Field-free Spin-orbit Torque Switching Research Articles

Field-free spin-orbit torque switching via out-of-plane spin-polarization induced by an antiferromagnetic insulator/heavy metal interface

Manipulating spin polarization orientation is challenging but crucial for field-free spintronic devices. Although such manipulation has been demonstrated in a limited number of antiferromagnetic metal-based systems, the inevitable shunting effects from the metallic layer can reduce the overall device efficiency. In this study, we propose an antiferromagnetic insulator-based heterostructure NiO/Ta/Pt/Co/Pt for such spin polarization control without any shunting effect in the antiferromagnetic layer. We show that zero-field magnetization switching can be realized and is related to the out-of-plane component of spin polarization modulated by the NiO/Pt interface. The zero-field magnetization switching ratio can be effectively tuned by the substrates, in which the easy axis of NiO can be manipulated by the tensile or compressive strain from the substrates. Our work demonstrates that the insulating antiferromagnet based heterostructure is a promising platform to enhance the spin-orbital torque efficiency and achieve field-free magnetization switching, thus opening an avenue towards energy-efficient spintronic devices.

Field-Free Current-Induced Switching of L 1 0 - FePt Using Interlayer Exchange Coupling for Neuromorphic Computing

$L{1}_{0}$-phase $\mathrm{Fe}\mathrm{Pt}$ is well known for its unusually robust perpendicular magnetic anisotropy (PMA) properties arising from strong conduction-electron spin-orbit coupling (SOC) with the $\mathrm{Fe}$ orbital moment. The strong PMA enables stable magnetic storage and memory devices with ultrahigh capacity. Meanwhile, SOC is also the premise of the recently discovered spin-orbit-torque (SOT) effect, which opens avenues for possible electrical manipulation of magnetization for $L{1}_{0}$-$\mathrm{Fe}\mathrm{Pt}$. The bulk SOT of the $L{1}_{0}$-$\mathrm{Fe}\mathrm{Pt}$ single layer was discovered recently; this leads to the magnetization of $L{1}_{0}$-$\mathrm{Fe}\mathrm{Pt}$ reversibly switching on itself. However, deterministic SOT switching of bulk perpendicularly magnetized $\mathrm{Fe}\mathrm{Pt}$ magnets relies on an external magnetic field to break the symmetry. Here, the symmetry-breaking issue is resolved by interlayer exchange coupling, where the $\mathrm{Fe}\mathrm{Pt}$ layer is coupled with an in-plane magnetized $\mathrm{Ni}\mathrm{Fe}$ layer through a $\mathrm{Ti}\mathrm{N}$ spacer layer. Furthermore, our device also presents memristive or gradual switching behaviors, even without an external field, offering the potential for constructing spin synapses and spin neurons for neuromorphic computing. An artificial neural network with high accuracy (\ensuremath{\sim}91.17%) is realized based on the constructed synapses and neurons. Our work paves the way for field-free SOT switching of single bulk PMA magnets and their potential applications in neuromorphic computing.

Field-free spin-orbit torque switching of GdCo ferrimagnet with broken lateral symmetry by He ion irradiation

Current-induced magnetization switching by spin-orbit torque (SOT) is of great importance for the energy-efficient operation of spin-based memory and logic devices. However, the requirement of an external in-plane magnetic field to deterministically switch the perpendicular magnetization of a device is a bottleneck for device application. There have been many efforts to realize field-free SOT switching using interlayer/exchange coupling, the spin valve structure, or materials with lateral symmetry breaking. However, limitations of material selection or layer structure modification hinder the application of these methods in practice. Here, we demonstrate the field-free SOT switching of a GdCo ferrimagnet with lateral symmetry breaking by He ion irradiation. Local control of the magnetic property with different He ion irradiation conditions induces a lateral magnetic gradient orthogonal to the current flow direction in the ferrimagnet. We also observe out-of-plane-SOT generation due to lateral symmetry breaking, which is essential for field-free switching. Since the He ion irradiation technique is utilized for the fabrication of complementary−metal−oxide−semiconductors, and resolution can reach the nanometer level, our findings have the potential to serve as the basis for new developments in the fabrication of wafer-scale spintronic memory and logic devices with high energy efficiency and high density.

Spintronic Physical Unclonable Functions Based on Field-Free Spin-Orbit-Torque Switching.

Physical unclonable function(PUFs) utilize inherent random physical variations of solid-state devices and are a core ingredient of hardware security primitives. PUFs promise more robust information security than that provided by the conventional software-based approaches. While silicon- and memristor-based PUFs are advancing, their reliability and scalability require further improvements. These are currently limited by output fluctuations and associated additional peripherals. Here, highly reliable spintronic PUFs that exploit field-free spin-orbit-torque switching in IrMn/CoFeB/Ta/CoFeB structures are demonstrated. It is shown that the stochastic switching polarity of the perpendicular magnetization of the top CoFeB can be achieved by manipulating the exchange bias directions of the bottom IrMn/CoFeB. This serves as an entropy source for the spintronic PUF, which is characterized by high entropy, uniqueness, reconfigurability, and digital output. Furthermore, the device ensures a zero bit-error-rate under repetitive operations and robustness against external magnetic fields, and offers scalable and energy-efficient device implementations.

Magnetization switching probability in the dynamical switching regime driven by spin-transfer torque

We study magnetization switching in the dynamical regime for in-plane magnetized systems, useful for field-free spin-orbit torque switching devices. We derive a formula for the switching probability, characterized by the thermal stability factor, critical current density, and the linewidth of ferromagnetic resonance. The formula agrees well with numerical simulations based on the Landau-Lifshitz-Gilbert equation. To study the viability of the theory developed, the switching probability of an in-plane magnetized ferromagnet in three-terminal spin-orbit torque switching devices is measured. We find that the transition width of the switching probability versus current increases with decreasing pulse width. The shape of the probability density, the current derivative of the switching probability, changes with varying pulse width. These characteristics are in good agreement with the theory developed. From the analyses, we show that the dynamical and thermally activated switching regimes can be distinguished simply from the shape of the probability density. The formula therefore provides useful means to analyze the switching probability of two-terminal spin torque and three-terminal spin-orbit torque switching devices.

Field-Free Type- x Spin-Orbit-Torque Switching by Easy-Axis Engineering

The current-induced type-x spin-orbit-torque (SOT) switching configuration describes the orthogonal relationship between the magnetic easy axis (EA) of a ferromagnetic (FM) layer and the injected spin polarization, $\ensuremath{\sigma}$, from a heavy-metal (HM) layer, which has the potential to eclipse the conventional type-y scenario (EA parallel to $\ensuremath{\sigma}$) at the sub-nanosecond pulse regime. Here, we show that, in HM/FM bilayer heterostructures, current-driven differential planar Hall signals can serve as efficient means to probe type-x SOT switching in a simple measurement fashion. Through this approach, we demonstrate field-free type-x SOT switching in all devices with a canted EA, which are engineered via field-annealing processes. By analyzing the switching-phase diagrams, we further verify that such field-free switching stems from a z-direction effective field contributed to by both the dampinglike torque and the canted EA. Our work indicates that the canted EA, as engineered by a field-annealing process, can give rise to a robust field-free SOT switching and further reduce the critical switching current density.

Field‐Free Switching of Magnetization by Tilting the Perpendicular Magnetic Anisotropy of Gd/Co Multilayers (Adv. Funct. Mater. 26/2022)

Field-Free Switching In article number 2112561, Jung-Il Hong, Chanyong Hwang, and co-workers demonstrate field-free spin-orbit torque switching of perpendicular magnetization in amorphous and ferrimagnetic Gd/Co multilayers accompanied by a tilted magnetic anisotropy axis. This tilted anisotropy could facilitate the development of magnetic memory and logic devices without the need for external application of a global magnetic field or manufacturing complex magnetic structures to induce symmetry breaking.

Controlling domain wall and field-free spin–orbit torque switching in synthetic antiferromagnets

Perpendicular magnetization switching driven by spin–orbit torques plays an increasingly important role for spintronic devices toward practical applications but is also hindered by the well-known technical challenge that an external in-plane magnetic field is required for deterministic switching. Here, we show that the deterministic switching can be achieved in synthetic antiferromagnets through the flexible domain control in the absence of external magnetic fields. Specifically, we have observed that the domain wall (DW) distorts under an applied electric current in contrast to the conventional rigid DW motion in a single ferromagnet. More importantly, the distorted DWs can be precisely controlled under zero magnetic field, leading to the deterministic switching. Our results indicate that the critical technical challenge may be addressed by employing a synthetic antiferromagnetic layer through the DW motion dominated field-free switching.

Field-free approaches for deterministic spin–orbit torque switching of the perpendicular magnet

Abstract All-electrical driven magnetization switching attracts much attention in next-generation spintronic memory and logic devices, particularly in magnetic random-access memory (MRAM) based on the spin–orbit torque (SOT), i.e. SOT-MRAM, due to its advantages of low power consumption, fast write/read speed, and improved endurance, etc. For conventional SOT-driven switching of the magnet with perpendicular magnetic anisotropy, an external assisted magnetic field is necessary to break the inversion symmetry of the magnet, which not only induces the additional power consumption but also makes the circuit more complicated. Over the last decade, significant effort has been devoted to field-free magnetization manipulation by using SOT. In this review, we introduce the basic concepts of SOT. After that, we mainly focus on several approaches to realize the field-free deterministic SOT switching of the perpendicular magnet. The mechanisms mainly include mirror symmetry breaking, chiral symmetry breaking, exchange bias, and interlayer exchange coupling. Furthermore, we show the recent progress in the study of SOT with unconventional origin and symmetry. The final section is devoted to the industrial-level approach for potential applications of field-free SOT switching in SOT-MRAM technology.

Role of an in-plane ferromagnet in a T-type structure for field-free magnetization switching

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.

Field-free spin–orbit torque switching in L1-FePt single layer with tilted anisotropy

For real-world applications, it is desirable to realize field-free spin–orbit torque (SOT) switching in thin films with high perpendicular magnetic anisotropy (PMA). In this paper, we report that field-free SOT switching in a L10-FePt single layer with a large switching ratio of 26% is obtained by using a MgO ⟨100⟩⋀8°/⟨100⟩ miscut substrate. It is found that field-free switching depends on the direction of the imposed pulse current. Only when the electric current is along the y (010)-direction but not along the x (100)-direction does field-free switching happen, which can be attributed to the tilted PMA induced symmetry breaking in the x–z plane. Furthermore, under the field-free condition, our FePt single layer system exhibits stable multi-state magnetic switching behavior and nonlinear synaptic characteristics. This work paves the way to realize field-free SOT switching in the L10-FePt single layer, which will have significant impact on spin memory devices and synaptic electronics.

Field-Free Spin-Orbit Torque Switching of GdCo Ferrimagnet with Broken Lateral Symmetry by He Ion Irradiation

Field-Free Spin-Orbit Torque Switching of GdCo Ferrimagnet with Broken Lateral Symmetry by He Ion Irradiation

Field-free spin–orbit torque switching induced by interlayer exchange coupling in Pt/Co/Ru/Ni/Pt multilayer

High-efficient operation of spin–orbit torque (SOT) is central for the exciting field of spin-orbitronics. To realize a deterministic field-free SOT switching, here, we investigate the enhancement mechanism of SOT with the interlayer exchange coupling in a Pt/Co/Ru/Ni/Pt multilayer. We find that the decrease of the domain wall energy in an interlayer exchange coupling system is the pivotal reason for achieving current-induced field-free domain wall switching. Comparing with the direct ferromagnetic couplings, the switching ratio and efficiency present a significant improvement in the interlayer exchange coupling system. Combining micromagnetic simulations and magneto-optical Kerr effect measurements, the domain wall motions are investigated and the switching mechanism is confirmed to be the domain wall depinning regime. Furthermore, the SOT switching direction is proven to be dominated by the field-modulated domain wall motions. This work paves a potential way toward practical spin–orbit torque-based memory, oscillating, and logic devices.

Electric-field control of field-free spin-orbit torque switching via laterally modulated Rashba effect in Pt/Co/AlOx structures

Spin-orbit coupling effect in structures with broken inversion symmetry, known as the Rashba effect, facilitates spin-orbit torques (SOTs) in heavy metal/ferromagnet/oxide structures, along with the spin Hall effect. Electric-field control of the Rashba effect is established for semiconductor interfaces, but it is challenging in structures involving metals owing to the screening effect. Here, we report that the Rashba effect in Pt/Co/AlOx structures is laterally modulated by electric voltages, generating out-of-plane SOTs. This enables field-free switching of the perpendicular magnetization and electrical control of the switching polarity. Changing the gate oxide reverses the sign of out-of-plane SOT while maintaining the same sign of voltage-controlled magnetic anisotropy, which confirms the Rashba effect at the Co/oxide interface is a key ingredient of the electric-field modulation. The electrical control of SOT switching polarity in a reversible and non-volatile manner can be utilized for programmable logic operations in spintronic logic-in-memory devices.

Spin–orbit torques of an in-plane magnetized system modulated by the spin transport in the ferromagnetic Co layer

Spin–orbit torque (SOT) magnetoresistive random-access memory (MRAM) devices have been proposed for energy efficient memory and computing applications. New classes of materials such as antiferromagnets, topological insulators, and semimetals can generate spins with unconventional polarization and improve the efficiency of field-free SOT switching. In this work, we report significant changes in SOTs due to a Co thin film inserted in the Pt/Co/Mg/CoFeB heterostructures. Remarkably, the damping-like effective field has been enhanced by 7.4 times after inserting a thin Co layer with weak perpendicular magnetic anisotropy (PMA), while the field-like effective field is reduced to near zero value. Independent characterizations were performed to verify the presence of the changes in SOTs following spin modulation by the Co insertion layer. In addition, we found that the dynamic spin pumping coupling between Pt/Co with weak PMA and the in-plane CoFeB could significantly modulate the effective SOTs in the heterostructure, and this effect is dependent on the thickness of the spacer Mg through long-range spin-wave mediated coupling. Our work has experimentally demonstrated a new avenue to modulate SOTs with physically sputtered metal layers, and this finding is promising to enable flexible and efficient spin polarizations for MRAM devices.

Spintronics for Energy- Efficient Computing: An Overview and Outlook

From the discovery of giant magnetoresistance (GMR) to tunnel magnetoresistance (TMR), their subsequent application in large capacity hard disk drives (HDDs) greatly speeded up the information era over the past decades. However, the growing demand for big-data storage and processing is limited by the von-Neumann architecture due to the memory bottleneck and power dissipation. Taking advantage of nonvolatility, high speed, and low power, magnetic random access memory (MRAM) becomes a promising candidate to overcome this limitation through processing-in-memory (PIM) architectures. In this article, we provide an overview of existing technology and give a roadmap of spintronic devices for future energy-efficient computing and its relevant integration architectures. We begin with the fundamentals of Toggle-MRAM and spin-transfer torque (STT)-MRAM, which already have commercial applications. We then introduce spin-orbit torque (SOT), a critical mechanism to realize low-power data manipulation in the next generation of MRAM and summarize the recent experimental breakthroughs of field-free SOT switching schemes. Finally, we present MRAM-based PIM architectures and novel spintronic devices, provide an application outlook, and deliver the future development potential of energy-efficient computing systems.

Optimization of a Spin-Orbit Torque Switching Scheme Based on Micromagnetic Simulations and Reinforcement Learning.

Spin-orbit torque memory is a suitable candidate for next generation nonvolatile magnetoresistive random access memory. It combines high-speed operation with excellent endurance, being particularly promising for application in caches. In this work, a two-current pulse magnetic field-free spin-orbit torque switching scheme is combined with reinforcement learning in order to determine current pulse parameters leading to the fastest magnetization switching for the scheme. Based on micromagnetic simulations, it is shown that the switching probability strongly depends on the configuration of the current pulses for cell operation with sub-nanosecond timing. We demonstrate that the implemented reinforcement learning setup is able to determine an optimal pulse configuration to achieve a switching time in the order of 150 ps, which is 50% shorter than the time obtained with non-optimized pulse parameters. Reinforcement learning is a promising tool to automate and further optimize the switching characteristics of the two-pulse scheme. An analysis of the impact of material parameter variations has shown that deterministic switching can be ensured for all cells within the variation space, provided that the current densities of the applied pulses are properly adjusted.

Spin–orbit torque controllable complete spin logic in a single magnetic heterojunction

The realization of complete spin logic within a single nonvolatile memory cell is a promising approach toward next-generation low-power stateful logic circuits. In this work, we demonstrate that all 16 Boolean logic functions can be realized within a single four-state nonvolatile IrMn/Co/Ru/CoPt magnetic heterojunction, where controllable field-free spin–orbit torque switching of the perpendicularly magnetized CoPt alloy is obtained, relying on the interlayer exchange coupling and exchange bias effect. By assigning different values to four variables of the four-state memory, that is, the initial control current pulse, the initial control magnetic field, and the input electrical potential of two terminals, in sequentially three steps, the complete Boolean logic functions are realized, while the anomalous Hall voltage of the devices is considered as logic output. The coexistence of nonvolatile four-state memory and complete spin logic functions holds promising application for future computing systems beyond von Neumann architecture.

Antiferromagnet-mediated spin–orbit torque induced magnetization switching in perpendicularly magnetized L1-MnGa

Current-induced magnetization switching plays an essential role in spintronic devices exhibiting nonvolatility, high-speed processing, and low-power consumption. Here, we report on the spin–orbit torque-induced magnetization switching in perpendicularly magnetized L10-MnGa/FeMn/Pt trilayers grown by molecular-beam epitaxy. An antiferromagnetic FeMn layer is inserted between the spin current generating Pt layer and spin absorbing MnGa layer. Due to the exchange bias effect, the trilayers show field-free spin–orbit torque switching. Overall, the spin transmission efficiency decreases monotonically as the FeMn thickness increases. It is found that the spin current can be transmitted through an 8 nm-thick FeMn layer as evidenced by partial switching of the L10-MnGa. The damping-like spin–orbit torque efficiency shows a peak value at tFeMn = 1.5 nm due to the enhanced interfacial spin transparency and crystalline quality of the FeMn. These results help demonstrate the efficacy of emerging spintronic devices containing antiferromagnetic elements.

Reconfigurable Physical Unclonable Function Based on Spin-Orbit Torque Induced Chiral Domain Wall Motion

A reliable design of physical unclonable function (PUF) based on spin-orbit torque (SOT) induced domain wall (DW) motion has been proposed and experimentally demonstrated. Magnetic DWs are nucleated and propagate in Ta/CoFeB/MgO heterostructures in which their device-to-device variation enables the PUF, while cycle-to-cycle variation in DW motion enables the reconfigurable design. Furthermore, field-free SOT switching has been observed in the chiral domain wall likely caused by the large Dzyaloshinskii-Moriya interaction (DMI) effect. The proposed DW motion based anti-counterfeiting device shows good promises as a CMOS-compatible PUF for hardware security.