Spintronic Memory Research Articles

Field-free switching model of spin–orbit torque (SOT)-MTJ device with thermal effect based on voltage-controlled magnetic anisotropy (VCMA)

Electrically driven magnetization switch has attracted much attention in the new spintronic memory, especially for spin–orbit torque (SOT)-based magnetic random access memory (MRAM). However, the published models are facing limitations with the continuous shrinkage of the feature size down to nanoscale. Also, the thermal effect caused by switching operation is non-negligible. Therefore, an effective model is needed to represent the switching dynamic of the device concerning the influences of the nanoscale and the thermal effect. In the paper, a compact model of three-terminal SOT-driven switching is established. The influence of the voltage-controlled magnetic anisotropy (VCMA) and spin transfer torque (STT) effect induced by bias voltage on the field-free SOT-driven switching is considered by numerically solving the LLG equations. Furthermore, a 3D model of the SOT-MTJ device is established by finite element method to trace the thermoelectric behavior inside the device. The thermoelectric behavior is integrated into the compact model to show the influence of the temperature on the switching behavior, highlighting the importance of the thermal effect for the realistic modelling of SOT-driven switching. Finally, a novel voltage pulse scheme is proposed, which can effectively shorten the switching time and improve the reliability of the device. The established model could provide strategies and guidelines for next-generation memory design and application.

Gate-voltage control of alternating-current-driven skyrmion propagation in ferromagnetic nanotrack devices

Magnetic skyrmions, with topologically protected particle-like magnetization configurations, are promising information carriers for future spintronics devices with ultralow energy consumption. Generally, during motion, skyrmions suffer from the skyrmion Hall effect (SkHE) wherein the skyrmions deflect away from the intended path of the driving force. Numerous methods have been proposed to avoid this detrimental effect. In this study, we propose controllable alternating current (AC)-driven skyrmion propagation in a ferromagnetic nanowire based on combination of gate-voltage-controlled magnetic anisotropy (VCMA) and SkHE. Micromagnetic simulations show that a skyrmion oscillatory closed-loop-like in situ motion driven by AC can be transformed into directional ratchet-like propagation along the nanotrack by creating a VCMA-gate barrier. Additionally, we show that the skyrmion propagation conditions depend on the gate barrier potential and driving AC parameters, and they can be used for the optimal design of nanotrack devices. Moreover, this mechanism could be used to control skyrmion macroscopic propagation directions by dynamically alternating the voltage of another series of gates. We further show the dynamic control of the long-distance propagation of skyrmions along with the pinning state. The study results provide a promising route for designing future skyrmion-based spintronics logical and memory devices.

Epitaxial growth of flexible 1T-VSe2 thin films on mica by pulsed laser deposition

Recently, vanadium diselenide (VSe2), a member of transition metal dichalcogenides, has attracted a great deal of interest in spintronic devices and memory devices due to its unique physical properties. However, it is still a challenge to prepare a continuous VSe2 thin film which is critical for its potential application. Here, we report a continuous single-crystalline 1T-VSe2 thin film grown on mica by pulsed laser deposition. Both x-ray diffraction and high-resolution transmission electron microscopy verify the van der Waals epitaxy of the VSe2/mica heterostructure. Free-standing and flexible VSe2 thin films can be obtained and combined with integrated circuit technology, which is of great significance for the application of two-dimensional materials in the field of multifunctional flexible electronic devices.

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.

Enhancing the limit of uniaxial magnetic anisotropy induced by ion beam erosion

Artificial tailoring of magnetic anisotropy by manipulating interfacial morphology and film structure is of fundamental interest from an application point of view in spintronic and magnetic memory devices. This Letter reports an approach to engineer and enhance the strength of oblique incidence ion beam erosion (IBE)-induced in-plane uniaxial magnetic anisotropy (UMA) by simultaneous modification of film morphology and film texture. Cobalt film and Si substrate have been taken as a model system to meet this objective. Unlike conventional thin film deposition on ripple patterned substrate or post-growth IBE of film, we direct our effort to the sequential deposition and subsequent IBE of the film. Detailed in situ investigation shows that the film grows in a textured polycrystalline state with the formation of nanometric surface ripples. The film also exhibits pronounced UMA with an easy axis oriented parallel to the surface ripple direction. Remarkably, the induced UMA is about one order of magnitude larger than the IBE-induced UMA reported earlier. The capability of imposing in-plane crystallographic texture throughout the film layer gives rise to magneto-crystalline anisotropy along with the shape anisotropy of nanometric surface ripples, which enhances the strength of the UMA and illustrates the universal applicability of the present method.

Enhanced spin-orbit torque and field-free switching in Au/TMDs/Ni hybrid structures.

Spin-orbit torque (SOT) plays a significant role in spintronic logic and memory devices. However, due to the limited spin Hall angle and SOT symmetry in a heavy-metal-ferromagnet bilayer, further improving SOT efficiency and all-electric magnetization manipulation remain a challenge. Here we report enhanced SOT efficiency and all-electric switching in Au based magnetic structures, by inserting two-dimensional transition metal dichalcogenides (2D TMDs) with large spin-orbit coupling. With the TMD spacer insert, both damping-like and field-like SOTs are improved, and an unconventional out-of-plane damping-like SOT is induced, due to the interface orbital hybridization, modified spin-mixing conductance and orbital current. Moreover, current induced field-free magnetization switching is demonstrated in Au/WTe2/Ni and Au/MoS2/Ni devices, and it shows multiple intermediate states and can be efficiently controlled by an electric current. Our results open a path for increasing torques and expand the application of 2D TMDs in spintronic devices for neuromorphic computing.

Cleavable crystals, crystal structure, and magnetic properties of the NbFe1+xTe3 layered van der Waals telluride.

Transition metal-based two-dimensional nanomaterials with competing magnetic states are at the cutting edge of spintronic and low-power memory devices. In this paper, we present a Fe-rich NbFe1+xTe3 layered telluride (x ≈ 0.5), which shows an interplay of spin-glass and antiferromagnetic states below the Néel temperature of 179 K. The compound has a layered crystal structure, where the NbFeTe3 layers are terminated by the Te atoms and van der Waals gaps. Bulk single crystals grown by chemical vapor transport reactions possess the (1̄01) cleavage plane suitable for the exfoliation of two-dimensional nanomaterials. Combination of high-resolution transmission electron microscopy and powder X-ray diffraction reveals the zigzag ladders of Fe atoms inside the structural layers, as well as complementary zigzag chains of the partially occupied Fe positions in the interstitial region. Fe atoms carry large effective magnetic moment of 4.85(3)μB per atom in the paramagnetic state yielding intriguing magnetic properties of NbFe1+xTe3. They include frozen spin-glass state at low temperatures and spin-flop transition in high magnetic fields indicating promising flexibility of the magnetic system and its potential control by magnetic field or gate tuning in the spintronic devices and heterostructures.

Sputter Gas Damage in Nanolayered Pt/Co/Ir-based Synthetic Antiferromagnets for Top-Pinned Magnetic Tunnel Junctions

We examine the role of sputter damage on limiting the perpendicular magnetic anisotropy and interlayer exchange coupling in Pt/Co/Ir-based synthetic antiferromagnets, a key building block used in the microelectronics community for standalone and embedded magnetic random access memories. By replacing the more conventional Ar process gas with Kr or Xe, we reduce the energy of backscattered sputter gas ions bombarding the substrate from in excess of 100 eV to a few 10 s of eV. Reducing this kinetic bombardment is critical to avoid intermixing of this nanolayered functional heterostructure, with the constituent Pt, Co, and Ir layer thicknesses each below 1 nm. Our approach leads to a simultaneous enhancement in the perpendicular magnetic anisotropy (>2 × 106 J/m3) and interlayer exchange coupling energy (>3.5 mJ/m2). This advantageous improvement in perpendicular anisotropy and interlayer exchange coupling is evident on a 5 nm-thick Pt buffer layer and additionally on an MgO/CoFeB/Mo underlayers complex, showing that this method can improve the performance of both top- and bottom-pinned reference layers in perpendicular magnetic tunnel junction devices. These experimental findings may help advance the processing of conventional and synthetic antiferromagnetic devices for emerging spintronic memory technologies.

Boosting Spin-Orbit-Torque Efficiency in Spin-Current-Generator/Magnet/Oxide Superlattices

Efficient manipulation of magnetic materials is essential for spintronics. In spin-current generator/magnet bilayers, the efficiency of spin-orbit torques per magnetic layer thickness scales inversely with the magnetic layer thickness, leading to considerable power increase in applications with large magnetic layer thickness. Here, we develop a 3D spin-orbit material scheme in which the spin torque efficiency can be remarkably boosted up by stacking [spin-current generator/magnet/oxide]n superlattices, with the oxide layers breaking the inversion symmetry. In contrast, the spin torque diminishes in [spin-current generator/magnet]n superlattices lacking inversion symmetry breaking. These results advances the understanding of spin-orbit torques in magnetic multilayers and establishes spin-current generator/magnet/oxide superlattices as advantageous bricks for development of high energy-efficiency, high-endurance, and high-density spintronic memory and computing.

Sign reversal of the anomalous Hall effect in antiperovskite (110)-oriented Mn3.19Ga0.81N1−δ film

Antiperovskite compounds with abundant magnetic phase transitions provide an ideal platform for exploring nontrivial magnetotransport responses. In this study, the anomalous Hall effect (AHE) in an antiperovskite (110)-oriented Mn3.19Ga0.81N1−δ film up to room temperature was observed, and an unusual sign reversal was detected in the Hall measurements. The AHE reversal suggests that the carrier reversal corresponds to a magnetic transition from a ferrimagnetic order to noncollinear antiferromagnetic order at about 240 K. Analysis of the scaling relation of AHE indicates that the sign reversal originates from the transition from the skew scattering dominated AHE to the intrinsic mechanism dominated AHE. These findings will inspire effective control of the magnetic configuration and innovative applications for manipulating carrier transport in spintronic memory devices based on antiperovskite films.

Ta thickness effect on field-free switching and spin–orbit torque efficiency in a ferromagnetically coupled Co/Ta/CoFeB trilayer

Current induced spin–orbit torque (SOT) switching of magnetization is a promising technology for nonvolatile spintronic memory and logic applications. In this work, we systematically investigated the effect of Ta thickness on the magnetic properties, field-free switching and SOT efficiency in a ferromagnetically coupled Co/Ta/CoFeB trilayer with perpendicular magnetic anisotropy. We found that both the anisotropy field and coercivity increase with increasing Ta thickness from 0.15 nm to 0.4 nm. With further increase of Ta thickness to 0.5 nm, two-step switching is observed, indicating that the two magnetic layers are magnetically decoupled. Measurements of pulse-current induced magnetization switching and harmonic Hall voltages show that the critical switching current density increases while the field-free switching ratio and SOT efficiency decrease with increasing Ta thickness. Both the enhanced spin memory loss and reduced interlayer exchange coupling might be responsible for the β DL decrease as the Ta spacer thickness increases. The studied structure with the incorporation of a CoFeB layer is able to realize field-free switching in the strong ferromagnetic coupling region, which may contribute to the further development of magnetic tunnel junctions for better memory applications.

Strain- and thickness-dependent magnetic properties of epitaxial La0.67Sr0.33CoO3/La0.67Sr0.33MnO3 bilayers

Magnetic properties and interfacial phenomena of epitaxial perovskite oxides depend sensitively on parameters such as film thickness and strain state. In this work, epitaxial La0.67Sr0.33CoO3 (LSCO)/La0.67Sr0.33MnO3 (LSMO) bilayers were grown on NdGaO3 (NGO) and LaAlO3 (LAO) substrates with a fixed LSMO thickness of 6 nm, and LSCO thickness (tLSCO) varying from 2 to 10 nm. Soft x-ray magnetic spectroscopy revealed that magnetically active Co2+ ions that strongly coupled to the LSMO layer were observed below a critical tLSCO for bilayers grown on both substrates. On LAO substrates, this critical thickness was 2 nm, above which the formation of Co2+ ions was quickly suppressed leaving only a soft LSCO layer with mixed valence Co3+/Co4+ ions. The magnetic properties of both LSCO and LSMO layers displayed strong tLSCO dependence. This critical tLSCO increased to 4 nm on NGO substrates, and the magnetic properties of only the LSCO layer displayed tLSCO dependence. A non-magnetic layer characterized by Co3+ ions and with a thickness below 2 nm exists at the LSCO/substrate interface for both substrates. The results contribute to the understanding of interfacial exchange spring behavior needed for applications in next generation spintronic and magnetic memory devices.

Chiral Spin Textures for Next-Generation Memory and Unconventional Computing

The realization of chiral spin textures - comprising myriad distinct, nanoscale arrangements of spins with topological properties - has established pathways for engineering robust, energy-efficient and scalable elements for non-volatile nanoelectronics. Particularly, current-induced manipulation of spin textures in nanowire racetracks and tunnel junction based devices are actively investigated for applications in memory, logic and unconventional computing. In this article, we paint a background on the progress of spin textures, as well as the relevant state-of-the-art techniques used for their development. In particular, we clarify the competing energy landscape of chiral spin textures, such as skyrmions and chiral domain walls, to tune their size, density and zero-field stability. Next, we discuss the spin texture phenomenology and their response to extrinsic factors arising from geometric constraints, inter-wire interactions and thermal-electrical effects. Finally, we reveal promising chiral spintronic memory and neuromorphic devices, and discuss emerging material and device engineering opportunities.

Suppression Effect on the Domain Wall Tilting in Synthetic Antiferromagnetic Racetrack Driven by the Spin–Orbit Torque

The perpendicularly magnetized synthetic antiferromagnet is an ideal candidate for racetrack memory due to the low stray field, allowing the closely packed domain wall (DW) and high recording density. Herein, the dynamics of DW motion driven by the magnetic field and current are investigated in the synthetic antiferromagnetic (SAF) Ta/Pt/Co/Pt/Ru/Pt/Co/Ta structures with different thicknesses of the top magnetic (TM) Co layers. The results illustrate that the direction of the DW motion will be reversed when the field pulse is below a critical field for the sample with a negligible remanence ratio. As for the current‐driven DW motion case, a significant enhancement of the DW velocity, as well as a strong suppression effect on the tilting of DW's plane due to the strong interlayer exchange coupling for the SAF structure, is found. The fast DW velocity and small tilting angle in SAF are valuable for spintronic racetrack memories.

Current-induced perpendicular effective magnetic field in magnetic heterostructures

The generation of perpendicular effective magnetic field or perpendicular spins (σz) is central for the development of energy-efficient, scalable, and external-magnetic-field-free spintronic memory and computing technologies. Here, we report the first identification and the profound impacts of a significant effective perpendicular magnetic field that can arise from asymmetric current spreading within magnetic microstrips and Hall bars. This effective perpendicular magnetic field can exhibit all the three characteristics that have been widely assumed in the literature to “signify” the presence of a flow of σz, i.e., external-magnetic-field-free current switching of uniform perpendicular magnetization, a sin 2φ-dependent contribution in spin-torque ferromagnetic resonance signal of in-plane magnetization (φ is the angle of the external magnetic field with respect to the current), and a φ-independent but field-dependent contribution in the second harmonic Hall voltage of in-plane magnetization. This finding suggests that it is critical to include current spreading effects in the analyses of various spin polarizations and spin–orbit torques in the magnetic heterostructure. Technologically, our results provide a perpendicular effective magnetic field induced by asymmetric current spreading as a novel, universally accessible mechanism for efficient, scalable, and external-magnetic-field-free magnetization switching in memory and computing technologies.

Tailoring skyrmion motion dynamics via magnetoelectric coupling: Toward highly energy-efficient and reliable non-volatile memory applications

Owing to the intriguing physical properties and significant spintronic applications, magnetic skyrmions have recently drawn intensive attention. Particularly, the skyrmion-based non-volatile memory (Sky-NVM) devices promise to be spintronic building blocks with high efficiency. However, tailoring Sky-NVM to achieve an energy-efficient and reliable operation in a synthetic, CMOS compatible, and magnetic-field-free integration is a challenging issue. Here, we report a new type of compact Sky-NVM with tailored skyrmion motion dynamics via in-plane strain gradient engineering. The skyrmion motion is merely driven by an in-plane electric field utilizing the magnetoelectric coupling effect, and the programmable switching is realized by gate biasing the potential barrier height via a voltage-controlled magnetic anisotropy. The proposed device is CMOS process compatible, and the comprehensive micromagnetic simulation results demonstrate that by applying a 0.3 V in-plane voltage combined with −0.17 V gate voltage, its write latency and the energy consumption reach 5.85 ns and 4.77 aJ/bit, respectively, superior to the state-of-the-art counterparts. Our work paves a new path toward ultra-low-power spintronic memory devices.

Field-free spin-orbit torque-induced perpendicularmagnetization switching in YIG/Ta/CoTb/Pt

<p indent="0mm">Spin-orbit torques (SOTs) provide a physical basis for designing new spintronic memory and logic devices. The key to the application is the realization of field-free SOT-driven magnetization switching. In this study, Ta/CoTb/Pt ferromagnetic multilayers were grown on yttrium iron garnet (YIG) magnetic insulating film. We achieved current-induced CoTb magnetization switching under zero magnetic field by using the magnetic dipole interaction field generated by the YIG magnetic insulating layer applied to the CoTb film with perpendicular magnetic anisotropy and the spin torque generated from Ta and Pt spin source layers. Experimentally, we measured the magnetization reversal behavior induced by pulse current through anomalous Hall effect and anomalous Hall resistance and observed the magnetic domain wall motion characteristics driven by pulse current under the magneto-optical Kerr microscope. Moreover, due to the influence of the magneto-optical Kerr microscope, it was found that for YIG/Ta/CoTb/Pt devices, the magnetic domain of ferromagnetic CoTb moves and completes magnetization switching by forming stripe-like features, which is significantly different from the magnetization switching characteristics of the Si/Ta/CoTb/Pt film. Our results may provide a new degree of freedom for the potential application value of the research of spintronic low-power devices, particularly field-free SOT-driven switching-based devices.

Electrostatic control of magnetism: Emergent opportunities with van der Waals materials

Since the first reports on the observation of magnetic order in atomically thin crystals of FePS3, CrI3, and CrGeTe3 in 2016 and 2017, there has been a greatly renewed interest in the magnetism of van der Waals (vdW) layered magnets. Due to their dimensionality and structure, ultrathin vdW magnets offer tantalizing prospects for electrostatic control of magnetism for energy-efficient spintronic logic and memory devices. Recent demonstrations revealed unusually high susceptibility of some vdW magnets to electrostatic fields and shed light on a path to room temperature devices, a long-standing goal in spintronics research. In this Perspective, we discuss the potential of different classes of vdW magnets for electrostatic control of magnetism by comparing their properties with those of non-vdW magnets such as dilute magnetic III–V semiconductors and perovskite manganites that have been intensively studied in the past two decades.

Convolutional neural networks with radio-frequency spintronic nano-devices

Convolutional neural networks (LeCun and Bengio 1998 The Handbook of Brain Theory and Neural Networks 255–58; LeCun, Bengio and Hinton 2015 Nature 521 436–44) are state-of-the-art and ubiquitous in modern signal processing and machine vision. Nowadays, hardware solutions based on emerging nanodevices are designed to reduce the power consumption of these networks. This is done either by using devices that implement convolutional filters and sequentially multiply consecutive subsets of the input, or by using different sets of devices to perform the different multiplications in parallel to avoid storing intermediate computational steps in memory. Spintronics devices are promising for information processing because of the various neural and synaptic functionalities they offer. However, due to their low OFF/ON ratio, performing all the multiplications required for convolutions in a single step with a crossbar array of spintronic memories would cause sneak-path currents. Here we present an architecture where synaptic communications are based on a resonance effect. These synaptic communications thus have a frequency selectivity that prevents crosstalk caused by sneak-path currents. We first demonstrate how a chain of spintronic resonators can function as synapses and make convolutions by sequentially rectifying radio-frequency signals encoding consecutive sets of inputs. We show that a parallel implementation is possible with multiple chains of spintronic resonators. We propose two different spatial arrangements for these chains. For each of them, we explain how to tune many artificial synapses simultaneously, exploiting the synaptic weight sharing specific to convolutions. We show how information can be transmitted between convolutional layers by using spintronic oscillators as artificial microwave neurons. Finally, we simulate a network of these radio-frequency resonators and spintronic oscillators to solve the MNIST handwritten digits dataset, and obtain results comparable to software convolutional neural networks. Since it can run convolutional neural networks fully in parallel in a single step with nano devices, the architecture proposed in this paper is promising for embedded applications requiring machine vision, such as autonomous driving.

Impact of Argon working pressure on the magnetic properties of sputtered Co60Fe20B20 thin films

We present thorough investigation of the influence of Argon gas working pressure (PAr) on Co60Fe20B20 (CFB) thin films deposited on Si. The longitudinal magneto-optic Kerr effect measurements indicate that all films exhibit uniaxial anisotropy caused by the self-steering effect arising due to the oblique angle deposition. The coercive field is found to be sensitive to PAr during growth and reached to a minimal value of 0.766 kA/m at 0.147 Pa. The ferromagnetic resonance measurements revealed that PAr has a direct impact on the inhomogeneous linewidth, anisotropy, and the effective Gilbert damping constant (αeff). The low value of αeff ∼(4.16±0.10)×10−3is achieved in film grown at PAr∼0.147 Pa with negligible inhomogeneity and low anisotropy. At this optimum value of PAr, the surface roughness of the films is found to be lowest (∼ 4 Å). The observed changes in the static and dynamic magnetization properties in these CFB films on varying the PAr are correlated to the growth-rate-dependent changes in internal stress in the film. The choice of the value of PAr employed during sputtering of CFB films at room temperature is thus demonstrated as the simple control parameter for tuning its structural and magnetic properties for the development of spintronic memory applications.