Controlling Magnetization in Ferromagnetic Semiconductors by Current-Induced Spin-Orbit Torque.

Antiferromagnets with zero net magnetic moment, strong anti-interference and ultrafast switching speed have potential competitiveness in high-density information storage [1], [2]. Electrical switching of antiferromagnets is at the heart of device application [3]. We will present our recent progress in the current-driven magnetization switching through spin-orbit torque (SOT) in three antiferromagnetic systems, including Mn 2 Au, and [Co/Pd]/Ru/[Co/Pd] synthetic antiferromagnets (SAF). Body centered tetragonal antiferromagnet Mn 2 Au with opposite spin sub-lattices is a unique metallic material for Neelorder spin-orbit torque (SOT) switching. The SOT switching in quasi-epitaxial (103), (101) and (204) Mn 2 Au films prepared by a simple magnetron sputtering method will be discussed. We demonstrate current induced antiferromagnetic moment switching in all the prepared Mn 2 Au films by a short current pulse at room temperature, whereas different orientated films exhibit distinguished switching characters. A direction-independent reversible switching is attained in Mn 2 Au (103) films due to negligible magnetocrystalline anisotropy energy, while for Mn 2 Au (101) and (204) films, the switching is invertible with the current applied along the in-plane easy axis and its vertical axis, but becomes attenuated seriously during initially switching circles when the current is applied along hard axis, because of the existence of magnetocrystalline anisotropy energy [4]. SAF were proposed to replace ferromagnets in magnetic memory devices to reduce the stray field, increase the storage density and improve the thermal stability. We will discuss the SOT in a perpendicularly magnetized Pt/[Co/Pd]/Ru/[Co/ Pd] SAF structure, which exhibits completely compensated magnetization and a high exchange coupling field of 2200 Oe. The magnetizations of two Co/Pd layers can be switched by spin-orbit torque between two antiparallel states simultaneously. The magnetization switching can be read out due to much stronger spin-orbit coupling at bottom Pt/[Co/Pd] interface compared to its upper counterpart without Pt. Both experimental and theoretical analyses unravel that the torque efficiency of antiferromagnetic coupled stacks is significantly higher than the ferromagnetic counterpart, which conquers the exchange coupling field, leading to the critical switching current of SAF comparable to the ferromagnetic coupled one [5], [6]. Besides the fundamental significance, the efficient switching of antiferromagnets by current would advance magnetic memory devices with high density, high speed and low power consumption.

Current-induced switching in CoGa/L10 MnGa/(CoGa)/Pt structure with different thicknesses

I report our effort on realizing three-terminal spintronics devices that can be used for integration with CMOS VLSI; three-terminal devices, in principle, allow higher speed and more reliable operation compared to their two-terminal counterpart [1]. Of particular interest are devices that utilize spin-orbit torque (SOT) switching, which does not require an antiferromagnetically aligned pair of magnetic electrodes like in current-induced domain wall motion devices [2]. The first topic I discuss is a high speed operation of an SOT switching device with a target ferromagnetic pillar having an in-plane magnetic easy axis collinear with the current flow direction in the underneath heavy-metal [3]. We show that one can switch magnetization as fast as 500 ps in this structure; a switching speed not readily available in two-terminal devices utilizing spin-transfer torque (STT) switching because STT requires switching current inversely proportional to the switching speed in this speed range. Another advantage of this scheme is that one can fabricate from exactly the same stack two different SOT devices; another being pillars having in-plane easy-axis but its direction perpendicular to the current direction, where conventional STT switching takes place. Using the two, we discuss the torques operating in the switching events. The second topic to be discussed is the use of an antiferromagnetic material as a source of spin flow as well as the exchange field: The former is for the switching and the latter is for the switching in the absence of external magnetic field. A small external magnetic field was required to induce SOT switching in structures other than ordinary STT switching took place, which was clearly an obstacle for future integration. It has been shown in a (Co/Ni)-multilayer/PtMn structure one can switch magnetization in the absence of external magnetic field [4].

Nonvolatile Memory Devices With Magnetic Nanowires Controlled by Spin-Transfer and Spin-Orbit Torques

The interfacial Dzyaloshinskii-Moriya interaction (DMI) holds promises for design and control of chiral spin textures in low-dimensional magnets with efficient current-driven dynamics. Recently, an interlayer DMI has been found to exist across magnetic multilayers with a heavy-metal spacer between magnetic layers. This opens the possibility of chirality in these three-dimensional magnetic structures. Here we show the existence of the interlayer DMI in a synthetic antiferromagnetic multilayer with both inversion and in-plane asymmetry. We analyse the interlayer DMI’s effects on the magnetization and the current-induced spin-orbit torque (SOT) switching of magnetization through a combination of experimental and numerical studies. The chiral nature of the interlayer DMI leads to an asymmetric SOT switching of magnetization under an in-plane magnetic field. Our work paves the way for further explorations on controlling chiral magnetizations across magnetic multilayers through SOTs, which can provide a new path in the design of SOT devices.

Current-induced spin-orbit torques (SOT) have attracted great attention for their unique ability to manipulate the magnetization of ferromagnets and antiferromagnets [1]. SOT-induced switching offers speed and reliability comparable to or better than spin transfer torque (STT) switching, as was shown in nanodots [2,3] and magnetic tunnel junctions (MTJ) [4,5]. However, whereas the STT-driven switching dynamics of MTJs has been extensively studied [6-8], less is known about the transient dynamics and actual speed of the magnetization reversal induced by SOT. In this study we investigate the reversal dynamics of SOT switching in 3-terminal MTJs and show how the interplay of voltage control of magnetic anisotropy (VCMA) and STT can be used to optimize the switching efficiency [9,10]. We report real-time measurements of in-field and field-free SOT switching and disentangle the combined impact of SOT, STT, and VCMA on the switching speed and efficiency. We show that the combination of these effects leads to reproducible sub-ns switching with an extremely narrow distribution of the switching times without the aid of an external field. Our 3-terminal devices are fabricated using a fully compatible CMOS process [11] and are based on CoFeB/MgO/CoFeB MTJs grown on a b-W SOT-injection line and patterned into 80 nm pillars (Fig. 1a). The field-free switching functionality is enabled by an in-plane magnet embedded into a hard-mask [12], which provides a symmetry-breaking magnetic field to the magnetization of the free layer (Fig. 1b). Our electrical setup combines after-pulse and real-time detection [13] allowing for the exploration of individual SOT switching events in the time domain as well as of their statistical distributions. The voltage time traces of individual switching events exhibit a finite incubation delay followed by a single-jump reversal, after which the magnetization remains quiescent in the final state (Fig. 1c,d) [13]. Whereas complete magnetization reversal occurs within ≈1 ns, the incubation delay can take a substantial part of the total switching time in close-to-critical conditions. This incubation time, which is not expected for the “instant-on” SOT in perpendicular MTJs, can be minimized by a moderate increase of the SOT current (compare Figs. 1c and 1d), resulting in a very narrow distribution of the switching latency (Fig. 1e) [14]. A similar effect is achieved by increasing the external magnetic field strength or by biasing the MTJ. We attribute the two-steps switching process to the nucleation of a reversed domain and propagation of a domain-wall across the free layer, as supported by micromagnetic simulations (Fig. 1f). The model reveals that a key element to the observed dynamics is the reduction of magnetic anisotropy of the free layer related to the current-induced temperature rise [13]. The results further evidence a significant dependence of the switching speed and reversal onset on the MTJ bias. In Fig. 1g, averaged time traces obtained for different bias conditions clearly show that the bias accelerates the switching process. Finally, from symmetry considerations, we show that the contributions of STT and VCMA can be separated, unraveling their distinct impact on the magnetization dynamics.

As one of the next-generation memory technologies, magnetic random access memory (MRAM) is of great interest because of its non-volatility, high access speed, large integration density and low power consumption. Current-induced spin-orbit torque (SOT) magnetization switching has been proposed for improving the writing performance of MRAM. Meanwhile, it has been demonstrated that the Co-based full Heusler alloys show potential for increasing the magnetoresistance ratio and achieving efficient reading due to their relatively large spin polarization1. Therefore, achieving the SOT magnetization switching in full Huesler alloys will be promising for optimizing both the reading and writing performance of MRAM devices.Here, we report a successful full SOT magnetization switching in a perpendicularly magnetized full Heusler alloy Co2FeSi2 by using Pd as a spin current generating layer. To eliminate the heating effect, a large writing pulse current with a pulse width of 0.1 ms (red columns in Fig. 1) was applied to switch the magnetization, and a small reading current of 1 mA (blue columns in Fig. 1) was applied for 0.5 s to read the Hall resistance. As shown in Fig. 2, with the assistance of the external magnetic field of 500 Oe, the magnetization can be fully switched with a switching current density of 3.7×107 A cm-2, which is in the same order of magnitude as that required in the conventional heavy metal (HM)/ferromagnet system even though the Pd shows a relatively smaller spin Hall angle than that of HM3. By using harmonic Hall measurements, the damping-like and field-like effective fields per unit current density are estimated to be 56.9 (10–7 Oe A–1 cm2) and 39.8 (10–7 Oe A–1 cm2), respectively. Our findings will advance the development of SOT-MRAM devices with both better reading and writing performance.This work was partly supported by Grants-in-Aid for Scientific Research (Nos. 18H03860, 20H05650, and 20F20366), CREST Program of JST (JPMJCR1777), Spintronics Research Network of Japan (Spin-RNJ), and Grant-in-Aid for JSPS Fellows (20F20366).  Fig. 1 Pulse current train used for the SOT switching measurements.  Fig. 2 SOT switching in the MgO/Co2FeSi/Pd heterostructure at room temperature.

Current-induced spin-orbit torque (SOT) switching of magnetization has attracted great interest due to its potential application in magnetic memory devices, which offer low-energy consumption and high-speed writing. However, most of the SOT studies on perpendicularly magnetized anisotropy (PMA) magnets have been limited to heterostructures with interfacial PMA and poor thermal stability. Here, we experimentally demonstrate a SOT magnetization switching for a ferrimagnetic D022-Mn3Ge film with high bulk PMA and robust thermal stability factor under a critical current density of 6.6 × 1011 A m-2 through the spin Hall effect of an adjacent capping Pt and a buffer Cr layer. A large effective damping-like SOT efficiency of 2.37 mT/1010 A m-2 is determined using harmonic measurements in the structure. The effect of the double-spin source layers and the negative-exchange interaction of the ferrimagnet may explain the large SOT efficiency and the manifested magnetization switching of Mn3Ge. Our findings demonstrate that D022-Mn3Ge is a promising candidate for application in high-density SOT magnetic random-access memory devices.

This study explores the mechanisms of spin–orbit torque (SOT) switching in ferromagnetic semiconductors (FMS) with perpendicular magnetic anisotropy (PMA), emphasizing the impact of symmetry-breaking. Using micromagnetic simulations based on the Landau-Lifshitz-Gilbert (LLG) equation, we examine several symmetry-breaking factors, including bias field misalignment, interlayer exchange coupling, out-of-plane spin polarization, and tilted magnetic anisotropy. The results reveal that bias field misalignment relative to the film plane significantly distorts the SOT switching hysteresis. Additionally, intrinsic symmetry-breaking effects, such as internal coupling fields, out-of-plane spin polarization, and tilted anisotropy, facilitate field-free SOT (FF-SOT) switching without external bias fields. Each type of FF-SOT switching exhibits distinct characteristics, including hysteresis shifts, switching ratios, and saturated magnetization. The combine effects, such as interlayer exchange bias and tilted anisotropy, significantly change the switching current density depending on their constructive or destructive combination in a device. Furthermore, a new approach to symmetry breaking via the Oersted field is proposed, which is applicable only along the ⟨100⟩ crystallographic directions of the FMS. This work emphasizes the role of symmetry-breaking in FF-SOT switching and offers fundamental information for interpreting FF-SOT switching observed from FMS films in experiments, contributing to the optimization of SOT efficiency and the advancement of spintronics technologies.

Compensated ferrimagnets (FIMs) made of rare-earth transition-metal compounds have stimulated increasing interest for enabling fast spin dynamics. Taking $\mathrm{Fe}$-$\mathrm{Tb}$ compounds as an example, substantial efforts have been made on the study of compensated magnetism and its potential spintronic applications. Current-induced spin-orbit torque (SOT) switching and its evolution with compensated ferrimagnetism in these compounds, however, remain to be systematically explored, which motivates the present study. By combining magnetometry and anomalous Hall effect measurements, a compositional magnetization compensation point (${x}_{c}$ = 0.25) is determined for ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Tb}}_{x}$ films of a fixed thickness of 6.5 nm. The antiferromagnetic coupling between $\mathrm{Fe}$ and $\mathrm{Tb}$ sublattices is directly revealed by conducting element-specific x-ray magnetic circular dichroism measurements. The evolution of SOT switching as a function of $\mathrm{Tb}$ concentration (x) in $\mathrm{Pt}/{\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Tb}}_{x}/\mathrm{Ta}$ multilayers is subsequently investigated. An enhanced SOT efficiency (approximately 3 times) is obtained at ${x}_{c}$ = 0.25. By conducting an endurance test, reliable SOT switching is revealed for over ${10}^{4}$ cycles. Our work suggests that compensated FIMs of composition ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Tb}}_{x}$ could be implemented for realizing efficient and stable spin-orbitronic performances.

Current-induced spin-orbit torque (SOT) is regarded as a promising mechanism for driving neuromorphic behavior in spin-orbitronic devices. In principle, the strong SOT in a heavy-metal-based magnetic heterostructure is attributed to the spin-orbit coupling (SOC)-induced spin Hall effect and/or the spin Rashba-Edelstein effect. Recently, SOC-free mechanisms such as the orbital-angular-momentum-induced orbital Hall effect and/or the orbital Rashba-Edelstein effect have been proposed to generate sizable torques comparable to those from the conventional spin Hall mechanism. In this work, we show that the orbital current can be effectively generated by the nitrided light metal $\mathrm{Cu}$. The overall dampinglike SOT efficiency, which consists of both the spin and the orbital current contributions, can be tailored from ${\ensuremath{\xi}}_{\mathrm{DL}}\ensuremath{\approx}0.06\text{ to }0.4$ in a $\mathrm{Pt}/\mathrm{Co}/{\mathrm{Cu}\mathrm{N}}_{x}$ magnetic heterostructure by tuning the nitrogen doping concentration. Current-induced magnetization switching further verifies the efficacy of such orbital current with a critical switching current density as low as ${J}_{c}$ \ensuremath{\sim} 5 \ifmmode\times\else\texttimes\fi{} ${10}^{10}\phantom{\rule{0.25em}{0ex}}{\mathrm{A}/\mathrm{m}}^{2}$. Most importantly, orbital-current-mediated memristive switching behavior can be observed in such heterostructures, which reveals that gigantic SOT and efficient magnetization switching are the trade-offs for the applicable window of memristive switching. Our work provides insights into the role that orbital current might play in SOT neuromorphic devices for making energy-efficient spin-orbitronic devices.

Deterministic magnetization switching driven by current-induced spin–orbit torque (SOT) without an external magnetic field has potential applications in magnetic random access memory. Here, we realized the field-free magnetization switching in a T-type structure (CoFeB/W/CoFeB), where the two CoFeB layers have perpendicular magnetic anisotropy and in-plane magnetic anisotropy (IMA), respectively. We discovered that the direction of symmetry-breaking field is parallel to the magnetization of the bottom CoFeB (IMA), which cannot be explained by a stray field of this layer. In addition, by placing a 2.5-nm thick insulating layer of MgO between the bottom CoFeB and W layer (CoFeB/MgO/W/CoFeB) to block the interlayer exchange coupling and the spin current from the bottom CoFeB, the field-free SOT switching was still achieved, primarily due to the Néel orange-peel effect in our devices. By using micromagnetic simulations, the roughness of angstrom magnitude was introduced into the model to calculate the symmetry-breaking field, finding a qualitative agreement with experiments. Moreover, we obtained the spin Hall angle of CoFeB (θSH = −0.024) by the current-induced hysteresis loop shift method, and the contribution of the effective efficiency χ from the bottom CoFeB was accounted for about 26% of the total in the current-induced SOT switching process. These results indicated that an in-plane ferromagnet layer in the T-type structure provides not only the symmetry-breaking field but also spin current for the field-free SOT magnetization switching.

Current-induced spin-orbit torques (SOTs) can electrically switch magnetic films. The thickness of these films is usually limited to a few tenths of nanometers. Toward stable spintronic nanodevices, it is important to explore the upper thickness limit and to identify the associated SOT switching mechanism, if it is different from standard models. Here, we experimentally show that the SOT switching could occur in Pt(3 nm)/Fe_{0.80}Gd_{0.20}/Ta(3 nm) trilayers with a thickness of Fe_{0.80}Gd_{0.20} ferrimagnetic films up to 200nm, all at room temperature. The contributions from the Oersted field, bulk SOTs, and thermal activation induced by Joule heating are also discussed. Through performing atomistic spin simulations, we identify the critical role of nucleation and propagation of vertical magnetic solitons along the thickness direction, which could explain such unprecedented SOT switching behaviors in extremely thick ferrimagnets. The revelation of the vertical soliton-assisted SOT switching of the extremely thick ferrimagnets can be used for miniaturizing spintronic devices.

Current-induced spin-orbit torques (SOTs) are of interest for fast and energy-efficient manipulation of magnetic order in spintronic devices. To be deterministic, however, switching of perpendicularly magnetized materials by SOT requires a mechanism for in-plane symmetry breaking. Existing methods to do so involve the application of an in-plane bias magnetic field, or incorporation of in-plane structural asymmetry in the device, both of which can be difficult to implement in practical applications. Here, we report bias-field-free SOT switching in a single perpendicular CoTb layer with an engineered vertical composition gradient. The vertical structural inversion asymmetry induces strong intrinsic SOTs and a gradient-driven Dzyaloshinskii–Moriya interaction (g-DMI), which breaks the in-plane symmetry during the switching process. Micromagnetic simulations are in agreement with experimental results, and elucidate the role of g-DMI in the deterministic switching processes. This bias-field-free switching scheme for perpendicular ferrimagnets with g-DMI provides a strategy for efficient and compact SOT device design.

Research efforts in discovering and gaining better understanding of various spin-based physical phenomena over the past decades have propelled the innovation and developments of new generations of memory and logic devices. With utilization of non-volatility inherent in magnetism and low-power consumption characteristics, these novel device concepts present new opportunities for future electronics and computers. In recent years, magnetization switching via spin-orbit torques (SOTs) has come out as a promising candidate for advanced memory and computing applications, as it gives the advantages of low power consumption as well as ultrafast writing speed. The underlying mechanisms by which the SOTs induce the magnetization switching, however, turns out to be quite complex. Furthermore, for perpendicularly magnetized systems, i.e., perpendicular MRAM, the SOT driven magnetization switching of the free layer often requires an external in-plane field that significantly hinders the technological viability of commercial implementations.In this research work, we aim to gain a deeper understanding of the SOTs and their roles in inducing the magnetization switching; in turn, by means of material or device engineering, we can control the SOTs to achieve the desired switching outcomes. We particularly focus our study on the perpendicularly magnetized systems because the high perpendicular magnetic anisotropy (PMA) in these systems makes them appealing for practical applications.A major part of this research work emphasizes on the elimination of the need for an external magnetic field in the SOT switching of a perpendicular magnet. One strategy to achieve the field-free perpendicular SOT switching is through creating a magnetic field that’s localized within the device. The origin of such internal field can come from the interlayer exchange coupling. Based on this idea, we demonstrate robust field-free perpendicular magnetization switching by utilizing the spin Hall effect and interlayer exchange coupling of iridium (Ir). This is the first reported clear experimental demonstration that a heavy metal layer, Ir in particular, is capable of serving as both a spin current source and an interlayer exchange coupling layer. An additional important characteristic of Ir is that its interface with either Co or FeCoB facilitates strong perpendicular magnetic anisotropy. These combined properties allow us to achieve the SOT driven magnetization switching of a perpendicular Co layer in absence of an external field. Besides the field-free switching of a single layer, we also show that the switching scheme can be well integrated with the MgO-based magnetic tunnel junction (MTJ). We show that the three-terminal MTJ device with the Ir-enabled switching exhibits reliable writing and reading operations at zero external field, moving a step closer to the practical applications of the SOT-related magnetoresistive devices.In addition to engineering the SOT materials, we provide another solution by altering the device design. The idea is based on the well-known phenomenon that a current carrying wire produces an effective magnetic field around it. Compared to the conventional three-terminal device, our device contains an additional current line orthogonal to the write path, which can generate an in-plane Oersted field during the SOT writing. Facilitated by this Oersted field, reliable SOT switching of the perpendicular MTJs is obtained without applying an external field. The switching characteristic also renders our device unique advantages in terms of preventing the half selecting issue.In the study of the switching dynamics, we find the switching process in our devices often starts with domain nucleation followed by the domain wall motion (DWM) to expand the reversed domains. This inspires us to dig deeper into the SOT driven DWM and explore the ways in manipulating the DWM so as to control the magnetization state of a perpendicular magnet. In this work, we investigate the DWM in a system with two heavy metal underlayers that have the opposite spin Hall angles. By simply varying the relative thicknesses of these two underlayers, we can manipulate the polarity of the SOTs exerting on the DWs, which further allows us to control the direction of DWM. Based on our findings, we propose a wedge DW device where the SOT driven DWM can effectively give rise to the expansion of reversed domains and thereby realize the magnetization switching.Lastly, we show the initial experimental works for developing a novel DW device known as mCell, which can be used as the computing unit in non-volatile logic circuit without the integration with CMOS. We develop a magnetic oxide (FeOx) layer that can serve as the electric-insulating magnetic layer inserted in between the write path and read path of mCell. The FeOx insertion layer not only provides sufficient magnetic coupling between the adjacent magnetic layers, but also significantly enhances the DWM in terms of the DW velocity and power efficiency.