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We have successfully synthesized Y3Fe5O12 (yttrium iron garnet, YIG) nanotubes (NTs) of different diameters using a conventional sol-gel method in anodized aluminum oxide (AAO) templates. An annealing process at 800 °C is performed to get a pure phase of YIG. The room-temperature magnetization measurements show that the coercivity decreases significantly as the diameter of nanotubes increases. The temperature dependence of magnetic moments in field-cooled (FC) and zero-field-cooled (ZFC) modes indicates that YIG NTs transform from ferromagnetic-like (FM-like) state to superparamagnetic (SPM) state when temperature decreases (in the region of 310 K–400 K). The Blocking temperature dramatically decreases as the size of YIG NTs reduces. In the FM-like state, the temperature-dependent coercivity in three different samples yields a 1/2 power law of the temperature. The temperature dependence of saturation magnetization in three different samples can be well fitted to Bloch’s T3/2 law. Characterization of ferromagnetic resonance (FMR) is conducted, which indicates the Gilbert damping parameter of YIG NTs is around 7–9 × 10−3.

A chiral bobber is a localized three-dimensional magnetization configuration, terminated by a singularity. Chiral bobbers coexist with magnetic skyrmions in chiral magnets, lending themselves to new types of skyrmion-complementary bits of information. However, the on-demand creation of bobbers, as well as their direct observation remained elusive. Here, we introduce a new mechanism for creating a stable chiral bobber lattice state via the proximity of two skyrmion species with comparable size. This effect is experimentally demonstrated in a Cu$_2$OSeO$_3$/[Ta/CoFeB/MgO]$_4$ heterostructure in which an exotic bobber lattice state emerges in the phase diagram of Cu$_2$OSeO$_3$. To unambiguously reveal the existence of the chiral bobber lattice state, we have developed a novel characterization technique, magnetic truncation rod analysis, which is based on resonant elastic x-ray scattering.

Magnetic two-dimensional (2D) van der Waals (vdWs) materials and their heterostructures attract increasing attentionin the spintronics community due to their various degrees of freedom such as spin, charge, and energy valley, which maystimulate potential applications in the field of low-power and high-speed spintronic devices in the future. This reviewbegins with introducing the long-range magnetic order in 2D vdWs materials and the recent progress of tunning their properties by electrostatic doping and stress. Next, the proximity-effect, current-induced magnetization switching, and the related spintronic devices (such as magnetic tunnel junctions and spin valves) based on magnetic 2D vdWs materials are presented. Finally, the development trend of magnetic 2D vdWs materials is discussed. This review provides comprehensive understandings for the development of novel spintronic applications based on magnetic 2D vdWs materials.

Spin-orbit torque (SOT) switching of magnetization is a promising emerging technology for nonvolatile spintronic memory and logic applications. However, deterministic switching of perpendicular magnetization with SOTs requires an additional symmetry breaking, which is typically provided by an external magnetic field, making it impractical for applications. In this work, we disclose that by the insertion of a slightly asymmetric light-metal layer at the heavy metal-ferromagnet interface of SOT heterostructures, current-induced out-of-plane effective magnetic fields are introduced that enable deterministic switching without an external magnetic field. We obtain uniform perpendicular magnetic anisotropy and switching current density despite the asymmetry of the light-metal layer, and we show the scalability of our approach by studying device sizes that differ by 2 orders of magnitude. Our work provides a practical route for utilization of SOTs for magnetization switching on the wafer scale and paves the way for the practical application of SOT-based technology.

Characterization of spin-orbit torques (SOTs) in the perpendicular magnetic anisotropy (PMA) system is of great importance for fundamental studies and technological applications in spintronic devices. Here, we report a spin Hall magnetoresistance (SMR) based spin-torque ferromagnetic resonance (ST FMR) study of SOT efficiency in perpendicularly magnetized $\mathrm{W}$/${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$/$\mathrm{Mg}\mathrm{O}$ structures. A full analysis of the ST FMR spectrum is developed for the PMA sample. A typical ST FMR spectrum of the PMA system shows two resonance modes, i.e., in-plane and out-of-plane magnetization precession modes. By performing modulation of the damping measurement for the in-plane mode, the dampinglike torque efficiency is determined to be \ensuremath{-}0.38, which is consistent with the reference value obtained in samples with in-plane magnetic anisotropy. The dampinglike torque efficiency of \ensuremath{-}0.31 is also obtained from the out-of-plane resonance mode, in which the significant contribution of the spin-pumping-induced inverse spin Hall effect is carefully considered. The obtained values of dampinglike torque efficiency by different means are consistent with each other. The present work provides useful insights to determine the dampinglike torque efficiency of the PMA sample via ST FMR measurements.

Versatile spin-orbit-torque switching is potentially useful for spin logics and neuromorphic computing. Using two orthogonal spin-orbit torques, we nonvolatilely switch the magnetization of a $\mathrm{Pt}$/$\mathrm{Co}$/$\mathrm{Mg}\mathrm{O}$ crossbar with perpendicular magnetic anisotropy among multistates such as the spin-up, spin-down, and spin-random states. By electrical methods, we can even reverse the chirality of the current dependence of the magnetization from clockwise to counterclockwise. Realization of chirality-controllable switching among multistates could pave the way toward spin logics and spintronic neural computing.

Spin current generated in a ferromagnetic metal (FM) can be divided into two types. While one is magnetization dependent and induced by the well-known anomalous Hall effect, the other is a magnetization-independent spin Hall effect which is similar to that in a paramagnetic heavy metal (HM). Here, we study the magnetization-independent spin Hall current in YIG/FM (NiFe and CoFeB) via spin-torque ferromagnetic resonance (ST-FMR) technique. Our experiments reveal the existence of a magnetization-independent spin current. Although there is a strong exchange interaction in FM, the spin current does not dephase as quickly as expected. Furthermore, we estimate the spin-torque efficiency $\ensuremath{\xi}$ of NiFe was 0.009, which is about $25%$ of the spin-torque efficiency of Pt. These results indicate that the spin Hall effect of FM should also be taken into account when investigating FM/HM heterostructures, and furthermore this effect can also benefit from the development of spin-orbit torque devices.

Skyrmions, magnetic textures with topological stability, hold promises for high-density and energy-efficient information storage devices owing to their small size and low driving-current density. Precise creation of a single nanoscale skyrmion is a prerequisite to further understand the skyrmion physics and tailor skyrmion-based applications. Here, we demonstrate the creation of individual skyrmions at zero-field in an exchange-biased magnetic multilayer with exposure to soft X-rays. In particular, a single skyrmion with 100-nm size can be created at the desired position using a focused X-ray spot of sub-50-nm size. This single skyrmion creation is driven by the X-ray-induced modification of the antiferromagnetic order and the corresponding exchange bias. Furthermore, artificial skyrmion lattices with various arrangements can be patterned using X-ray. These results demonstrate the potential of accurate optical control of single skyrmion at sub-100 nm scale. We envision that X-ray could serve as a versatile tool for local manipulation of magnetic orders., Skyrmions are objects with whirled magnetization protected by their topology that can be created by different means, however, without control of their position. Here, the authors present a method exploiting x-rays to create skyrmions at the beam position allowing for creation of artificial skyrmion lattices.

Magnetic two-dimensional (2D) van der Waals (vdWs) materials are receiving increased attention due to their exceptional properties and potential applications in spintronic devices. Because exchange bias and spin–orbit torque (SOT)-driven magnetization switching are basic ingredients for spintronic devices, it is of pivotal importance to demonstrate these effects in the 2D vdWs material-based magnetic heterostructures. In this work, we employ a vacuum exfoliation approach to fabricate Fe3GeTe2 (FGT)/Ir22Mn78 (IrMn) and FGT/Pt bilayers, which have high-quality interfaces. An out-of-plane exchange bias of up to 895 Oe is obtained in the former bilayer, which is larger than that of the previously studied bilayers. In the latter bilayer, the SOT switching of the perpendicularly magnetized FGT is realized, which exhibits higher SOT-driven switching performance compared to the previously studied FGT/Pt bilayer devices with interfacial oxidation. The minimum of SOT efficiency is further determined to be 0.18 ± 0.04, comparable to the previously reported values for the Pt/Co and Pt/CoFeB bilayers. This work highlights the importance of the high-quality interface for the exchange bias and SOT effect and may pave the way for implementing 2D vdWs in spintronic devices.

Spintronics is regarded as a promising solution for resolving the major challenging issues related to the scaling of Si-based complementary metal–oxide–semiconductor (CMOS) technology as it offers the advantages of combing the spin and charge degrees of freedom. After decades of progress, the quintessence to achieve practical low-dissipation applications lies in the ability to manipulate magnetic states by electric field via several different physical mechanisms. Among them, the emergence of the spin-orbit coupling engineering has been shown to dramatically reduce energy dissipation and improve the performance as well as to multiply spintronic device possibilities and functionalities for a new generation of ultralow-power nonvolatile spintronic systems. This article provides a review of the current development including fundamental physics and experimental implementations of electric-field-controlled ferromagnetism in dilute magnetic semiconductors, voltage control of magnetic anisotropy, spin-orbit-torque-assisted magnetization switching, and antiferromagnetic (AFM) material-based spin-orbitronic systems. We provide an assessment in terms of scaling of energy, speed, and size. Finally, we offer an outlook of electric-field-controlled spintronic applications, particularly in view of their integration with CMOS to form hybrid spintronic circuits.

International audience; charge-spin conversion and ii) the speed of SOT switching. Generally, SOT in a magnetic layer originates from the spin current injection from the adjacent layer with strong spin-orbit coupling (SOC). The charge-spin conversion efficiency is vital and can be quantified as the θ = / SHE s 3D e 3D J J (dimensionless) or q J J t θ = = / / ICS s 3D e 2D SHE s , where s 3D J represents the 3D spin current density; e 3D J and e 2D J represent the 3D and 2D electric (charge) current density, respectively; and t s represents the effective SOC thickness. In the conventional SOC materials such as HMs, in principle, the θ SHE should be much less than 1, which limits their potential applications in the ultralow power magnetization manipulation. [4] In topological insulators (TIs), SOC from topologically protected surface states, where the spin and orbital angular momenta are locked (spin-momentum locking [5-7]), gives rise to a very large θ SHE [8-12] (or q ICS) and the resulting ultralow switching current density [13-15] at low temperature. Recently, several works have reported the room-temperature SOT switching by TIs, [16-19] which opens the door for the applications of topological insulators. However, there are fundamental limitations of FMs: the low switching speed (≈ns) and the stray-field interaction, which limit the operation speed and the density of magnetic memory, respectively. Antiferromagnets (AFMs) can afford the THz ultrafast spin dynamics; [20] however, AFMs produce zero stray field and zero spin polarization because of the opposite coupled spin lattices from the same element, which makes it difficult to detect the antiferromagnetic order efficiently. [21] Ferrimagnets have two antiferromagnetically coupled spin sublattices, and the contribution of each spin sublattice to their properties can be tuned by the composition or temperature. At the magnetic compensation point, ferrimagnets show similar properties of AFMs, such as ultrafast spin dynamics, [22,23] while the detection is still feasible because of the different responses to the optical or electrical excitations from two spin sublattices. [23-25] Here, we combine TIs [Bi 2 Se 3 and (BiSb) 2 Te 3 ] with nearly compensated ferrimagnets [Gd x (FeCo) 1−x ], and investigate the room-temperature SOT in TI/Gd x (FeCo) 1−x systems. By changing the composition of Gd x (FeCo) 1−x , we can tune the net magnetic moment and the dominated spin sublattice (CoFe-rich and Gd-rich). The robust room-temperature SOT switching Utilizing spin-orbit torque (SOT) to switch a magnetic moment provides a promising route for low-power-dissipation spintronic devices. Here, the SOT switching of a nearly compensated ferrimagnet Gd x (FeCo) 1−x by the topological insulator [Bi 2 Se 3 and (BiSb) 2 Te 3 ] is investigated at room temperature. The switching current density of (BiSb) 2 Te 3 (1.20 × 10 5 A cm −2) is more than one order of magnitude smaller than that in conventional heavy-metal-based structures, which indicates the ultrahigh efficiency of charge-spin conversion (>1) in topological surface states. By tuning the net magnetic moment of Gd x (FeCo) 1−x via changing the composition, the SOT efficiency has a significant enhancement (6.5 times) near the magnetic compensation point, and at the same time the switching speed can be as fast as several picoseconds. Combining the topological surface states and the nearly compensated ferrimagnets provides a promising route for practical energy-efficient and high-speed spintronic devices. Topological Spintronics Spintronic devices have been considered as one of the promising candidates for the next-generation memory and logic devices, and spin-orbit torque (SOT) provides an efficient way to manipulate the magnetic moment by electrical method with ultralow power dissipation and ultrafast operating speed. [1-3] Beyond previous studies of SOT switching based on conventional heavy metal/ferromagnet (HM/FM) heterostructures, two crucial issues need to be resolved: improving: i) the efficiency of

We use temperature-dependent Hall measurements to identify contributions of spin Hall, magnetic proximity, and sublattice effects to the anomalous Hall signal in heavy metal/ferrimagnetic insulator heterostructures with perpendicular magnetic anisotropy. This approach enables detection of both the magnetic proximity effect onset temperature and the magnetization compensation temperature and provides essential information regarding the interfacial exchange coupling. Onset of a magnetic proximity effect yields a local extremum in the temperature-dependent anomalous Hall signal, which occurs at higher temperature as magnetic insulator thickness increases. This magnetic proximity effect onset occurs at much higher temperature in Pt than W. The magnetization compensation point is identified by a sharp anomalous Hall sign change and divergent coercive field. We directly probe the magnetic proximity effect using x-ray magnetic circular dichroism and polarized neutron reflectometry, which reveal an antiferromagnetic coupling between W and the magnetic insulator. Finally, we summarize the exchange-coupling configurations and the anomalous Hall-effect sign of the magnetized heavy metal in various heavy metal/magnetic insulator heterostructures.

Spin-orbit torque flips the perpendicular magnetic moment of a 2D van der Waals magnet., The recent discovery of ferromagnetism in two-dimensional (2D) van der Waals (vdW) materials holds promises for spintronic devices with exceptional properties. However, to use 2D vdW magnets for building spintronic nanodevices such as magnetic memories, key challenges remain in terms of effectively switching the magnetization from one state to the other electrically. Here, we devise a bilayer structure of Fe3GeTe2/Pt, in which the magnetization of few-layered Fe3GeTe2 can be effectively switched by the spin-orbit torques (SOTs) originated from the current flowing in the Pt layer. The effective magnetic fields corresponding to the SOTs are further quantitatively characterized using harmonic measurements. Our demonstration of the SOT-driven magnetization switching in a 2D vdW magnet could pave the way for implementing low-dimensional materials in the next-generation spintronic applications.

Gradual magnetization switching driven by spin–orbit torque (SOT) is preferred for neuromorphic computing in a spintronic manner. Here we have applied focused ion beam to selectively illuminate patterned regions in a Pt/Co/MgO strip with perpendicular magnetic anisotropy, soften the illuminated areas, and realize the gradual switching by a SOT-driven nucleation process. It is found that a large in-plane field is helpful to reduce the nucleation barrier, increase the number of nucleated domains and intermediate states during the switching progress, and finally flatten the switching curve. We proposed a phenomenological model for descripting the current dependence of magnetization and the dependence of the number of nucleation domains on the applied current and magnetic field. This study may promote the birth of SOT devices applicable in neuromorphic computing applications.

A phenomenon of Y3Fe5O12 (YIG)-ferromagnetic-resonance (FMR) linewidth broadening in magnetically coupled YIG-Co films is reported. The broadened linewidth cannot be attributed to previous effects which only depend on the crystalline quality of YIG. Through micromagnetic simulation, the origin of this broadened linewidth was found to be a new type of inhomogeneity effect induced by the inhomogeneous dipolar field of Co layer. Furthermore, it shows that the extent of the linewidth broadening depends on the magnetization state of Co. The broadening is remarkable when the Co domains are random. The influence of Co on YIG-FMR linewidth still exists when the spacer between YIG and Co is 315 nm, indicating it is a long-range effect due to the dipolar origin.

Current-induced spin–orbit torques (SOTs) in heavy metal/ferromagnet heterostructures have emerged as an efficient method for magnetization switching with applications in nonvolatile magnetic memory and logic devices. However, experimental realization of SOT switching of perpendicular magnetization requires an additional inversion symmetry breaking, calling for modifications of the conventional SOT heterostructures. In this work, we study SOTs and deterministic switching of perpendicular magnetization by inserting different asymmetric dusting layers at the heavy metal/ferromagnet interface. Similar to the previous works with lateral structural asymmetry, we study the emergence of current-induced perpendicular effective magnetic fields (Hzeff). By examining three different material combinations of heavy metal/dusting layers (W/IrMn, Pt/IrMn, and W/Ta), we shed light on the origins of Hzeff; we show that Hzeff is generically created in all the studied asymmetric structures, has a close correlation with the interfacial magnetic anisotropy, and is independent of the signs of spin Hall angles of the materials. Furthermore, we show that the induction of Hzeff enables field-free deterministic SOT switching of perpendicular magnetization. Our results can be used in designing SOT heterostructures for practical applications in nonvolatile technologies.

Spin orbit torque-driven magnetization switching in perpendicularly magnetized thin film relies on an extra in-plane magnetic field to break the in-plane magnetic symmetry, which is an obstacle for the integration of spin–orbit torque-based spintronic devices. Here, we propose a simple method to realize the field-free spin–orbit torque-driven magnetization switching by exploiting a tilted magnetic anisotropy, which is caused by the direct coupling of two ferromagnetic layers in the Pt/Co/FeNi/Ta structure. When preparing the sample, a 1000 Oe in-plane magnetic field was applied to ensure the magnetic moment deviating a small angle from the out-of-plane direction to this in-plane field direction. We experimentally demonstrate the deterministic field-free magnetization switching in Pt/Co/FeNi/Ta by the field-like spin–orbit torques when the electric current is applied perpendicular to this in-plane field direction. The switching performance is slightly degraded with the critical switching current density and thermal stability factor, respectively, reaching 6.4 × 106 A/cm2 and 25 due to the slightly decreased spin–orbit torque efficiency and perpendicular magnetic anisotropy with introducing the FeNi layers. Our work paves the way for realizing the field-free magnetization switching by spin–orbit torques.

In this work, we present an ingenious method to fabricate self-aligned nanoscale Hall devices using chemically synthesized nanowires as both etching and deposition masks. This versatile method can be extensively used to make nanoribbons out of arbitrary thin films without the need for extremely high alignment accuracy to define the metal contacts. The fabricated nanoribbon width scales with the mask nanowire width (diameter), and it can be easily reduced down to tens of nanometers. The self-aligned metal contacts from the sidewall extend to the top surface of the nanoribbon, and the overlap can be controlled by tuning the deposition recipe. To demonstrate the feasibility, we have fabricated Ta/CoFeB/MgO nanoribbons sputtered on a SiO2/Si substrate with different metal contacts, using synthesized SnO2 nanowires as masks. Anomalous Hall effect measurements have been carried out on the fabricated nanoscale Hall device in order to study the current-induced magnetization switching in the nanoscale heavy metal/ferromagnet heterostructure, which has shown distinct switching behaviors from micron-scale devices. The developed method provides a useful fabrication platform to probe the charge and spin transport in the nanoscale regime.

The perpendicular magnetic anisotropy (PMA), current-induced spin–orbit torques (SOTs), and Dzyaloshinskii–Moriya interaction (DMI) in the as-grown W or Ta/Ir22Mn78(IrMn)/CoFeB/MgO stacks with varying IrMn layer thicknesses were investigated. The in-plane magnetized W/CoFeB/MgO sample becomes perpendicularly magnetized after inserting the IrMn layer without the requirement of the annealing process. The effective magnetization fields 4πMeff show a nonmonotonic dependence on the IrMn layer thickness, which reaches the maximum in magnitude at a thickness of tIrMn = 0.75 nm. The SOT effective fields corresponding to damping-like and field-like torques decrease with the insertion layer thickness. Moreover, the variation of the IrMn layer thickness leads to the change of the DMI in magnitude and sign change from positive (favoring right-handed chirality) to negative (favoring left-handed chirality). The realization of changing the PMA, SOTs, and DMI by inserting the IrMn layer provides more flexibility in the design of spintronic devices.

Manipulation of magnetization by electric-current-induced spin-orbit torque (SOT) is of great importance for spintronic applications because of its merits in energy-efficient and high-speed operation. An ideal material for SOT applications should possess high charge-spin conversion efficiency and high electrical conductivity. Recently, transition metal dichalcogenides (TMDs) emerge as intriguing platforms for SOT study because of their controllability in spin-orbit coupling, conductivity, and energy band topology. Although TMDs show great potentials in SOT applications, the present study is restricted to the mechanically exfoliated samples with small sizes and relatively low conductivities. Here, a manufacturable recipe is developed to fabricate large-area thin films of PtTe2 , a type-II Dirac semimetal, to study their capability of generating SOT. Large SOT efficiency together with high conductivity results in a giant spin Hall conductivity of PtTe2 thin films, which is the largest value among the presently reported TMDs. It is further demonstrated that the SOT from PtTe2 layer can switch a perpendicularly magnetized CoTb layer efficiently. This work paves the way for employing PtTe2 -like TMDs for wafer-scale spintronic device applications.

Two-dimensional layered materials with remarkable thermoelectric coefficients are promising candidates for sustainable thermopower batteries. Here, we investigate the anomalous Nernst effect and anomalous Hall effect in the polycrystalline Fe 3 GeTe 2 thin films. While its Seebeck coefficient and anomalous Hall angle (or magnetization) monotonously increase and decrease with temperature, respectively, the anomalous Nernst coefficient of the Fe 3 GeTe 2 films exhibits a peak value of 0.28 μV K–1 T–1 at 150 K, which is the compromised outcome between the enhanced Seebeck effect and the gradually weakened magnetism with elevating temperature. A noticeable anomalous Nernst effect observed in Fe 3 GeTe 2 sheds light on the low-temperature application of two-dimensional layered materials in spin-caloritronics.

Heavy metals and topological insulators are promising materials for converting charge current into spin current for efficient manipulation of magnetization states in magnetic devices [1]–[5]. One of the most important parameters is the charge-to-spin conversion (CS) efficiency. Improving CS efficiency is critical for reducing write current of the emerging nonvolatile memory technology, spin-orbit torque MRAM (SOT-MRAM) [2], which provides comparable speed with SRAM but with a much higher memory capacity. Here, we measure CS efficiency in various topological insulators (TIs) using second-harmonic method (2eo-method) and obtain a record-high value 8.33±0.65 for insulating (BiSb)2 Te3 at room temperature. We first establish the consistency of CS efficiency obtained between spin-torque ferromagnetic resonance (ST-FMR) and 2eo-method. Then, we systematically investigate the CS efficiency in a bilayer consisting of a metallic Bi2Se3 and a CoFeB thin film using 2eo-method. By tuning the Fermi level of TI layer into bulk band gap using (BiSb)2 Te3, we improve the CS efficiency by an order of magnitude.

The key physics of the spin valve involves spin-polarized conduction electrons propagating between two magnetic layers such that the device conductance is controlled by the relative magnetization orientation of two magnetic layers. Here, we report the effect of a magnon valve which is made of two ferromagnetic insulators (YIG) separated by a nonmagnetic spacer layer (Au). When a thermal gradient is applied perpendicular to the layers, the inverse spin Hall voltage output detected by a Pt bar placed on top of the magnon valve depends on the relative orientation of the magnetization of two YIG layers, indicating the magnon current induced by spin Seebeck effect at one layer affects the magnon current in the other layer separated by Au. We interpret the magnon valve effect by the angular momentum conversion and propagation between magnons in two YIG layers and conduction electrons in the Au layer. The temperature dependence of magnon valve ratio shows approximately a power law, supporting the above magnon-electron spin conversion mechanism. This work opens a new class of valve structures beyond the conventional spin valves., 13 pages, 4 figures

Deterministic switching of magnetization via spin-orbit torque, without an external magnetic field, is an attractive approach for nonvolatile memory and logic devices. This can be accomplished using in-plane exchange bias instead of an applied field. The authors study the mechanism of this switching, and expose the importance of Joule heating: Field-free current-driven switching is achieved in experiments, but care must be taken so that heating does not decrease exchange bias and spoil the switching.

Current-induced spin-orbit torques (SOTs) in structurally asymmetric multilayers have been used to efficiently manipulate magnetization. In a structure with vertical symmetry breaking, a damping-like SOT can deterministically switch a perpendicular magnet, provided an in-plane magnetic field is applied. Recently, it has been further demonstrated that the in-plane magnetic field can be eliminated by introducing a new type of perpendicular field-like SOT via incorporating a lateral structural asymmetry into the device. Typically, however, when a current is applied to such devices with combined vertical and lateral asymmetries, both the perpendicular field-like torque and the damping-like torque coexist, hence jointly affecting the magnetization switching behavior. Here, we study perpendicular magnetization switching driven by the combination of the perpendicular field-like and the damping-like SOTs, which exhibits deterministic switching mediated through domain wall propagation. It is demonstrated that the role of the damping-like SOT in the deterministic switching is highly dependent on the magnetization direction in the domain wall. By contrast, the perpendicular field-like SOT is solely determined by the relative orientation between the lateral structural asymmetry and the current direction, regardless of the magnetization direction in the domain wall. The experimental results further the understanding of SOTs-induced switching, with implications for spintronic devices.

Ferromagnetism in topological insulators (TIs) opens a topologically non-trivial exchange band gap, providing an exciting platform to manipulate the topological order through an external magnetic field. Here, we experimentally show that the surface of an antiferromagnetic thin film can independently control the topological order of the top and the bottom surface states of a TI thin film through proximity couplings. During the magnetization reversal in a field scan, two intermediate spin configurations stem from unsynchronized magnetic switchings of the top and the bottom AFM/TI interfaces. These magnetic configurations are shown to result in new topological phases with non-zero Chern numbers for each surface, introducing two counter-propagating chiral edge modes inside the exchange gap. This change in the number of transport channels, as the result of the topological transitions, induces antisymmetric magneto-resistance spikes during the magnetization reversal. With the high Neel ordering temperature provided by the antiferromagnetic layers, the signature of the induced topological transition persists in transport measurements up to a temperature of around 90 K, a factor of three over the Curie temperature in a typical magnetically doped TI thin film.

Electric-field manipulation of magnetic order has proved of both fundamental and technological importance in spintronic devices. So far, electric-field control of ferromagnetism, magnetization and magnetic anisotropy has been explored in various magnetic materials, but the efficient electric-field control of spin-orbit torque (SOT) still remains elusive. Here, we report the effective electric-field control of a giant SOT in a Cr-doped topological insulator (TI) thin film using a top-gate FET structure. The SOT strength can be modulated by a factor of 4 within the accessible gate voltage range, and it shows strong correlation with the spin-polarized surface current in the film. Furthermore, we demonstrate the magnetization switching by scanning gate voltage with constant current and in-plane magnetic field applied in the film. The effective electric-field control of SOT and the giant spin-torque efficiency in Cr-doped TI may lead to the development of energy-efficient gate-controlled spin-torque devices compatible with modern field-effect semiconductor technologies., 22 pages, 4 figures

We theoretically study equilibrium and dynamic properties of nanosized magnetic skyrmions in thin magnetic films with broken inversion symmetry, where electric field couples to magnetization via spin-orbit coupling. Based on a symmetry-based phenomenology and micromagnetic simulations we show that this electric-field coupling, via renormalizing the micromagnetic energy, modifies the equilibrium properties of the skyrmion. This change, in turn, results in a significant alteration of the current-induced skyrmion motion. Particularly, speed and direction of the skyrmion can be manipulated by designing a desired energy landscape electrically, which we describe within Thiele's analytical model and demonstrate in micromagnetic simulations including electric-field-controlled magnetic anisotropy. We additionally use this electric-field control to construct gates for controlling skyrmion motion exhibiting a transistor-like and multiplexer-like function. The proposed electric-field effect can thus provide a low energy electrical knob to extend the reach of information processing with skyrmions., Comment: 9 pages, 7 figures

The current-induced spin-orbit torques (SOTs) in the perpendicularly magnetized Ir22Mn78/Co20Fe60B20/MgO structures are investigated. The damping- and field-like torques are characterized using a harmonic technique. The spin Hall angle of Ir22Mn78 is determined to be θSHE = +0.057 ± 0.002. The SOT-driven magnetization switching is also demonstrated with the assistance of an external in-plane field. Furthermore, the magneto-optical Kerr effect imaging experiments show that the magnetization switching is realized through domain nucleation and domain wall motion. These results may promise potential practical applications in high-performance SOT devices based on the antiferromagnetic materials.

We study the heavy metal layer thickness dependence of the current-induced spin-orbit torque (SOT) in perpendicularly magnetized Hf|CoFeB|MgO multilayer structures. The damping-like (DL) current-induced SOT is determined by vector anomalous Hall effect measurements. A non-monotonic behavior in the DL-SOT is found as a function of the thickness of the heavy-metal layer. The sign of the DL-SOT changes with increasing the thickness of the Hf layer in the trilayer structure. As a result, in the current-driven magnetization switching, the preferred direction of switching for a given current direction changes when the Hf thickness is increased above ~7 nm. Although there might be a couple of reasons for this unexpected behavior in DL-SOT, such as the roughness in the interfaces and/or impurity based electric potential in the heavy metal, one can deduce a roughness dependence sign reversal in DL-SOT in our trilayer structure. © 2016 Author(s).

We study spin-orbit-torque (SOT)-driven magnetization switching in perpendicularly magnetized Ta/Mo/Co40Fe40B20 (CoFeB)/MgO films. The thermal tolerance of the perpendicular magnetic anisotropy (PMA) is enhanced, and the films sustain the PMA at annealing temperatures of up to 430 °C, due to the ultra-thin Mo layer inserted between the Ta and CoFeB layers. More importantly, the Mo insertion layer also allows for the transmission of the spin current generated in the Ta layer due to spin Hall effect, which generates a damping-like SOT and is able to switch the perpendicular magnetization. When the Ta layer is replaced by a Pt layer, i.e., in a Pt/Mo/CoFeB/MgO multilayer, the direction of the SOT-induced damping-like effective field becomes opposite because of the opposite sign of spin Hall angle in Pt, which indicates that the SOT-driven switching is dominated by the spin current generated in the Ta or Pt layer rather than the Mo layer. Quantitative characterization through harmonic measurements reveals that the large SOT effective field is preserved for high annealing temperatures. This work provides a route to applying SOT in devices requiring high temperature processing steps during the back-end-of-line processes.

Compared with current-controlled magnetization switching in a perpendicular magnetic tunnel junction (MTJ), electric field- or voltage-induced magnetization switching reduces the writing energy of the memory cell, which also results in increased memory density. In this work, an ultra-thin PZT film with high dielectric constant was integrated into the tunneling oxide layer to enhance the voltage-controlled magnetic anisotropy (VCMA) effect. The growth of MTJ stacks with an MgO/PZT/MgO tunnel barrier was performed using a combination of sputtering and atomic layer deposition techniques. The fabricated MTJs with the MgO/PZT/MgO barrier demonstrate a VCMA coefficient, which is ∼40% higher (19.8 ± 1.3 fJ/V m) than the control sample MTJs with an MgO barrier (14.3 ± 2.7 fJ/V m). The MTJs with the MgO/PZT/MgO barrier also possess a sizeable tunneling magnetoresistance (TMR) of more than 50% at room temperature, comparable to the control MTJs with an MgO barrier. The TMR and enhanced VCMA effect demonstrated si...