SOT-induced Magnetization Switching Research Articles

Unconventional angular dependence of spin-orbit torque-induced harmonic Hall resistance in Pt/YIG bilayers

Spin-orbit torque (SOT), arising from spin-orbit coupling-induced spin currents, provides efficient control of magnetization. SOT characterization involving harmonic Hall resistances is typically done in low-current regimes, distinct from high-current regimes, where SOT-induced magnetization switching occurs. In this study, we investigate azimuthal angle (ϕ)-dependent harmonic Hall resistances of a Pt/yttrium iron garnet (YIG) layer across a wide range of measurement currents. Under low-current conditions, conventional ϕ-dependent Hall resistances are observed; the first harmonic Hall resistance exhibits sin 2ϕ behavior and the second harmonic Hall resistance comprises cos ϕ and cos 3ϕ terms. Interestingly, with increasing current, higher-order angular-dependent terms become non-negligible, referring to the sin 4ϕ and sin 6ϕ terms for the first harmonic and the cos 5ϕ and cos 7ϕ terms for the second harmonic Hall resistances. We attribute this unconventional angular dependence to the nonlinear response of magnetization direction to SOT, emphasizing its relevance to understanding the magnetization dynamics during SOT-induced switching under large currents.

Noncollinear spin texture-driven torque in deterministic spin–orbit torque-induced magnetization switching

To reveal the role of chirality on field-free spin–orbit torque (SOT) induced magnetization switching, we propose an existence of z-torque through the formation of noncollinear spin texture during SOT-induced magnetization switching in a laterally two-level perpendicular magnetic anisotropy (PMA) system. For the investigation of torque, we simulate magnetization dynamics in the two-level PMA system with SOT, which generates the noncollinear spin texture. From the spatial distribution of magnetic energy, we reveal the additional z-directional torque contribution in the noncollinear spin texture, which is unexpected in the conventional SOT-induced magnetization switching in collinear spin texture. The z-directional torque originates from the interaction between the chirality of the noncollinear spin texture and the interfacial Dzyaloshinskii-Moriya interaction of the system. Furthermore, the experimental observation of the asymmetric magnetization switching to the direction of the current flow in the two-level PMA system supports our theoretical expectation.

An energy efficient way for quantitative magnetization switching

Recent advancements in electrically controlled spin devices have been made possible through the use of multiferroic systems comprising ferroelectric (FE) and ferromagnetic (FM) materials. This progress provides a promising avenue for developing energy-efficient devices that allow for electrically controlled magnetization switching. In this study, we fabricated spin orbit torque (SOT) devices using multiferroic composites and examined the angular dependence of SOT effects on localized in-plane strain induced by an out-of-plane electric field applied to the piezoelectric substrate. The induced strain precisely modulates magnetization switching via the SOT effect in multiferroic heterostructures, which also exhibit remarkable capability to modulate strain along different orientations – a feature with great potential for future applications in logic device arrays. To investigate the influence of electric fields on magnetization switching, harmonic Hall measurements, synchrotron-powered x-ray magnetic circular dichroism-photoemission electron microscopy (XMCD-PEEM), x-ray diffraction (XRD), magnetic force microscopy (MFM), and micromagnetic simulation were conducted. The results demonstrate that electric-field-induced strain enables precise control of SOT-induced magnetization switching with significantly reduced energy consumption, making it highly suitable for next-generation spin logic devices.

Tilted Perpendicular Anisotropy-Induced Spin-Orbit Ratchet Effects

Abstract Using micromagnetic simulations, in this study we demonstrate the tilted perpendicular anisotropy -induced spin-orbit ratchet effect. During spin-orbit torque (SOT)-induced magnetization switching, the critical currents required to switch between the two magnetization states (upward and downward magnetization) are asymmetric. In addition, in the nanowire structure, tilted anisotropy induces formation of tilted domain walls (DWs). The tilted DWs exhibit a ratchet behavior during motion. The ratchet effect during switching and DW motions can be tuned by changing the current direction with respect to the tilting direction of anisotropy. The ratchet motion of the DWs can be used to mimic the Leaky–Integrate–Fire function of a biological neuron, especially the asymmetric property of the “potential” and “reset” processes. Our results provide a full understanding of the influence of tilted perpendicular anisotropy on SOT-induced magnetization switching and DW motion, and are beneficial for design of further SOT-based 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.

Current-Induced Magnetization Switching Behavior in Perpendicular Magnetized L10-MnAl/B2-CoGa Bilayer

Rare-earth-free Mn-based binary alloy L10-MnAl with bulk perpendicular magnetic anisotropy (PMA) holds promise for high-performance magnetic random access memory (MRAM) devices driven by spin-orbit torque (SOT). However, the lattice-mismatch issue makes it challenging to place conventional spin current sources, such as heavy metals, between L10-MnAl layers and substrates. In this work, we propose a solution by using the B2-CoGa alloy as the spin current source. The lattice-matching enables high-quality epitaxial growth of 2-nm-thick L10-MnAl on B2-CoGa, and the L10-MnAl exhibits a large PMA constant of 1.04 × 106 J/m3. Subsequently, the considerable spin Hall effect in B2-CoGa enables the achievement of SOT-induced deterministic magnetization switching. Moreover, we quantitatively determine the SOT efficiency in the bilayer. Furthermore, we design an L10-MnAl/B2-CoGa/Co2MnGa structure to achieve field-free magnetic switching. Our results provide valuable insights for achieving high-performance SOT-MRAM devices based on L10-MnAl alloy.

Modulation of switching current density in T-type magnetic structure through magnetic anisotropy

We investigate magnetic field driven domain wall motion and spin-orbit torque (SOT) induced perpendicular magnetization switching of the ferrimagnetic layer in a T-type magnetic heterojunction with the structure of Pt/Co/Ta/Co81Tb19/Ta, where the bottom Co layer has in-plane magnetic anisotropy and the top Co81Tb19 layer has perpendicular magnetic anisotropy. It is found that the magnetic field driven domain wall motion and the depinning field of the Co81Tb19 layer can be effectively tuned by the thickness of the Co layer (tCo). Meanwhile, SOT-induced perpendicular magnetization switching of the Co81Tb19 layer is observed, and the critical switching current density first decreases and then increases with increasing the thickness of the Co layer. A lower critical switching current density was obtained at the optimized thickness (tCo = 1.2 nm), resulting in a 37% reduction in critical switching current density. Meanwhile, the research reveals a strong correlation between critical switching current density and magnetic anisotropy field, indicating that tuning the magnetic anisotropy is an efficient way to modulate the switching current density in a T-type magnetic structure. These results provide a new perspective on SOT manipulation and will be useful for the design of SOT-based devices.

Spin–orbit torque true random number generator with thermal stability

Interfacial Dzyaloshinskii–Moriya interaction (DMI) plays a pivotal role in spin–orbit torque (SOT)-induced magnetization switching, notably seen in deterministic switching even in the absence of an external magnetic field at 0 K. However, in SOT devices operating at room temperature, thermal fluctuations contribute significantly to magnetization switching due to the altered energy profile caused by DMI. In this work, we unveil that unlike the deterministic magnetization switching observed at 0 K, SOT-induced magnetization switching assisted by DMI is highly stochastic. Following the SOT-induced nucleation of a domain wall (DW), thermal fluctuations can induce rapid back-and-forth DW motion under the influence of a current pulse, resulting in stochastic switching. Furthermore, our findings indicate that the switching probability remains stable as the temperature increases. These results illustrate that SOT-induced magnetization switching assisted by DMI is well-suited for a true random number generator with robust thermal stability.

Symmetry of spin–orbit torque induced meron annihilation

Electrical manipulation of the topological charges of magnetic vortices is of vital importance for the development of vortex-based devices. Here, we show that the spin–orbit torque (SOT) effect can be employed to deterministically and selectively annihilate the vortex core, and this process exhibits a symmetry that is consistent with that of the SOT-induced magnetization switching in perpendicularly magnetized systems. By changing the SOT current pulse direction, it is also possible to write back a vortex with random topological charges from the quasi-single-domain state after annihilation. These intriguing results can be utilized as a random topological charge generator and applied in stochastic computing where a tunable random stream source plays a central role.

Field-Free Spin-Orbit Torque-Induced Magnetization Switching in the IrMn/CoTb Bilayers with a Spontaneous In-Plane Exchange Bias.

Spin-orbit torque (SOT)-induced magnetization switching in ferrimagnetic materials is promising for application in a new generation of information storage devices. Here, we demonstrate SOT-induced field-free magnetization switching of the perpendicularly magnetized CoTb ferrimagnet layer in the IrMn/CoTb bilayer, in which the in-plane magnetic inversion symmetry is broken by a spontaneous in-plane exchange bias (IEB) established by isothermal crystallization of the IrMn layer. We obtain a significant SOT effective field acting on the CoTb layer and a large effective spin Hall angle in this system, derived by the second harmonic voltage measurement method. Moreover, the IrMn/CoTb bilayer demonstrates multistate magnetic switching behavior with different amplitudes of current pulses at zero field, which can mimic the synaptic weight updates in the neuromorphic network. These findings make the IrMn/CoTb bilayer with spontaneous IEB a promising candidate for potential applications in multilevel storage and neuromorphic computing.

Micromagnetic simulation for random magnetization switching process of a spin–orbit true random number generator

True random number generator (TRNG) is an important component for modern information security technologies. Among the candidates, TRNG with spin–orbit torque (SOT)-induced probabilistic magnetization switching is expected to be competitive for its advantages in anti-radiation, unlimited endurance, robust stability, and broad temperature range. However, specific mechanism of the randomness in SOT-induced magnetization switching are still not clear, which limits the demonstration of applicable SOT-TRNG. Here, we performed micromagnetic simulation of the SOT-induced probabilistic magnetization switching by using MuMax3. When various thermal noise seeds were introduced, not only stochastic precession trails within the current pulses, but also stochastic precession tendency after removing the current pulse were obtained, together resulting in random final magnetization states. Particularly, the consistency between mz orientations at the current withdraw moment and at the final stabilized magnetization state shows a positive correlation with the fluctuation range of mz during the current pulse duration, the latter of which narrows with the increasing of channel current density. Our work suggests the importance of thermal noise-related magnetization precession variations on the SOT-induced probabilistic magnetization switching, and suggesting the MuMax3 to be a practical tool for simulating the SOT-TRNG.

Field-free switching and high spin–orbit torque efficiency in Co/Ir/CoFeB synthetic antiferromagnets deposited on miscut Al2O3 substrates

Ir-CoFeB-based synthetic antiferromagnets (SAFs) are potential candidates as the free layer of the next-generation magnetic tunnel junctions (MTJs) for high speed and density memories due to their perpendicular magnetic anisotropy and strong interlayer exchange coupling. However, the field-free spin–orbit torque (SOT) switching of Ir-CoFeB-based SAFs has rarely been reported, especially in the Co/Ir/CoFeB system with high anti-interference capability and being readily integrated with MTJs. In this paper, SOT-induced magnetization switching and SOT efficiency in Co/Ir/CoFeB SAFs with perpendicular anisotropy and tunable exchange coupling are systemically investigated. A full field-free switching of perpendicular Co/Ir/CoFeB SAFs is realized by depositing them onto crystal miscut Al2O3 substrates, which induce a tilted magnetic anisotropy. Furthermore, by introducing crystalline MgO or amorphous HfO2/SiO2 as the seed layers, the source of the tilted magnetic anisotropy was proved to be from the transverse asymmetry caused by the crystal miscut. Moreover, the crystal miscut enhances the SOT efficiency. The findings provide an approach to reliable field-free switching and high SOT efficiency of Ir-CoFeB-based SAFs for memories as well as logics with low power, fast speed, and high density.

Inversion symmetry breaking in spin–orbit torque-induced magnetization switching to improve the recording density of multi-level magnetoresistive random-access memory

In this study, we prepared a multi-layer Tb–Fe/Pt/Tb–Fe wire to develop a multi-level magnetic memory. By applying current, magnetizations of the Tb–Fe layers were inversion symmetrically switched by spin– orbit torque (SOT) generated from the middle Pt layer. Measurements of SOT showed that its efficiency had opposite polarities in the top and bottom Tb–Fe layers. The switching current density of the top and bottom Tb–Fe layers shifted in opposite directions under a uniform perpendicular magnetic field. Because the perpendicular magnetic field broke the inversion symmetry of SOT generated from the middle Pt layer, it could be used to control the switching current. Our results prove that the additional uniform and perpendicular magnetic field can enhance the controllability of the magnetization state in case of multi-level SOT-induced magnetization switching.

Magnetization switching of a nearly compensated ferrimagnet by enhanced spin-orbit torque

Spin–orbit torques (SOTs) provide an efficient way to electrically manipulate the magnetic order in spintronic devices. Compared with conventional ferromagnetic materials, ferrimagnetic materials have the advantages of antiferromagnetically coupled sublattices and induced ultrafast spin dynamics. In this paper, we study the current-induced magnetization switching in the ferrimagnetic Ta/GdFeCo/MgO system. Robust SOT-induced magnetization switching can be achieved at the magnetic compensation temperature point of around 70 K, where the magnetization is nearly zero and the coercivity can reach almost 3 T. The temperature dependence of the SOT efficiency is quantified by the second harmonic method, and the enhanced SOT efficiency near the magnetic compensation temperature is attributed to the negative exchange coupling between the two sublattices of CoFe and Gd. This work demonstrates the SOT switching of the nearly compensated ferrimagnet, with great potential for future magnetic interaction-free and ultrafast ferrimagnetic applications.

Origin of the anomalous Hall effect at the magnetic insulator/heavy metals interface

Ferrimagnetic insulators (FIMIs) are considered to be promising candidates in spin–orbit torque (SOT) devices due to their ability to propagate a spin current by magnons without Ohmic losses owing to the absence of electronic scattering. Moreover, any electrical current shunt is avoided in magnetic insulating materials. On the other hand, SOT-induced magnetization switching is generally measured through the anomalous Hall effect (AHE) in FIMI/heavy metal (HM) systems. However, the origin of AHE in FIMI/HM remains elusive since charges flow only in the HM. Here, we experimentally demonstrate that the AHE has the same origin as the spin Hall magnetoresistance (SMR). To this end, we have studied two bilayer heterostructures, Tm3Fe5O12(TmIG)/W and TmIG/Pt, where we ensure opposite spin Hall effect (SHE) signs for two heavy metals (W and Pt). The magnitudes of AHE and SMR are found to be larger for TmIG/W than TmIG/Pt. We have also evidenced the identical polarity of AHE hysteresis in both systems revealing a square dependency on the spin Hall angle whereas the current-induced magnetization switching polarity in TmIG/W is opposite to that of TmIG/Pt as expected for opposite spin Hall angle signs. Our results establish that the AHE and the spin-Hall magnetoresistance in TmIG insulating ferromagnets and heavy metal bilayers originate from the same mechanism.

Spin–Orbit Torques Based on Topological Materials

ConspectusWidespread applications of magnetic devices, such as magnetic random-access memory (MRAM), logic-in memory, and neuromorphic computing devices, require an efficient method to manipulate the local magnetization. One superior mechanism is the spin–orbit torque (SOT) associated with spin currents generated by charge currents in a material with strong spin–orbit coupling. A higher SOT efficiency (i.e., a higher charge-to-spin conversion efficiency) is crucial for low-power SOT device operation. Therefore, materials possessing the merit of a high SOT are urgently desired for the innovation of SOT devices. Among all of the SOT materials, topological materials such as topological insulators and topological semimetals have attracted considerable attention recently because of their nontrivial band structures with strong spin–orbit coupling and expected giant SOT. Therefore, topological materials are regarded as promising candidates for future energy-efficient device applications. However, research in the area of topological material-based spintronics is still in an early stage, and the related physical, material and device issues still need to be addressed. In this Account, we review our recent progress regarding the charge-to-spin conversion and SOT-driven magnetization switching using the emerging topological materials, with an emphasis on topological insulators and Weyl semimetals. First, we introduce the extraordinary physical features of single-layer topological insulators associated with electron transport, charge-spin interconversion, and unique spin textures and subsequently present our SOT results for topological materials mainly for Bi2Se3. This material shows pronounced SOT with the in-plane and out-of-plane component due to the spin momentum locking and hexagonal warping effect, respectively. The SOT can be efficiently modulated by varying the film crystal directions or by interface engineering between Bi2Se3 and a ferromagnet. Thereafter, room-temperature magnetization switching by the electron-mediated or magnon-mediated spin torque is demonstrated in topological insulator-based devices, and the switching current density JC is on the order of ∼105 A/cm2, which is approximately 1 to 2 orders of magnitude smaller than that in heavy metals. Second, we summarize our SOT results in Weyl semimetals. Due to the broken crystal symmetry, Weyl semimetals (e.g., WTe2) can show an additional sizable out-of-plane spin polarization, which can be detected by electrical and optical techniques. Therefore, the Weyl semimetals can possess unconventional out-of-plane damping-like SOT, which will facilitate the field-free magnetization switching. The interface Dzyaloshinskii–Moriya interaction (DMI) is observed in Weyl semimetal/ferromagnet heterostructures, which can affect the domain wall motion. We also demonstrate the energy-efficient SOT-induced magnetization switching in Weyl semimetal-based devices. Third, we discuss the advances in SOT devices with wafer-scale topological materials prepared by industry-compatible techniques such as magnetron sputtering and chemical vapor deposition. These films may have stoichiometry similar to that of single-crystalline topological materials and SOTs as large as in single-crystalline topological materials. Finally, we present our perspectives for practical applications using this emerging family of quantum materials. We anticipate that this Account will deepen the understanding of SOT on topological materials, and our insights will guide the design and applications of high-performance topological material-based SOT devices.

Field-Free Magnetization Switching in a Ferromagnetic Single Layer through Multiple Inversion Asymmetry Engineering.

A simple, reliable, and self-switchable spin-orbit torque (SOT)-induced magnetization switching in a ferromagnetic single layer is needed for the development of next generation fully electrical controllable spintronic devices. In this work, field-free SOT-induced magnetization switching in a CoPt single layer is realized by broken multiple inversion symmetry through simultaneously introducing both oblique sputtering and a vertical composition gradient. A quantitative analysis indicates that multiple inversion asymmetries can produce dynamical bias fields along both z- and x-axes, leading to the observed field-free deterministic magnetization switching. Our study provides a method to accomplish fully electrical manipulation of magnetization in a ferromagnetic single layer without the external magnetic field and auxiliary heavy metal layer, enabling flexible design for future spin-orbit torque-based memory and logic devices.

Role of the chiral spin configuration in field-free spin–orbit torque-induced magnetization switching by a locally injected spin current

For deterministic magnetization switching by spin–orbit torque (SOT) in a perpendicular magnetic anisotropy system, an additional in-plane direction magnetic field is essential to break the lateral symmetry. Realizing chirality in a magnetic ordering system can be one approach for achieving asymmetry in the lateral direction for field-free magnetization switching. However, systematic analysis of the influence of the chiral spin system on deterministic switching is still scarce. We investigate the field-free SOT-induced magnetization switching by using a chiral spin configuration experimentally and theoretically with micromagnetic simulations. We designed a system in which only part of the ferromagnetic layer overlaps with the heavy metal layer in the Pt/Co/MgO structure. Therefore, a spin current exerts only on a local area of the ferromagnetic layer, which results in a Néel-type chiral spin configuration. The induced chiral spin configuration can be stabilized (or destabilized) depending on the sign of the interfacial Dzyaloshinskii–Moriya interaction and the direction of the current. The stabilized spin configuration plays a crucial role in the deterministic switching in the zero field. We expect our findings to widen the perspective on chirality-based all-electrical SOT device applications.

Probing the magnetization switching with in-plane magnetic anisotropy through field-modified magnetoresistance measurement

Effective probing current-induced magnetization switching is highly required in the study of emerging spin–orbit torque (SOT) effect. However, the measurement of in-plane magnetization switching typically relies on the giant/tunneling magnetoresistance measurement in a spin valve structure calling for complicated fabrication process, or the non-electric approach of Kerr imaging technique. Here, we present a reliable and convenient method to electrically probe the SOT-induced in-plane magnetization switching in a simple Hall bar device through analyzing the MR signal modified by a magnetic field. In this case, the symmetry of MR is broken, resulting in a resistance difference for opposite magnetization orientations. Moreover, the feasibility of our method is widely evidenced in heavy metal/ferromagnet (Pt/Ni20Fe80 and W/Co20Fe60B20) and the topological insulator/ferromagnet (Bi2Se3/Ni20Fe80). Our work simplifies the characterization process of the in-plane magnetization switching, which can promote the development of SOT-based devices.

Spin-Orbit Torque in Van der Waals-Layered Materials and Heterostructures.

Spin‐orbit torque (SOT) opens an efficient and versatile avenue for the electrical manipulation of magnetization in spintronic devices. The enhancement of SOT efficiency and reduction of power consumption are key points for the implementation of high‐performance SOT devices, which strongly rely on the spin‐orbit coupling (SOC) strength and magnetic properties of ferromagnetic/non‐magnetic heterostructures. Recently, van der Waals‐layered materials have shown appealing properties for use in efficient SOT applications. On the one hand, transition‐metal dichalcogenides, topological insulators, and graphene‐based heterostructures possess appreciable SOC strength. This feature can efficiently converse the charge current into spin current and result in large SOT. On the other hand, the newly discovered layered magnetic materials provide ultra‐thin and gate‐tunable ferromagnetic candidates for high‐performance SOT devices. In this review, the latest advancements of SOT research in various layered materials are summarized. First, a brief introduction of SOT is given. Second, SOT studies of various layered materials and heterostructures are summarized. Subsequently, progresses on SOT‐induced magnetization switching are presented. Finally, current challenges and prospects for future development are suggested.