Field-free Magnetization Switching Research Articles

Electric field control of spin-orbit torque in annealed Ta/CoFeB/HfO x heterostructures via interfacial oxidation modulation

Electric field control of spin-orbit torque (SOT) exhibits promising potential in advanced spintronic devices through interfacial modulation. In this work, we investigate the influence of electric field and interfacial oxidation on SOT efficiency in annealed Ta/CoFeB/HfO x heterostructures. By varying annealing temperatures, the damping-like SOT efficiency reaches its peak at the annealing temperature of 320 °C, with an 80% field-free magnetization switching ratio induced by SOT having been demonstrated. This enhancement is ascribed to the annealing-induced modulation of oxygen ion migration at the CoFeB/HfO x interface. By applying voltages across the Ta/CoFeB/HfO x heterostructures, which drives the O2‒ migration across the interface, a reversible, bipolar, and non-volatile modulation of SOT efficiency was observed. The collective influence of annealing temperature and electric field effects on SOT carried out in this work provides an effective approach into facilitating the optimization and control of SOT in spintronic devices.

Large out-of-plane spin–orbit torque in topological Weyl semimetal TaIrTe4

The unique electronic properties of topological quantum materials, such as protected surface states and exotic quasiparticles, can provide an out-of-plane spin-polarized current needed for external field-free magnetization switching of magnets with perpendicular magnetic anisotropy. Conventional spin–orbit torque (SOT) materials provide only an in-plane spin-polarized current, and recently explored materials with lower crystal symmetries provide very low out-of-plane spin-polarized current components, which are not suitable for energy-efficient SOT applications. Here, we demonstrate a large out-of-plane damping-like SOT at room temperature using the topological Weyl semimetal candidate TaIrTe4 with a lower crystal symmetry. We performed spin–torque ferromagnetic resonance (STFMR) and second harmonic Hall measurements on devices based on TaIrTe4/Ni80Fe20 heterostructures and observed a large out-of-plane damping-like SOT efficiency. The out-of-plane spin Hall conductivity is estimated to be (4.05 ± 0.23)×104 (ℏ ⁄ 2e) (Ωm)−1, which is an order of magnitude higher than the reported values in other materials.

Field-free magnetization switching through large out-of-plane spin–orbit torque in the ferromagnetic CoPt single layers

Spin–orbit torque (SOT) induced magnetization switching in an energy-efficient and fast way has exhibited great application potential in next generation magnetic memories. However, a complicated layer structure is usually needed to break the mirror symmetry for achieving SOT induced field-free magnetization switching. Here, we report a sizeable field-free magnetization switching through large out-of-plane SOT in the chemically disordered A1-CoxPt100−x single layers within a Co composition range from 40 to 70. The largest absolute out-of-plane SOT efficiency is found at its equiatomic concentration (Co50Pt50), in which the absolute in-plane SOT efficiency also reaches the maximum value, 22.7 Oe/107 A cm−2. We further demonstrate that the symmetry dependence of field-free magnetization switching might arise from the chemically ordered L11-CoPt nano-scaled platelets formed during the sample deposition. We expect that the experimental identification of the field-free magnetization switching in the ferromagnetic CoPt single layer is desirable to simplify the applications of spin logic devices.

Highly efficient field-free switching of perpendicular yttrium iron garnet with collinear spin current

Yttrium iron garnet, a material possessing ultralow magnetic damping and extraordinarily long magnon diffusion length, is the most widely studied magnetic insulator in spintronics and magnonics. Field-free electrical control of perpendicular yttrium iron garnet magnetization with considerable efficiency is highly desired for excellent device performance. Here, we demonstrate such an accomplishment with a collinear spin current, whose spin polarization and propagation direction are both perpendicular to the interface. Remarkably, the field-free magnetization switching is achieved not only with a heavy-metal-free material, Permalloy, but also with a higher efficiency as compared with a typical heavy metal, Pt. Combined with the direct and inverse effect measurements, we ascribe the collinear spin current to the anomalous spin Hall effect in Permalloy. Our findings provide a new insight into spin current generation in Permalloy and open an avenue in spintronic devices.

Field-free magnetization switching with full scale in Pt/Tm3Fe5O12 bilayer on vicinal substrate

The spin–orbit torques within a Pt/Tm3Fe5O12 (TmIG) bilayer offer an expedient method for manipulating the magnetization of TmIG. However, the practical application of TmIG is hindered by the presence of an external field during switching. Here, we demonstrate field-free magnetization switching in Pt/TmIG bilayer on a vicinal substrate with minimal sacrifice to the perpendicular magnetic anisotropy (PMA) of TmIG. With the assistance of tilt PMA, reversible perpendicular magnetization switching is realized in the absence of an external field. Our results offer an alternative solution for achieving field-free perpendicular magnetization switching in a Pt/TmIG bilayer, thereby fostering the advancement of emerging SOT-based devices.

Field-free magnetic switching dependence on lateral interfaces in synthetic antiferromagnets by ion implantation

Field-free spin–orbit torque switching in synthetic antiferromagnets (SAF) holds significant promise for high-density spintronic memory and logic devices. In this paper, we realize the field-free magnetization switching in SAFs due to the local ion implantation-induced 45° lateral interface and symmetry breaking. Moreover, the magnetization switching ratio is enlarged by the lateral interface owing to the superimposition of a damping-like effective field and a symmetry-breaking effective field. Our work is significant for the development of magnetic random-access memory technology with high-speed and anti-interference ability.

Efficient current-induced spin torques and field-free magnetization switching in a room-temperature van der Waals magnet

The discovery of magnetism in van der Waals (vdW) materials has established unique building blocks for the research of emergent spintronic phenomena. In particular, owing to their intrinsically clean surface without dangling bonds, the vdW magnets hold the potential to construct a superior interface that allows for efficient electrical manipulation of magnetism. Despite several attempts in this direction, it usually requires a cryogenic condition and the assistance of external magnetic fields, which is detrimental to the real application. Here, we fabricate heterostructures based on Fe 3 GaTe 2 flakes that have room-temperature ferromagnetism with excellent perpendicular magnetic anisotropy. The current-driven nonreciprocal modulation of coercive fields reveals a high spin-torque efficiency in the Fe 3 GaTe 2 /Pt heterostructures, which further leads to a full magnetization switching by current. Moreover, we demonstrate the field-free magnetization switching resulting from out-of-plane polarized spin currents by asymmetric geometry design. Our work could expedite the development of efficient vdW spintronic logic, memory, and neuromorphic computing devices.

Perpendicular magnetic anisotropy tilting for spin–orbit torque-induced field-free switching of magnetization

Since the discovery of spin–orbit torque, it has garnered significant attention as one of potential spintronics building blocks, owing to its distinct advantages, such as fast switching capabilities. For the practical application of spin–orbit torque, several technical challenges remain; a primary one being the elimination of the necessity for an external magnetic field during switching process. To address this issue, here, we thoroughly explore the nature of perpendicular magnetic anisotropy tilting via wedge sputtering deposition incorporating a physical barrier. We precisely quantify the tilted angle of magnetic anisotropy, and introduce a way to determine the direction of the magnetic anisotropy tilting. In addition, the realization of field-free magnetization switching using the tilted perpendicular magnetic anisotropy is experimentally demonstrated, ensuring the controllability of polarity of the switching.

Field-free magnetization switching through modulation of zero-field spin–orbit torque efficacy

To make spin–orbit torque magnetic random access memory (SOT-MRAM) practical, current-induced magnetization switching without an external bias field is essential. Given that the CoFeB/MgO structure has already been used in typical spin-transfer torque-MRAM for its high tunneling magnetoresistance, leveraging a similar material system to achieve field-free SOT switching is of great importance. In this work, we systematically investigate the field-free switching mechanism in CoFeB/W/CoFeB T-type structures, where the two CoFeB layers are in-plane and perpendicularly magnetized, respectively. Initial SOT characterization shows a sizable zero-field SOT efficacy (χHx=0) for such T-type devices. Furthermore, field-free angle-dependent SOT measurement confirms the parallel relationship between the built-in bias field and the magnetization of the in-plane magnetized CoFeB. Based on thorough verification and exclusion of other potential mechanisms, the Néel orange-peel effect emerges as the dominant origin for such a built-in bias field, where a positive correlation between the deposited film surface roughness and χHx=0 is found. Based on this discovery, the field-free switching efficacy in T-type structures is further optimized via film roughness tuning and examined with pillar-shaped devices. Our results provide insights into the tentative approach to improve field-free switching using T-type devices and the feasibility of downscaling.

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.

High efficient field-free magnetization switching via exchange bias effect induced by antiferromagnetic insulator interface

Spin–orbit torque induced deterministic magnetization switching typically requires the assistance of an external magnetic field for symmetry breaking. However, achieving field-free switching in perpendicular magnetized layers is crucial for expanding the market of high-density memory. Previous reports have utilized exchange bias, an antiferromagnetic interfacial effect, to realize field-free magnetization switching. However, metallic antiferromagnetic layers will introduce shunting effects that reduce switching efficiency and the Néel vector becomes unstable when current flows through the antiferromagnetic layer. In this study, we achieved the zero-field magnetization switching in NiO/Pt/Co/Pt multilayers. Simulation results demonstrate higher efficiency compared to metallic antiferromagnetic IrMn-based structures. Our findings highlight that the insulator antiferromagnetic can provide an exchange bias field, eliminating the need for an external magnetic field. By avoiding shunting effects, our designed structure offers a more efficient approach for spintronic devices.

Electric field controlled perpendicular exchange bias in Ta/Pt/Co/IrMn/Pt heterostructure

Perpendicular exchange bias (PEB) is an essential effect of antiferromagnetic materials. It has the advantages over the in-plane exchange bias in application of magnetic random access memory (MRAM) technology, particularly in terms of higher packing density. The influence of the strain effect on PEB of Ta/Pt/Co/Ir20Mn80/Pt multilayers deposited on the piezoelectric substrate is studied in this work. It is found that the exchange bias field (Hex) can be regularly regulated by the strain effect introduced by the piezoelectric substrate. It indicates that the uncompensated spins at the interface of Ir20Mn80/Co can be reoriented under the strain effect, resulting in the variation of Hex. Furthermore, we propose a MRAM device structure based on the strain-mediated PEB, which enables field-free magnetization switching driven by the pure strain. Our study demonstrates the controllability of PEB and its application in spintronics, providing a method for the development of antiferromagnetic material-based next generation MRAM.

Field-free switching of VG-SOT-pMTJ device through the interplay of SOT, exchange bias, and VCMA effects

Field-free magnetization switching via the interplay of spin orbit torque (SOT), exchange bias field (HEX), and voltage controlled magnetic anisotropy (VCMA) is crucial for the development of scalable, high speed, and energy-efficient spintronic memories. This has been experimentally demonstrated by the rapid evolution of the voltage gated-spin orbit torque-magnetic random access memory (VG-SOT-MRAM) cell, in which perpendicular spin current is fed along with the in-plane HEX and VCMA assistance for cell programming. Here, we have examined the writing properties of a three terminal voltage gated-spin orbit torque-perpendicularly magnetized magnetic tunnel junction (VG-SOT-pMTJ) device structure (IrMn/CoFeB/MgO/CoFeB) in-depth through simulation. We observed that SOT critical switching current (I_SOT) decreases either by increasing the VCMA voltage or FL thickness. Even SOT field-like torque can accelerate the switching process and modulate the critical switching current. As the VCMA voltage rises, I_SOT falls by nearly 60%. In our experimental setup, VCMA/SOT optimal pulse width and amplitude for better write delay are 1 ns and 0.3 V, respectively. Furthermore, the impacts of free layer thickness, pMTJ radius, HEX, and noise are analyzed. Finally, we demonstrate the dependency of material parameters on temperature and VCMA voltage.

Ion-irradiation-induced field-free magnetization switching in synthetic antiferromagnets by spin–orbit torque

The perpendicularly magnetized synthetic antiferromagnets (SAFs) are expected to be ideal materials for magnetic random-access memory (MRAM) due to the low cross-talk noise and the high recording density. Normally, an in-plane magnetic field is required to break the inversion symmetry for deterministic magnetization switching in perpendicular SAFs by SOTs, which is undesirable for applications. In this work, the ion-irradiation method was adopted to engineer the interface of a perpendicularly magnetized Pt/Co/Pt/Ru/Pt/Co/Ta SAF to achieve the filed-free switching of the magnetization via the in-plane magnetization component of top-Co layer, which provides an in-plane effective field to bottom-Co layer and breaks the symmetry. Furthermore, with the increase of ion fluence, the critical switching current density decreases, which is consistent with the increase of spin Hall angle induced by ion-irradiation measured by the harmonic Hall voltage method. These results indicate that ion irradiation is a promising approach for realizing field-free magnetization switching in SAFs driven by SOTs and has potential application in SOT-based spintronic devices.

Electrical Control of Spin Hall Efficiency and Field-Free Magnetization Switching in W/Pt/Co/Pt Heterostructures with Competing Spin Currents.

Reversal of magnetization via current-induced spin-orbit torque (SOT) is one of the core issues in spintronics. However, an in-plane assistant field is usually required for the deterministic switching of a perpendicularly magnetized system. Additionally, the efficiency of SOT is low, which is detrimental to device applications. This study achieved a reversible and non-volatile control of the critical current for magnetization switching and spin Hall efficiency in the TaN/W/Pt/Co/Pt/TaN heterostructures by ionic liquid (IL) gating-induced hydrogen ion adsorption and desorption in the upper Pt layer. Furthermore, the thinning of the Pt and TaN capping layers activated the oxygen ion migration toward the Co layer under IL gating, resulting in an exchange bias field and allowing field-free magnetization switching and Boolean logic operation. The results of this study offer an intriguing opportunity to promote the development of SOT-based spintronic devices from the perspective of iontronics with low energy dissipation.

Field-Free Magnetization Switching in A1 CoPt Single-Layer Nanostructures for Neuromorphic Computing

Current-induced field-free magnetization switching in single-layer films has been widely investigated for the application of spin–orbit torque (SOT) drivers in low-power, high-density, and nano-sized memory. In this paper, we report field-free SOT switching with threefold angle dependence in A1 disordered single-layer CoPt using the MgO (111) substrate. It is found that the magnetization could not switch in the absence of an external in-plane magnetic field when the current was flowing in the direction of the mirror symmetry high-symmetry axis ([11–2]). While along the direction of the low-symmetry axis ([1–10]) which destroyed the mirror symmetry to the greatest extent, field-free switching could be performed, and the switching ratio was up to 30%. This was mainly attributed to the composition gradient-induced in-plane inversion asymmetry and the low symmetry at the interface of Co/Pt broken out-of-plane inversion. Furthermore, CoPt devices with nanoscale thickness exhibited stable multistate magnetic switching behavior without an external magnetic field, and simultaneously nonlinear synaptic characteristics were obtained. A neural network was established, in which the synaptic weights between the hidden layer and the output layer are updated by using the resistance state of SOT synaptic devices, and the network could achieve about 90.54% recognition accuracy when applied to MNIST’s handwritten digital dataset. This work confirmed that a field-free SOT switch could be realized in single-layer CoPt, which provided theoretical guidance and data support for spin devices.

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.

Chiral Interlayer Exchange Coupling for Asymmetric Domain Wall Propagation in Field-Free Magnetization Switching

The discovery of chiral spin texture has unveiled many unusual yet extraordinary physical phenomena, such as the Néel type domain walls and magnetic skyrmions. A recent theoretical study suggests that a chiral exchange interaction is not limited to a single ferromagnetic layer; instead, three-dimensional spin textures can arise from an interlayer Dzyaloshinskii-Moriya interaction. However, the influence of chiral interlayer exchange coupling on the electrical manipulation of magnetization has rarely been addressed. Here, the coexistence of both symmetric and chiral interlayer exchange coupling between two orthogonally magnetized CoFeB layers in PtMn/CoFeB/W/CoFeB/MgO is demonstrated. Images from polar magneto-optical Kerr effect microscopy indicate that the two types of coupling act concurrently to induce asymmetric domain wall propagation, where the velocities of domain walls with opposite chiralities are substantially different. Based on this microscopic mechanism, field-free switching of the perpendicularly magnetized CoFeB is achieved with a wide range of W thicknesses of 0.6-4.5 nm. This work enriches the understanding of interlayer exchange coupling for spintronic applications.

Field-free spin–orbit devices via heavy-metal alloy with opposite spin Hall angles for in-memory computing

With the advantages of high speed, low energy consumption, and non-volatility, spin–orbit devices are promising to be used in the field of in-memory computing. However, for large-scale integration, a simpler field-free switching scheme needs to be further explored. Here, we prepared field-free spin–orbit devices based on the PtW alloy layer with competing spin currents. The preparation of such devices is friendly to integration, because there is no requirement of introducing additional processing technology. Only the traditional heavy-metal layer is needed to be replaced by an alloy layer with opposite spin Hall angles. A series of positive and negative pulsed current tests have shown a stable field-free magnetization switching in the Ta/PtW/Co/AlOx/Pt device. The programmable Boolean logic of NAND and NOR were performed in a single device by changing the initial magnetization state. In addition, a pair of devices were connected with always opposite magnetizations to implement the XNOR logic gate, which can be applied to perform the dot product operation in the binary neural network. Based on the spin XNOR gates, a three-layer binary neural network achieves 89% recognition accuracy of handwritten digits. Our findings pave the way to efficient in-memory computing applications.

Observation of anti-damping spin–orbit torques generated by in-plane and out-of-plane spin polarizations in MnPd3

Large spin-orbit torques (SOTs) generated by topological materials and heavy metals interfaced with ferromagnets are promising for next-generation magnetic memory and logic devices. SOTs generated from y spin originating from spin Hall and Edelstein effects can realize field-free magnetization switching only when the magnetization and spin are collinear. Here we circumvent the above limitation by utilizing unconventional spins generated in a MnPd3 thin film grown on an oxidized silicon substrate. We observe conventional SOT due to y spin, and out-of-plane and in-plane anti-damping-like torques originated from z spin and x spin, respectively, in MnPd3/CoFeB heterostructures. Notably, we have demonstrated complete field-free switching of perpendicular cobalt via out-of-plane anti-damping-like SOT. Density functional theory calculations show that the observed unconventional torques are due to the low symmetry of the (114)-oriented MnPd3 films. Altogether our results provide a path toward realization of a practical spin channel in ultrafast magnetic memory and logic devices.