Damping-like Spin-orbit Torque Research Articles

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.

Efficient magnetization switching induced by spin–orbit torque in perpendicularly magnetized Mn1+xCo2−xAl Heusler alloys

Perpendicularly magnetized Co-based Heusler alloys are promising candidates in high-performance spintronic devices. However, there is a contradiction between thermal stability and damping-like spin–orbit torque (SOT) efficiency in Co-based Heusler alloys with interface-induced perpendicular magnetic anisotropy (PMA). Here, we present epitaxially grown perpendicularly magnetized Mn1+xCo2−xAl (MCA) Heusler alloys through tetragonal distortion. Ferrimagnetism and distortion-induced PMA are obtained in MCA Heusler alloys. A large effective spin Hall angle (θeff) up to 0.33 is experimentally demonstrated in Pt/MCA bilayers, markedly surpassing that in conventional Pt/FM bilayers. Consequently, SOT-induced efficient magnetization switching is realized in Pt/MCA bilayers, with a critical switching current density (Jc) as low as 4 × 107 A/cm2. The large θeff and high SOT efficiency would be attributed to the staggered magnetic exchange torques in the ferrimagnetic MCA Heusler alloys. These findings demonstrate that perpendicularly magnetized ferrimagnetic MCA Heusler alloys are highly promising for high-density SOT-magnetic random-access memory with superior thermal stability and low power consumption.

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.

Enhanced spin Hall effect at high temperature in non-centrosymmetric silicide TaSi2 driven by Berry phase monopoles

We demonstrate the concept of Berry phase monopole engineering of the spin Hall effect in non-centrosymmetric silicide TaSi2. We show that while the effective damping-like spin–orbit torque (SOT) efficiency θDLeff of TaSi2 is nearly unchanged from 62 to 288 K (−0.049 to −0.069), θDLeff suddenly increases at high temperatures and becomes nearly double (−0.12) at 346 K. The corresponding intrinsic spin Hall conductivity σDLeff significantly increases at high temperatures, which can be attributed to the increasing contribution from the four degenerate points near the Fermi level via thermal excitation. Our results provide a strategy to enhance θDLeff at high temperatures via Berry phase monopole engineering and pave the way for SOT spintronic devices working at high temperatures.

Magnetic-Proximity-Induced Efficient Charge-to-Spin Conversion in Large-Area PtSe2/Ni80Fe20 Heterostructures.

As a topological Dirac semimetal with controllable spin-orbit coupling and conductivity, PtSe2, a transition-metal dichalcogenide, is a promising material for several applications, from optoelectrics to sensors. However, its potential for spintronics applications has yet to be explored. In this work, we demonstrate that the PtSe2/Ni80Fe20 heterostructure can generate large damping-like current-induced spin-orbit torques (SOT), despite the absence of spin-splitting in bulk PtSe2. The efficiency of charge-to-spin conversion is found to be -0.1 ± 0.02 nm-1 in PtSe2/Ni80Fe20, which is 3 times that of the control sample, Ni80Fe20/Pt. Our band structure calculations show that the SOT due to PtSe2 arises from an unexpectedly large spin splitting in the interfacial region of PtSe2 introduced by the proximity magnetic field of the Ni80Fe20 layer. Our results open up the possibilities of using large-area PtSe2 for energy-efficient nanoscale devices by utilizing proximity-induced SOT.

Giant, Linearly Increasing Spin-Orbit Torque Efficiency in Symmetry-Broken Spin-Orbit Torque Superlattices.

Magnetic heterostructures with high spin-orbit torque efficiency and low impedance have great promise for low-power spintronic technologies. We report a symmetry-broken spin-orbit superlattice [Pt0.75Cu0.25/Co/Ta]n, in which the dampinglike spin-orbit torque efficiency accumulates linearly with the repeat number n and achieves a giant value of >200% when n = 16, which is 100 times stronger than that of a conventional magnetic heterostructure with a clean Pt (e.g., 2% at a resistivity of 7 μΩ cm). The giant spin-orbit torque effect arises predominantly from the spin Hall effect of Pt0.75Cu0.25. The anomalous Nernst effect increases remarkably as the repeat number n increases, implying a critical need to include the thermal effect in the analysis of magnetic superlattices and multilayers. The giant spin-orbit torque, low resistivity, and strong anomalous Nernst effect suggest the great potential of the superlattice [Pt0.75Cu0.25/Co/Ta]n for low-power memory and logic technologies as well as high-performance thermoelectric battery and sensor applications.

Quantitative characterization of current-induced self-spin–orbit torques in a perpendicularly magnetized (Ga,Mn)As single thin film

Quantitative characterizations of the current-induced spin–orbit torques (SOTs) are vitally important for both fundamental understanding and practical applications of SOT-based spintronic devices. Here, we study effective SOT magnetic fields in a (Ga,Mn)As single film with perpendicular magnetic anisotropy, where we can achieve highly efficient full-magnetization switching with a small critical switching current density Jc as low as 105 A/cm2. Using second harmonic Hall measurements, we estimate the SOT effective fields; the damping-like SOT effective field HDL and the field-like SOT effective field HFL are 22.1 and 18.1 Oe, respectively, at 4 K when a current of 1.43 × 105 A/cm2 is applied to the device. Based on this result, we estimate the corresponding spin-torque efficiencies ξDL and ξFL to be 1.32 and 1.08, respectively, which are one order of magnitude higher than those in conventional metal systems. The high efficiency can be partly attributed to the simple single-functional-layer structure, which can avoid the loss from spin scattering at the interface between different functional layers as observed in conventional SOT devices. Our findings will lay the foundation for studying SOT physics and devices based on ferromagnetic semiconductors.

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.

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.

Effects of thermal annealing on the spin–orbit torque and unidirectional spin Hall magnetoresistance in Pt/CoFe/Ta trilayers

We investigated the spin–orbit torque (SOT) and unidirectional spin Hall magnetoresistance (USMR) in Pt/CoFe/Ta trilayer as well as Pt/CoFe and CoFe/Ta bilayers with in-plane magnetic anisotropy by performing transverse and longitudinal second harmonic resistance measurements. Compared to the two bilayers, we found that the trilayer exhibits enhanced SOT and USMR due to the opposite spin Hall angles of Pt and Ta, which work together to enhance the spin accumulation in the trilayer. Furthermore, we found that thermal annealing has a significant influence on the magnitude and sign of the SOT and USMR in the Pt/CoFe/Ta trilayers. Specifically, we observed that both the damping-like SOT and USMR of the trilayer decrease as the annealing temperature increases, and they even change signs at an annealing temperature between 235 and 265 °C. In contrast, the sign change of the SOT and USMR upon annealing is absent in the Pt/CoFe and CoFe/Ta bilayers. These findings suggest that the sign of the SOT and USMR in the Pt/CoFe/Ta trilayer can be easily manipulated by using an appropriate thermal annealing treatment, which has important implications for the development of novel spintronic devices.

Enhancement of interfacial spin transparency in Py/NiO/Pt heterostructure

This work reports the enhancement of damping-like and field-like spin–orbit torque (SOT) efficiencies and interfacial spin transparency (Tin) in the Py/NiO/Pt heterostructure. The SOT efficiencies and Tin are characterized by combining the spin–torque ferromagnetic resonance (ST-FMR) and the spin-pumping (SP) techniques. The inevitable inverse spin Hall voltage contamination induced by SP in the ST-FMR spectrum is extracted and subtracted by combining additional SP measurements, which allows obtaining accurate SOT efficiencies and Tin. The damping-like and field-like SOT efficiencies vary with the NiO insertion layer thickness, which is a result of the change of Tin. The maximum Tin reaches ∼0.82 for a 0.6 nm-thick NiO layer. This work shows that NiO insertion is an effective method for enhancing Tin and, hence, the SOT efficiency.

Enhancement of spin–orbit torque in Pt/Co/HfOx heterostructures with voltage-controlled oxygen ion migration

Spin–orbit torque (SOT) induced magnetization switching and SOT modulation by interfacial coupling exhibit good potential in spintronic devices. In this work, we report the enhancement of damping-like field and SOT efficiency of up to 60% and 23%, respectively, in perpendicularly magnetized Pt/Co/HfOx heterostructures over a Pt/Co system at an optimal thickness of 2 nm HfOx. The SOT improvement is primarily attributed to the interfacial oxidization of the Co layer, and the strength is tunable via voltage-induced oxygen ion migration at the Co/HfOx interface. Our measurement reveals that by controlling gate voltages, the Co oxidation can be increased, which leads to the SOT efficiency enhancement. Our work promotes the SOT enhancement and modulation by oxidation effects for energy-efficient spintronic devices.

Perpendicular effective field induced by spin-orbit torque and magnetization damping in chiral domain walls

The effective magnetic fields induced by spin-orbit torque (SOT), Dzyaloshinsky-Moriya exchange interaction, and magnetization damping in chiral domain wall systems are very important to manipulate magnetization switching, stimulate magnetization oscillation, and drive domain wall motion in the applications of various spintronic devices. However, magnetization damping as an effective magnetic field is usually ignored in magnetization switching measured by magnetic hysteresis loops due to its dynamical and instantaneous feature. Here, we demonstrate that in addition to the dampinglike SOT effective field, the perpendicular effective field measured by the shift of Hall magnetic hysteresis curves also contains the $z$-direction effective fields originating from magnetization damping under the total static magnetic field. These findings provide key insights for understanding the effective magnetic fields originated from the magnetic damping and dampinglike SOT effective field in chiral domain walls, and pave the path to practical application of the chiral domain wall systems.

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.

Interfacial Engineering Strategies for Efficient Spin–Orbit Torque Devices with Pt Alloys

To make spin–orbit torque magnetic random-access memories (SOT-MRAMs) become competitive enough to replace the contemporary memory architecture, it is of great importance to utilize conductive SOT materials that possess large damping-like spin–orbit torque (DL-SOT) efficiencies and back-end-of-line (BEOL) compatibility. In this work, we report a BEOL-compatible CoFeB/MgO-based heterostructure with the Pt–Cr alloy as the spin current source (SCS) and inserted a Pt/Co/Pt composite layer to achieve texture decoupling and ferromagnetic coupling. A thermally robust perpendicular magnetic anisotropy (PMA) of the hybrid heterostructure can be retained after annealing at 400 °C for 1 h, while maintaining large DL-SOT efficiencies of the annealed Pt–Cr alloys (ξDL ∼ 0.34 for Pt0.70Cr0.30). Additionally, by optimizing the current distribution with a resistive Pt spacer layer, it is possible to simultaneously enhance the current-to-effective field conversion ratio (Hzeff/I) and retain the apparent ξDL (ξDL ∼ 0.37 for Pt0.79Cr0.21). Moreover, the critical switching current density can be reduced from JcPt ∼ 2.0 × 107 A·cm–2 for Pt to JcPt0.70Cr0.30 ∼ 4.9 × 106 A·cm–2 for Pt0.70Cr0.30 without tampering the thermal stability of the annealed heterostructure. These interfacial engineering strategies provide directions to integrate Pt-based SCSs with large ξDL into a BEOL-compatible platform for future SOT-MRAM devices with PMA.

Enhanced spin-orbit torque and field-free switching in Au/TMDs/Ni hybrid structures.

Spin-orbit torque (SOT) plays a significant role in spintronic logic and memory devices. However, due to the limited spin Hall angle and SOT symmetry in a heavy-metal-ferromagnet bilayer, further improving SOT efficiency and all-electric magnetization manipulation remain a challenge. Here we report enhanced SOT efficiency and all-electric switching in Au based magnetic structures, by inserting two-dimensional transition metal dichalcogenides (2D TMDs) with large spin-orbit coupling. With the TMD spacer insert, both damping-like and field-like SOTs are improved, and an unconventional out-of-plane damping-like SOT is induced, due to the interface orbital hybridization, modified spin-mixing conductance and orbital current. Moreover, current induced field-free magnetization switching is demonstrated in Au/WTe2/Ni and Au/MoS2/Ni devices, and it shows multiple intermediate states and can be efficiently controlled by an electric current. Our results open a path for increasing torques and expand the application of 2D TMDs in spintronic devices for neuromorphic computing.