Current-induced effective magnetic fields can provide efficient ways of electrically manipulating the magnetization of ultrathin magnetic heterostructures. Two effects, known as the Rashba spin orbit field and the spin Hall spin torque, have been reported to be responsible for the generation of the effective field. However, a quantitative understanding of the effective field, including its direction with respect to the current flow, is lacking. Here we describe vector measurements of the current-induced effective field in Ta|CoFeB|MgO heterostructrures. The effective field exhibits a significant dependence on the Ta and CoFeB layer thicknesses. In particular, a 1 nm thickness variation of the Ta layer can change the magnitude of the effective field by nearly two orders of magnitude. Moreover, its sign changes when the Ta layer thickness is reduced, indicating that there are two competing effects contributing to it. Our results illustrate that the presence of atomically thin metals can profoundly change the landscape for controlling magnetic moments in magnetic heterostructures electrically.

▲Current-induced effective field vector in Ta│CoFeB│MgO

The current-induced domain wall motion along a thin cobalt ferromagnetic strip sandwiched in a multilayer (Pt/Co/AlO) is theoretically studied with emphasis on the roles of the Rashba field, the spin Hall effect, and the Dzyaloshinskii-Moriya interaction. The results point out that these ingredients, originated from the spin-orbit coupling, are consistent with recent experimental observations in three different scenarios. With the aim of clarifying which is the most plausible the influence of in-plane longitudinal and transversal fields is evaluated.

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Current-driven domain wall motion along high perpendicular anisotropy multilayers: The role of the Rashba field, the spin Hall effect, and the Dzyaloshinskii-Moriya interaction

Magnetic devices are a leading contender for the implementation of memory and logic technologies that are non-volatile, that can scale to high density and high speed, and that do not wear out. However, widespread application of magnetic memory and logic devices will require the development of efficient mechanisms for reorienting their magnetization using the least possible current and power. There has been considerable recent progress in this effort; in particular, it has been discovered that spin–orbit interactions in heavy-metal/ferromagnet bilayers can produce strong current-driven torques on the magnetic layer, via the spin Hall effect in the heavy metal or the Rashba–Edelstein effect in the ferromagnet. In the search for materials to provide even more efficient spin–orbit-induced torques, some proposals have suggested topological insulators, which possess a surface state in which the effects of spin–orbit coupling are maximal in the sense that an electron’s spin orientation is fixed relative to its propagation direction. Here we report experiments showing that charge current flowing in-plane in a thin film of the topological insulator bismuth selenide (Bi2Se3) at room temperature can indeed exert a strong spin-transfer torque on an adjacent ferromagnetic permalloy (Ni81Fe19) thin film, with a direction consistent with that expected from the topological surface state. We find that the strength of the torque per unit charge current density in Bi2Se3 is greater than for any source of spin-transfer torque measured so far, even for non-ideal topological insulator films in which the surface states coexist with bulk conduction. Our data suggest that topological insulators could enable very efficient electrical manipulation of magnetic materials at room temperature, for memory and logic applications.

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Spin-transfer torque generated by a topological insulator

Spin–orbit coupling (SOC) in two-dimensional (2D) materials has emerged as a powerful tool for designing spintronic devices. On the one hand, the interest in this respect for graphene, the most popular 2D material with numerous fascinating and exciting properties, is fading due to the absence of SOC. On the other hand, 2D transition metal dichalcogenides (TMDs) are known to exhibit rich physics including large SOC. TMDs have been used for decades in a variety of applications such as nano-electronics, photonics, optoelectronics, sensing, and recently also in spintronics. Here, we review the current progress in research on 2D TMDs for generating spin–orbit torques in spin-logic devices. Several challenges connecting to thin film growth, film thickness, layer symmetry, and transport properties and their impact on the efficiency of spintronic devices are reviewed. How different TMDs generate spin–orbit torques in magnetic heterostructures is discussed in detail. Relevant aspects for improving the quality of the thin film growth as well as the efficiency of the generated spin–orbit torques are discussed together with future perspectives in the field of spin-orbitronics.

Emergence of spin–orbit torques in 2D transition metal dichalcogenides: A status update

Pure spin current has transformed the research field of conventional spintronics due to its various advantages, including energy efficiency. An efficient mechanism for generation of pure spin current is spin pumping, and high effective spin-mixing conductance (Geff) and interfacial spin transparency (T) are essential for its higher efficiency. By employing the time-resolved magneto-optical Kerr effect technique, we report here a giant value of T in substrate/W (t)/Co20Fe60B20 (d)/SiO2 (2 nm) thin-film heterostructures in the beta-tungsten (β-W) phase. We extract the spin diffusion length of W and spin-mixing conductance of the W/CoFeB interface from the variation of damping as a function of W and CoFeB thickness. This leads to a value of T = 0.81 ± 0.03 for the β-W/CoFeB interface. A stark variation of Geff and T with the thickness of the W layer is obtained in accordance with the structural phase transition and resistivity variation of W with its thickness. Effects such as spin memory loss and two-magnon scattering are found to have minor contributions to damping modulation in comparison to the spin pumping effect which is reconfirmed from the unchanged damping constant with the variation of Cu spacer layer thickness inserted between W and CoFeB. The giant interfacial spin transparency and its strong dependence on crystal structures of W will be important for future spin-orbitronic devices based on pure spin current.

Structural Phase-Dependent Giant Interfacial Spin Transparency in W/CoFeB Thin-Film Heterostructures.

Spintronic materials with two-dimensional (2D) transition-metal dichalcogenide (TMD)/ferromagnet (FM) interfaces have received a great deal of interest recently due to strong modulation of the magnetic properties, which provide attractive opportunities for magnetic information storage. Here, we demonstrate the strong spin-orbit coupling (SOC) effect and great interfacial tuning of magnetization dynamics in $M{X}_{2}$/$\mathrm{Co}\text{\ensuremath{-}}\mathrm{Fe}\text{\ensuremath{-}}\mathrm{B}$ thin films by means of a time-resolved magneto-optical Kerr approach, where M is chosen as $\mathrm{Mo}$ or $\mathrm{W}$ and X is $\mathrm{S}$ or $\mathrm{Se}$. The significant drop of demagnetization time, ${\ensuremath{\tau}}_{m}$, and increase of magnetic damping factor, ${\ensuremath{\alpha}}_{s}$, clearly highlight the presence of high interfacial SOC strength in just one monolayer of ${\mathrm{Mo}\mathrm{S}}_{2}$. Compared with the single $\mathrm{Co}\text{\ensuremath{-}}\mathrm{Fe}\text{\ensuremath{-}}\mathrm{B}$ film, the precession frequency, f, is lowered after inserting a $M{X}_{2}$ layer, suggesting a reduction of the effective magnetization, $4\ensuremath{\pi}{M}_{\mathrm{eff}}$, originating from the interfacial d-d hybridization effect. The role of SOC strength on ${\ensuremath{\tau}}_{m}$, ${M}_{\mathrm{eff}}$, and ${\ensuremath{\alpha}}_{s}$ is confirmed by using different TMD materials, and thus, demonstrates that the element M plays a dominant role. The samples with ${\mathrm{WS}}_{2}$ or ${\mathrm{W}\mathrm{Se}}_{2}$ have much shorter ${\ensuremath{\tau}}_{m}$, smaller ${M}_{\mathrm{eff}}$, and larger ${\ensuremath{\alpha}}_{s}$ values due to the strong SOC interaction of heavier atoms. The observed efficient control of dynamic magnetic behavior will further promote the development of TMD/FM materials for practical spintronic applications with ultrafast manipulation speed and low energy consumption.

Tuning Magnetization Dynamics with Strong Spin-Orbit Coupling in Transition-Metal Dichalcogenide/ Co - Fe - B Heterostructures

Topological insulators (TIs) with spin-momentum-locked metallic surface states can exert giant spin-orbit torques, offering great potential in energy-efficient magnetic memory devices. In this work, temperature (T)-dependent SOT efficiencies are investigated in Sb2Te3/Ta/TbCo heterostructures with perpendicular magnetic anisotropy. The spin Hall angle θSH is around 0.16 at room temperature (RT), which is much higher than that of the control sample without TI. Moreover, as T decreases from RT down to 10 K, θSH exhibits a conspicuous 5-fold enhancement. Detailed analysis indicates that the θSH enhancement at reduced temperatures mainly results from the improved spin-polarized surface states, as evidenced from the continuously increased ratio of surface-to-bulk conduction. The θSH difference between 20 and 10 nm Sb2Te3 gradually shrinks with the increase of T, which is due to the increase of bulk state contribution. Our findings provide a deep insight into the spin transport mechanisms and robust charge-spin conversion in TIs.

Temperature Dependence of Spin-Orbit Torques in Nearly Compensated Tb21Co79 Films by a Topological Insulator Sb2Te3.

Giant Enhancement of Spin-Orbit Torque Efficiency in Pt/Co Bilayers by Inserting a WSe2 under Layer

Magnetization dynamics of the epitaxially-grown Co2FeAl (CFA) thin films have been systematically investigated by the time-resolved magneto-optical Kerr effect (TR-MOKE). The dependences of precession frequency f, relaxation time τ and magnetic damping factor α upon the orientation of applied magnetic field are found to have a strong four-fold symmetry. Two series of samples with various substrate temperatures (Ts) and thickness (tCFA) were prepared and a large Gilbert damping difference between the hard and easy axes is extracted to be 3.3 × 10−3 after subtracting the extrinsic contributions of spin pumping, two-magnon scattering and magnetic inhomogeneities. The four-fold variation of Gilbert damping relates closely to the in-plane magnetocrystalline anisotropy and can be attributed to the anisotropic distribution of spin–orbit coupling. Our findings provide new insights into the anisotropic properties of magnetization and damping, which is very helpful for designing and optimizing advanced spintronic devices on different demands.

Controllable magnetization precession dynamics and damping anisotropy in Co2FeAl Heusler-alloy films

Static and dynamic magnetic properties of $\mathrm{Co}$-$\mathrm{Fe}$(10 nm)/$\mathrm{Ru}$$({t}_{\mathrm{Ru}}=0\ensuremath{-}3\phantom{\rule{0.25em}{0ex}}\mathrm{nm})$/$\mathrm{Co}$-$\mathrm{Fe}$(5 nm) asymmetric trilayers are systematically investigated. The interlayer exchange coupling (IEC) strengths of bilinear $({J}_{1})$, biquadratic $({J}_{2})$, and their equivalent $({J}_{\mathrm{eff}})$ terms are determined; they show a weak nonmonotonic behavior with ${t}_{\mathrm{Ru}}$. Interestingly, the magnetic remanence ratio \ensuremath{\eta} of hard axis to easy axis has two distinct peaks; this can reasonably be interpreted by taking the strong ${J}_{2}$ term into account. Various pump-laser fluences are utilized to modulate the static IEC during the time-resolved magneto-optical Kerr effect measurements, by which individual magnetization precessions of the two $\mathrm{Co}$-$\mathrm{Fe}$ layers are achieved and the effects of dynamic IEC through mutual spin currents are highlighted. With the increase in ${t}_{\mathrm{Ru}}$, the magnetic damping factors of both layers display the same nonmonotonic behavior, which has been mainly ascribed to the spin pumping damping ${\ensuremath{\alpha}}_{\mathrm{SP}}$ associated with the dynamic IEC. Moreover, it is found that the variation trend of damping difference $\mathrm{\ensuremath{\Delta}}{\ensuremath{\alpha}}_{\mathrm{SP}}$ between the two $\mathrm{Co}$-$\mathrm{Fe}$ layers is similar to that of ${J}_{\mathrm{eff}}$, revealing that the dynamic IEC effect is actually dominated by the static IEC and thereupon a theoretical formula is proposed to describe the correlation between $\mathrm{\ensuremath{\Delta}}{\ensuremath{\alpha}}_{\mathrm{SP}}$ and ${J}_{\mathrm{eff}}$. These results suggest the feasibility of efficient control of spin pumping damping through controlled IEC, which has great significance for microwave spintronic devices based on asymmetric trilayer structures. 

Significant and Nonmonotonic Dynamic Magnetic Damping in Asymmetric 
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Zhihao Ji

Papers

Tuning Magnetization Dynamics with Strong Spin-Orbit Coupling in Transition-Metal Dichalcogenide/ Co - Fe - B Heterostructures

Temperature Dependence of Spin-Orbit Torques in Nearly Compensated Tb21Co79 Films by a Topological Insulator Sb2Te3.

Giant Enhancement of Spin-Orbit Torque Efficiency in Pt/Co Bilayers by Inserting a WSe2 under Layer

Controllable magnetization precession dynamics and damping anisotropy in Co2FeAl Heusler-alloy films

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