Field-like Spin-orbit Torque Research Articles

Field-like spin\u2013orbit torque induced by bulk Rashba channels in GeTe/NiFe bilayers

Most studies of the Rashba effect have focused on interfacial Rashba spin–orbit coupling. Recently, bulk Rashba materials have attracted considerable interest owing to their potential to enhance the Rashba spin–orbit torque. By employing a bulk Rashba material, GeTe, as a spin–orbit channel in GeTe/NiFe bilayers, a large field-like spin–orbit torque up to 15.8 mT/(107 A cm−2) is measured. This value is one of the largest reported field-like torques and is attributed to the interfacial spin–orbit coupling being enhanced by the bulk Rashba effect in the GeTe channel. Furthermore, the large field-like torque is maintained even for a 20-nm-thick NiFe layer. This unconventional dependence on the thickness of both the GeTe and NiFe layers cannot be described by conventional theory, but it is believed to stem from the additional bulk Rashba effect-induced term. The large field-like torque over a wide range of ferromagnet thicknesses results in scalable in-plane spin–orbit torque devices. This result calls for a further theoretical study on spin transport in heterostructures, including bulk Rashba materials.

Modification of interfacial spin-orbit torque in Co/Pt/oxide hybrid structures

We investigate current-induced spin-orbit torque (SOT) in Co/Pt/oxide systems by varying a sort of the oxide layer. When the Pt thickness is less than \ensuremath{\sim}2 nm, Pt at the interface of the oxide layer is magnetically polarized owing to the magnetic proximity effect. In these systems, fieldlike (FL) SOT depends significantly on the adjacent oxide material, whereas dampinglike SOT is almost irrelevant to oxides. The FL SOT efficiency of the $\mathrm{Pt}/\mathrm{Hf}{\mathrm{O}}_{2}$ sample is 2.2 times greater than that of the Pt/MgO sample at the maximum. X-ray magnetic circular dichroism spectroscopy reveals that the anisotropy of the Pt orbital magnetic moment varies with the oxide material, suggesting that the modulation of the electronic structure at the Pt/oxide interface contributes to SOT enhancement.

Large modulation of spin-orbit torques in IrMn/CoFeB/MgO heterostructures with optimized perpendicular magnetic anisotropy

Spin-orbit torques (SOTs) and perpendicular magnetic anisotropy (PMA) are of great technical relevance for new generation spintronics applications. Here, we present a prominent tuning effect of annealing on SOT and PMA in IrMn/CoFeB/MgO/SiO2 heterostructures. By systematically changing the thicknesses of Co40Fe40B20 and IrMn, and annealing temperature, the phase diagrams of PMA are obtained. The SOTs are quantified by harmonic Hall voltage measurements. Both the damping-like and field-like SOT efficiencies show a nonmonotonic variation with annealing temperature, which can be further correlated with the device resistivity. Our work contributes a reference for fabricating perpendicularly magnetized CoFeB on seed layer-free IrMn and for tuning SOTs by annealing.

Probing Charge Density Wave Effects in 1T-TaS2 Monolayer/Ni81Fe19 Heterostructure: A Spin Dynamics Approach

The transition metal dichalcogenide 1T-TaS2 is known to exhibit a number of collective electronic states known as charge density wave (CDW) instabilities. Intriguing phenomena such as a large damping-like spin–orbit torque (SOT) have been reported in monolayer 1T-TaS2 [Nano Lett. 2020, 20 (9), 6372–6380]. Probing of CDWs in monolayer thick 1T-TaS2 has been an inconceivable task. Here, the temperature-dependent spin dynamics and the effect of CDWs in the 1T-TaS2(monolayer)/Ni81Fe19 (Py) (7 nm) heterostructure are reported. Employing ferromagnetic resonance, the effect of the different commensurate (C) and nearly commensurate (NC) CDW states on the spin dynamics during heating and cooling cycles has been characterized by use of the effective damping constant and the spin mixing conductance of the heterostructure. In addition, these CCDW and NCCDW states, which affect the SOT efficiencies due to damping- and field-like SOTs, have been evaluated by using angle-dependent planar Hall effect measurements in controlled cooling and heating cycles. Our findings provide a fundamental understanding of the effect of different CDW states on the spin dynamics in two-dimensional 1T-TaS2 monolayer interfaced Py.

Unveiling the Mechanism of Bulk Spin‐Orbit Torques within Chemically Disordered FexPt1‐x Single Layers

AbstractThe recent discovery of spin‐orbit torques (SOTs) within magnetic single‐layers has attracted attention. However, it remains elusive as to how to understand and how to tune the SOTs. Here, utilizing the single layers of chemically disordered FexPt1‐x, the mechanism of the “unexpected” bulk SOTs is unveiled by studying their dependence on the introduction of a controlled vertical composition gradient and temperature. The bulk dampinglike SOT is found to arise from an imbalanced internal spin current that is transversely polarized and independent of the magnetization orientation. The torque can be strong only in the presence of a vertical composition gradient. The SOT efficiency per electric field is insensitive to temperature but changes sign upon reversal of the orientation of the composition gradient, which is analog to the strain behaviors. These characteristics suggest that the imbalanced internal spin current originates from a bulk spin Hall effect and that the associated inversion asymmetry that allows for a non‐zero net torque is most likely a strain non‐uniformity induced by the composition gradient. The fieldlike SOT is a relatively small bulk effect compared to the dampinglike SOT. This study points to the possibility of developing low‐power single‐layer SOT devices by strain engineering.

Spin-orbit torques and magnetotransport properties of α−Sn and β−Sn heterostructures

Topological insulators have emerged as an important material class for efficient spin-charge interconversion. Most topological insulators considered to date are binary or ternary compounds, with the exception of $\alpha$-Sn. Here we report a comprehensive characterization of the growth, magnetotransport properties, and current-induced spin-orbit torques of $\alpha$-Sn and $\beta$-Sn-based ferromagnetic heterostructures. We show that $\alpha$-Sn grown with a Bi surfactant on CdTe(001) promotes large spin-orbit torques in a ferromagnetic FeCo layer at room temperature, comparable to Pt, whereas $\alpha$-Sn grown without Bi surfactant and the non-topological phase, $\beta$-Sn, induce lower torques. The dampinglike and fieldlike spin-orbit torque efficiency in $\alpha$-Sn with Bi are 0.12 and 0.18, respectively. Further, we show that $\alpha$-Sn grown with and without Bi presents a spin Hall-like magnetoresistance comparable to that found in heavy metal/ferromagnet bilayers. Our work demonstrates direct and efficient charge-to-spin conversion in $\alpha$-Sn ferromagnetic heterostructures, showing that $\alpha$-Sn is a promising material for current-induced magnetization control in spintronic devices.

Spin-orbit torques in strained PtMnSb from first principles

We compute spin-orbit torques (SOTs) in strained PtMnSb from first principles. We consider both tetragonal strain and shear strain. We find a strong linear dependence of the field-like SOTs on these strains, while the antidamping SOT is only moderately sensitive to shear strain and even insensitive to tetragonal strain. We also study the dependence of the SOT on the magnetization direction. In order to obtain analytical expressions suitable for fitting our numerical \textit{ab-initio} results we derive a general expansion of the SOT in terms of all response tensors that are allowed by crystal symmetry. Our expansion includes also higher-order terms beyond the usually considered lowest order. We find that the dependence on the strain is much smaller for the higher-order terms than for the lowest order terms. In order to judge the sensitivity of the SOT on the exchange correlation potential we compute the SOT in both GGA and LDA. We find that the higher-order terms depend significantly on the exchange-correlation potential, while the lowest order terms are insensitive to it. Since the higher-order terms are small in comparison to the lowest order terms the total SOT is insensitive to the exchange correlation potential in strained PtMnSb.

Charge-spin interconversion in epitaxial Pt probed by spin-orbit torques in a magnetic insulator

We measure spin-orbit torques (SOTs) in a unique model system of all-epitaxial ferrite/Pt bilayers to gain insights into charge-spin interconversion in Pt. With negligible electronic conduction in the insulating ferrite, the crystalline Pt film acts as the sole source of charge-to-spin conversion. A small field-like SOT independent of Pt thickness suggests a weak Rashba-Edelstein effect at the ferrite/Pt interface. By contrast, we observe a sizable damping-like SOT that depends on the Pt thickness, from which we deduce the dominance of an extrinsic spin-Hall effect (skew scattering) and Dyakonov-Perel spin relaxation in the crystalline Pt film. Furthermore, our results point to a large internal spin-Hall ratio of $\approx$0.8 in epitaxial Pt. Our experimental work takes an essential step towards understanding the mechanisms of charge-spin interconversion and SOTs in Pt-based heterostructures, which are crucial for power-efficient spintronic devices.

Interfacial and bulk spin Hall contributions to fieldlike spin-orbit torque generated by iridium

We present measurements of spin orbit torques generated by Ir as a function of film thickness in sputtered Ir/CoFeB and Ir/Co samples. We find that Ir provides a damping-like component of spin orbit torque with a maximum spin torque conductivity 1.4e5 in SI unit and a maximum spin-torque efficiency of 0.04, which is sufficient to drive switching in an 0.8 nm film of CoFeB with perpendicular magnetic anisotropy. We also observe a surprisingly large field like spin orbit torque. Measurements as a function of Ir thickness indicate a substantial contribution to the FLT from an interface mechanism so that in the ultrathin limit there is a non-zero FLT with a maximum torque conductivity -5.0E4 in the SI unit. When the Ir film thickness becomes comparable to or greater than its spin diffusion length, 1.6 nm, there is also a smaller bulk contribution to the fieldlike torque.

Effects of Interfacial Oxidization on Magnetic Damping and Spin-Orbit Torques.

We investigate the effects of interfacial oxidation on the perpendicular magnetic anisotropy, magnetic damping, and spin-orbit torques in heavy-metal (Pt)/ferromagnet (Co or NiFe)/capping (MgO/Ta, HfOx, or TaN) structures. At room temperature, the capping materials influence the effective surface magnetic anisotropy energy density, which is associated with the formation of interfacial magnetic oxides. The magnetic damping parameter of Co is considerably influenced by the capping material (especially MgO) while that of NiFe is not. This is possibly due to extra magnetic damping via spin-pumping process across the Co/CoO interface and incoherent magnon generation (spin fluctuation) developed in the antiferromagnetic CoO. It is also observed that both antidamping and field-like spin-orbit torque efficiencies vary with the capping material in the thickness ranges we examined. Our results reveal the crucial role of interfacial oxides on the perpendicular magnetic anisotropy, magnetic damping, and spin-orbit torques.

W layer thickness dependence of the spin–orbit effective fields in NiFe/W bilayers

Spin–orbit torques (SOTs) generated by in-plane current injection in a ferromagnetic metal (FM)/heavy metal (HM) bilayer offers a new route to electrically manipulate magnetization. Here, we report on two sizable spin–orbit field contributions from the spin Hall effect and Rashba effect in NiFe/W bilayers by using the planar Hall effect. Both spin–orbit fields decrease with increasing W layer thickness. Importantly, the spin–orbit field contributing from the spin Hall effect decreases faster than the one from the Rashba effect as the thickness of W layer increases, leading to the sign change of the field-like torque at thicker W. Our results illustrate the co-contributions of the Rashba effect and the spin Hall effect to the field-like SOT in NiFe/W bilayer giving more insight into the effect of the field-like SOT in a FM/HM bilayer.

Hall-bar-width dependence of the field-like spin-orbit torque in NiFe/Pt bilayers

Current-induced spin-orbit torques (SOTs) can cause an obvious manipulation of magnetization in a heavy metal/ferromagnetic metal bilayer. We report here the peculiarities of field-like SOT by a planar Hall effect (PHE) measurement for a number of stripe samples with various widths (w), consisting of a thermally oxidized Si sub./NiFe/Pt. The separation of interfacial spin-orbit effective field, bulk spin-orbit effective field, and Oersted field was performed by using the longitudinal and transverse PHE configurations. The interfacial effective field is much larger and plays a dominant role than the bulk one in the films. All the effective fields follow the 1/w dependence and obey the linear relationship with the current density. Furthermore, both the interfacial and bulk SOT coefficients remain constant and are independence of Hall-bar-width. The dominant interfacial spin-orbit effective field in the NiFe/Pt bilayers suggests the necessity of further analysis of interfacial effect on the current-induced manipulation of magnetization.

Large damping-like spin–orbit torque and perpendicular magnetization switching in sputtered WTex films

We report on the study of charge-spin conversion in sputter-deposited WTex films using harmonic Hall measurements. Strong damping-like (DL) spin–orbit torque (SOT) is observed in WTex thin films with the largest SOT efficiency of 0.67 × 105 Ω−1 m−1 at a WTex thickness of 5 nm, which quickly decreases to around zero as the thickness increases to 10 nm. In comparison, the field-like SOT is around one order smaller than the DL SOT. Making use of the large DL SOT, we realize efficient current-induced perpendicular magnetization switching in WTex/Ti/CoFeB multilayers. Our findings suggest the great potential of sputter-deposited WTex in spintronics applications.

Impact of the interplay of piezoelectric strain and current-induced heating on the field-like spin–orbit torque in perpendicularly magnetized Ta/Co20Fe60B20/Ta/MgO film

Spin–orbit torques (SOTs) are known to be the most efficient way to manipulate the magnetization direction by electrical currents. While, conventionally, one symmetry component of the SOTs, namely, the damping-like torque, was considered to play a primary role, recently, the significance of the other component, the field-like torque, has been revised, owing to the non-trivial dynamics it can induce in heavy metal/ferromagnet multilayers. In this work, we first discuss the unusual behavior of the field-like SOT in a Ta/CoFeB/Ta/MgO multilayer system with a reduced magnetic anisotropy and demonstrate an energy-efficient approach to manipulate the magnitude of the SOT effective fields. Finally, our results show a possibility to engineer the anisotropy of the field-like SOTs by piezoelectric strain, which can be potentially attractive for application in spintronics.

Walker solution for a magnetic domain wall driven by spin-orbit torques

A domain-wall configuration and a current-injection scheme are proposed which ensure the satisfaction of the boundary condition of the dynamic Walker wall driven by the spin-orbit torques (SOTs). The analytic solutions of the wall are obtained in the whole region of current by the traveling-wave method. In the steady regime, the initial velocity mainly depends on the Slonczewski SOT. Meanwhile, the final steady velocity largely relies on the fieldlike one. The critical behavior of velocity is closely linked to the ratio of the fieldlike SOT parameter $\ensuremath{\beta}$ to the Gilbert damping constant $\ensuremath{\alpha}$. Reducing the difference between $\ensuremath{\beta}$ and $\ensuremath{\alpha}$, the Walker breakdown is postponed and the Walker velocity is enhanced. Additionally, by adjusting an applied magnetic field, $\ensuremath{\beta}$ can be measured by halting the wall. In the periodic regime, the wall propagates back and forth with an average velocity increasing almost linearly for high currents.

Voltage control of spin–orbit torque in Pd/Co/Pd/HfO x

Voltage control of current-induced spin–orbit torque (SOT) in an in-plane-magnetized Pd/Co/Pd system with a low-temperature-deposited HfO x and a gate electrode on top is studied. An application of the gate electric field to the HfO x layer is to induce a non-volatile electrochemical effect from the Pd/HfO x interface. By means of low-frequency harmonic Hall measurements, the voltage modulation of both damping-like and field-like SOT is obtained. The sign of the voltage-induced SOT modification is found to be reversed by changing the top Pd thickness. Our finding is expected to facilitate the efficient active manipulation of SOT.

Time-resolved visualization of the magnetization canting induced by field-like spin–orbit torques

We report on the use of time-resolved scanning transmission x-ray microscopy imaging for the visualization of the dynamical canting of the magnetization induced by field-like spin–orbit torques in a perpendicularly magnetized microwire. In particular, we show how the contributions to the dynamical canting of the magnetization arising from the field-like spin–orbit torque can be separated from the heating-induced effects on the magnetization of the microwire. This method will allow for the imaging of the dynamical effects of spin–orbit torques in device-like structures and buried layers.

Spin-orbit torque generation in NiFe/IrO2 bilayers

The 5d transition-metal oxides have a unique electronic structure dominated by strong spin-orbit coupling and hence they can be an intriguing platform to explore spin-current physics. Here, we report on room-temperature generation of spin-orbit torque (SOT) from a conductive 5d iridium oxide, IrO2. By measuring second-harmonic Hall resistance of Ni81Fe19/IrO2 bilayers, we find both dampinglike and fieldlike SOTs. The former is larger than the latter, enabling easier control of magnetization. We also observe that the dampinglike SOT efficiency has a significant dependence on IrO2 thickness, which is well described by the drift-diffusion model based on the bulk spin Hall effect. We deduce the effective spin Hall angle of +0.093 +- 0.003 and the spin-diffusion length of 1.7 +- 0.2 nm. By comparison with control samples Pt and Ir, we show that the effective spin Hall angle of IrO2 is comparable to that of Pt and seven times higher than that of Ir. The fieldlike SOT efficiency has a negative sign without appreciable dependence on the thickness, in contrast to the dampinglike SOT. This suggests that the fieldlike SOT likely stems from the interface. These experimental findings suggest that the uniqueness of the electronic structure of 5d transition-metal oxides is crucial for highly efficient charge to spin-current conversion.

Composition dependence of spin−orbit torque in Pt1−xMnx/CoFeB heterostructures

We investigate spin–orbit torques (SOTs) in heterostructures with Pt1−xMnx alloy and CoFeB as a function of Mn composition (x) by using an extended harmonic Hall measurement. Slonczewski-like and field-like SOT efficiencies (ξSL and ξFL) show non-monotonic variation and a different trend with respect to x, and considerably large ξSL up to 0.21 is obtained at x = 0.20. Compared to the x dependence of longitudinal resistivity, the Slonczewski-like SOT in the low x region is mainly attributed to an intrinsic spin-Hall mechanism, whereas a non-monotonic variation in the higher x region suggests the presence of additional factors. The present findings deliver useful clues to understand the physics behind SOT generation in antiferromagnetic heterostructures and offer a route to realize efficient devices.

Direct detection of spin-orbit effective fields through magneto-optical Kerr effect

Current-induced effective magnetic fields provide efficient methods to electrically control the magnetization switching in ultrathin magnetic films. It is getting clearer that the two terms of spin-orbit torque (SOT), i.e., the dampinglike SOT and the fieldlike SOT, play different roles during the magnetization switching process. Fast and direct estimation of both effective fields are crucial for highly efficient magnetization switching. Methods to extract the amplitudes of the effective fields have been intensively investigated, but they are either complex or require multiparameter fitting, especially for the fieldlike field. We introduce the direct detection of SOT effective fields using hysteresis-loop shift through the magneto-optical Kerr effect in the out-of-plane Pt/Co/W and in-plane Pt/Fe/MgO trilayer systems. The results show strong agreement with transport measurements. Benefiting from noncontact optical detection, our approach in the detection of the spin-orbit effective field has great advantages in the simplified device fabrication processing, and it is suitable not only for metallic systems but also for insulating systems, which paves the way to direct detection of SOT effective fields.