic number n = 3, even reaches 0.257 (∼360% enhancement) determined by damping dc modulation method at [Pt/Ru] total thickness of 32 nm with n = 10. Our findings will benefit various SOT devices by significantly reducing energy consumption.

Manipulation of spin–orbit torque and Dzyaloshinskii-Moriya interaction by varying Mn concentration in Pt1-xMnx/Co bilayer

Reducing critical current density (JC) of current-driven magnetization switching via power-efficient spin–orbit torque (SOT) is a key challenge in spin-based memory and logic devices. The Pt1-xMnx alloy shows prospect for enhancing the damping-like SOT efficiency (ξDL) while maintaining relatively low resistivity and power consumption. In this work, we investigated the Mn concentration x dependence of ξDL in Pt1-xMnx/Co bilayer and observed a nonmonotonic variation with the maximum ξDL ≈ 0.25 appearing around x = 0.2. The increase of the Pt1-xMnx layer resistivity is the main contribution to the enhancement of ξDL. Mn atoms doped in Pt not only modulate ξDL originating from the spin Hall effect of the Pt1-xMnx layer, but also affect the interfacial phenomena at the Pt1-xMnx/Co interface. We find that the related interfacial Dzyaloshinskii-Moriya interaction (DMI) is also tunable by x with the maximum appearing around x = 0.15. The significantly reduced JC of Pt1-xMnx/Co bilayer is mainly due to the enhancement of ξDL and the attenuation of effective perpendicular magnetic anisotropy energy density. Meanwhile, Pt1-xMnx/Co bilayer maintains low power consumption and necessary thermal stability. Our work highlights the potential of alloying Pt with Mn as a valuable means to achieve high-performance SOT spintronic devices.

Spin–orbit torque in perpendicularly magnetized [Pt/Ni] multilayers

The performance of spin–orbit torque (SOT) in heavy metal/ferromagnetic metal periodic multilayers has attracted widespread attention. In this paper, we have successfully fabricated a series of perpendicular magnetized [Pt(2–t)/Ni(t)]4 multilayers, and studied the SOT in the multilayers by varying the thickness of Ni layer t. The current induced magnetization switching was achieved with a critical current density of 1 × 107 A/cm2. The damping-like SOT efficiency ξ DL was extracted from an extended harmonic Hall measurement. We demonstrated that the ξ DL can be effectively modulated by t Pt/t Ni ratio of Pt and Ni in the multilayers. The SOT investigation about the [Pt/Ni] N multilayers might provide new material candidates for practical perpendicular SOT-MRAM devices.

Process gas dependence of spin–orbit torque in Pt/NiO/Co structures

Recently, to enlarge spin-current-induced phenomena, such as spin–orbit torque (SOT) in a ferromagnet (FM)/heavy-metal (HM) bilayer, enhancement of spin-current transmittance by inserting an antiferromagnetic insulator at the FM/HM interface has been extensively studied. In this study, we have investigated the SOT modulation in a Pt/NiO/Co structure in which the NiO layer was deposited using Ar and Xe process gases. Consequently, the quality of the NiO layer could be regulated. An increase in the damping-like SOT efficiency was observed by inserting the 1 nm NiO layer in the Ar type while it monotonically decreased with an increase in the NiO thickness in the Xe type. Significant temperature dependence of the SOT efficiency in the Ar type indicates that thermal magnon largely contributes to the spin-current propagation. X-ray diffraction measurement result suggests that the difference in crystallinity of the NiO layer between the Ar and Xe types attributes to the different SOT modulations.

Spin current generation from an epitaxial tungsten dioxide WO2

We report on efficient spin current generation at room temperature in rutile-type WO2 grown on an Al2O3(0001) substrate. The optimal WO2 film has a (010)-oriented monoclinically distorted rutile structure with metallic conductivity due to 5d2 electrons, as characterized by x-ray diffraction, electronic transport, and x-ray photoelectron spectroscopy. By conducting harmonic Hall measurement in a Ni81Fe19/WO2 bilayer, we estimate two symmetries of the spin–orbit torque (SOT), i.e., dampinglike (DL) and fieldlike ones, to find that the former is larger than the latter. By comparison with the Ni81Fe19/W control sample, the observed DL SOT efficiency ξDL of WO2 (+0.174) is about two-thirds of that of W (−0.281) in magnitude, with a striking difference in their signs. The magnitude of the ξDL of WO2 exhibits a comparable value to those of widely reported Pt and Ta, and Ir oxide IrO2. The positive sign of the ξDL of WO2 can be explained by the preceding theoretical study based on the 4d oxides. These results highlight that the epitaxial WO2 offers a great opportunity of rutile oxides with spintronic functionalities, leading to future spin–orbit torque-controlled devices.

Enhancement of spin-orbit torque in WS2/Co/Pt trilayers via spin-orbit proximity effect

Enhancement of spin-orbit torque (SOT) efficiency in ferromagnet/heavy-metal bilayer is promising for the realization of low-power spintronic devices. Here we show that inserting a single-layer ${\mathrm{WS}}_{2}$ between the substrate and Co/Pt layers, can reduce Co coercivity by $\ensuremath{\sim}28%$ and increase dampinglike SOT efficiency by $\ensuremath{\sim}30%$, up to 35.07 Oe/$({10}^{7}\mathrm{A}\text{/}{\mathrm{cm}}^{2})$. When inserting ${\mathrm{WS}}_{2}$ with different layers, we further demonstrate that these phenomena only exist for odd ${\mathrm{WS}}_{2}$ layers, i.e., monolayer and trilayer, while they disappear for even ${\mathrm{WS}}_{2}$ layers, i.e., bilayer. Theoretical analysis based on the first-principles calculations suggests that the results originate from the thickness-controlled charge transfer between ${\mathrm{WS}}_{2}$ and Co, which is consistent with the spin-orbit proximity effect.

Enhanced spin–orbit torque efficiency with low resistivity in perpendicularly magnetized heterostructures consisting of Si-alloyed β-W layers

Spin-orbit torque (SOT) based magnetization switching is of current technological interest to demonstrate its utilization in nonvolatile embedded memory and logic devices. These devices require perpendicular magnetic anisotropy (PMA) for high bit density, significant SOT efficiency to warrant low power consumption, and external field-free magnetization switching. Above all, materials associated with these devices must be semiconductor fabrication friendly. However, only a few materials and their heterostructures previously explored fulfill the requirements. Here, we propose a W–Si alloy, a widely used material in semiconductor devices, as the spin current-generating layer. First, we investigate the spin Hall conductivity of W–Si alloys by adding Si atoms to the β-W matrix using the first-principles calculations. Then, experimentally, we confirm that the heterostructure consisting of W-Si (4 at%)/CoFeB exhibits PMA, a high damping-like SOT efficiency (∼0.58), and low longitudinal resistivity (∼135 μΩ cm). Furthermore, we estimate ten times smaller write power consumption than the heterostructure based on the pristine β-W. The proposed W–Si/CoFeB heterostructures can withstand post-deposition heat treatment up to 500 °C.

Enhancing the Spin-Orbit Torque Efficiency by the Insertion of a Sub-nanometer β-W Layer.

Spin-orbit torque (SOT) efficiency is one of the key issues of spintronics. However, enhancing the SOT efficiency is usually limited by the positive correlation between resistivity and the spin Hall ratio, where a high resistivity often accompanies a large spin Hall ratio. Here, we demonstrate that sub-nanometer β-W intercalation has a considerable impact on the SOT efficiency in α-W (6 nm)/Co (8 nm)/Pt (3 nm) samples. The damping-like SOT efficiency per unit current density, ξDLj, of α-W (5.7 nm)/β-W (0.3 nm)/Co (8 nm)/Pt (3 nm) shows a ∼ 296% enhancement compared to that of the α-W/Co/Pt system. Meanwhile, a resistivity similar to that of α-W and the spin Hall ratio larger than β-W induce a giant damping-like SOT efficiency per applied electric field, ξDLE, which is about 12.1 times larger than that of β-W. Our findings will benefit the SOT devices by reducing energy consumption.

Spin–orbit torque generation in bilayers composed of CoFeB and epitaxial SrIrO3 grown on an orthorhombic DyScO3 substrate

We report on the highly efficient spin–orbit torque (SOT) generation in epitaxial SrIrO3 (SIO), which is grown on an orthorhombic DyScO3(110) substrate. By conducting harmonic Hall measurement in Co20Fe60B20 (CoFeB)/SIO bilayers, we characterize two kinds of the SOTs, i.e., dampinglike (DL) and fieldlike ones to find that the former is much larger than the latter. By comparison with the Pt control sample with the same CoFeB thickness, the observed DL SOT efficiency ξDL of SIO (∼0.32) is three times higher than that of Pt (∼0.093). The ξDL is nearly constant as a function of the CoFeB thickness, suggesting that the SIO plays a crucial role in the large SOT generation. These results on the CoFeB/SIO bilayers highlight that the epitaxial SIO is promising for low-current and reliable spin–orbit torque-controlled devices.

Tailoring Neuromorphic Switching by CuNx -Mediated Orbital Currents

Current-induced spin-orbit torque (SOT) is regarded as a promising mechanism for driving neuromorphic behavior in spin-orbitronic devices. In principle, the strong SOT in a heavy-metal-based magnetic heterostructure is attributed to the spin-orbit coupling (SOC)-induced spin Hall effect and/or the spin Rashba-Edelstein effect. Recently, SOC-free mechanisms such as the orbital-angular-momentum-induced orbital Hall effect and/or the orbital Rashba-Edelstein effect have been proposed to generate sizable torques comparable to those from the conventional spin Hall mechanism. In this work, we show that the orbital current can be effectively generated by the nitrided light metal $\mathrm{Cu}$. The overall dampinglike SOT efficiency, which consists of both the spin and the orbital current contributions, can be tailored from ${\ensuremath{\xi}}_{\mathrm{DL}}\ensuremath{\approx}0.06\text{ to }0.4$ in a $\mathrm{Pt}/\mathrm{Co}/{\mathrm{Cu}\mathrm{N}}_{x}$ magnetic heterostructure by tuning the nitrogen doping concentration. Current-induced magnetization switching further verifies the efficacy of such orbital current with a critical switching current density as low as ${J}_{c}$ \ensuremath{\sim} 5 \ifmmode\times\else\texttimes\fi{} ${10}^{10}\phantom{\rule{0.25em}{0ex}}{\mathrm{A}/\mathrm{m}}^{2}$. Most importantly, orbital-current-mediated memristive switching behavior can be observed in such heterostructures, which reveals that gigantic SOT and efficient magnetization switching are the trade-offs for the applicable window of memristive switching. Our work provides insights into the role that orbital current might play in SOT neuromorphic devices for making energy-efficient spin-orbitronic devices.

Room-temperature spin-orbit torque switching in a manganite-based heterostructure

The oxide ferromagnet ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}\mathrm{Mn}{\mathrm{O}}_{3}$ (LSMO) possesses low saturation magnetization $({M}_{s})$, a low magnetic damping constant $(\ensuremath{\alpha})$, and high carrier spin polarization, which promise superior functionalities in spintronics devices. For applications, these devices are to be integrated into electronic circuits, which require the control of the magnetization of LSMO by electrical means. However, a reliable electrical switching method remains largely unexplored. Here, we study the current-induced spin-orbit torque (SOT) in an all-oxide $\mathrm{SrIr}{\mathrm{O}}_{3}/\mathrm{LSMO}$ bilayer. We found the magnetization of a 15 nm LSMO can be switched by electrical current, as confirmed from both the electrical transport measurement and the magnetic optical measurement. By taking advantage of the strain-mediated magnetic anisotropy, the magnetic easy axis of LSMO was controlled to be either perpendicular or parallel to the charge current direction such that a type-$y$ or a type-$x$ SOT switching was achieved, respectively. The dampinglike SOT efficiency is characterized to be 0.15 at room temperature according to the harmonic Hall voltage analysis. We also studied the temperature dependences of the SOT effective field and current-induced switching. Our demonstration of the electrical switching of LSMO magnetization at room temperature may stimulate SOT studies in a wide variety of all-oxide systems.

Spin-Orbit Torque Switching of a High-Quality Perpendicularly Magnetized Ferrimagnetic Heusler Mn3Ge Film.

Current-induced spin-orbit torque (SOT) switching of magnetization has attracted great interest due to its potential application in magnetic memory devices, which offer low-energy consumption and high-speed writing. However, most of the SOT studies on perpendicularly magnetized anisotropy (PMA) magnets have been limited to heterostructures with interfacial PMA and poor thermal stability. Here, we experimentally demonstrate a SOT magnetization switching for a ferrimagnetic D022-Mn3Ge film with high bulk PMA and robust thermal stability factor under a critical current density of 6.6 × 1011 A m-2 through the spin Hall effect of an adjacent capping Pt and a buffer Cr layer. A large effective damping-like SOT efficiency of 2.37 mT/1010 A m-2 is determined using harmonic measurements in the structure. The effect of the double-spin source layers and the negative-exchange interaction of the ferrimagnet may explain the large SOT efficiency and the manifested magnetization switching of Mn3Ge. Our findings demonstrate that D022-Mn3Ge is a promising candidate for application in high-density SOT magnetic random-access memory devices.

In-plane crystallographic orientations related spin-orbit torque in epitaxial Pt(111)/Co/Ta heterostructures

Current induced spin–orbit torque (SOT) in heavy metals with strong spin–orbit coupling strength has attracted considerable attention due to its potential applications in spintronic technology. Pt, as one of the mostly used heavy metals in SOT-based spintronic devices, shows large spin Hall angle (θSH) with its single phase and alloy counterparts. In this work, the in-plane crystallographic orientations related θSH of epitaxial Pt(111) layer is reported in MgO(111)/Pt(111)/Co/Ta heterostructures with strong perpendicular magnetic anisotropy. The θSH shows a quite large difference with values, respectively, around 0.083 and 0.057 when the current applied along the [11¯0] and [112¯] crystallographic directions of Pt(111) by the damping-like SOT efficiency using the harmonic Hall voltage measurement technique. The critical switching current densities also show large difference between these two orthogonal crystallographic orientations with the trend of that the larger SOT efficiency leads to the smaller critical switching current density. It independently confirms the generation of different damping-like SOT efficiency when current along [11¯0] and [112¯] directions of Pt(111). Moreover, a perpendicularly magnetized Pt/Co/Ta reference heterostructures with Pt having polycrystalline phase shows tiny variation of damping-like SOT efficiency in in-plane two orthogonal directions, which also indirectly indicates the crystallographic orientations related θSH in epitaxial Pt(111) layers. This study indicates that the θSH of epitaxial Pt is a crystallographic orientations related parameter.

Spin-orbit torque and Dzyaloshinskii–Moriya interaction in 4d metal Rh-based magnetic heterostructures

The electrical switching of magnetization through spin–orbit torque (SOT) has potential applications for energy-efficient spintronic devices. Previous studies focused mostly on 5d heavy metals with strong spin–orbit coupling (SOC) to generate a spin current or a nonequilibrium spin accumulation and exert SOTs on the magnetization of a neighboring ferromagnetic layer. Recent theoretical and experimental studies indicated that 4d metals with weak SOC may also generate a sizable torque and realize the current-induced magnetization switching. In this work, we studied the current-induced SOTs in 4d metal Rh-based magnetic heterostructures with a perpendicular magnetic anisotropy. The damping-like SOT efficiency ξDL of [Ni/Co]3/Rh multilayers increases with the Rh thickness tRh and becomes saturated at tRh = 5 nm. Although the spin-Hall angle of Rh is rather small about 0.028 ± 0.005, a reversible current-induced SOT switching can still be achieved. In addition, the interfacial Dzyaloshinskii-Moriya interaction (iDMI) in Rh/Co heterostructures was quantitatively characterized by using Brillouin light scattering. The iDMI constant D increases with tRh and reaches 224 ± 39 μJ/m2 at tRh = 5 nm. Our results indicated that even for a weak SOC 4d metal Rh, it is still possible to obtain a current-induced magnetization switching and observe an obvious iDMI effect in the Rh-based magnetic heterostructures, which may broaden the scope of spintronic materials used for SOT devices.