Heavy Metal Layer Research Articles

Field‐Free Switching and Enhanced Electrical Detection of Ferrimagnetic Insulators Through an Intermediate Ultrathin Ferromagnetic Metal Layer

AbstractPerpendicularly magnetized ferrimagnetic insulators offer great potential for the development of fast and energy‐efficient spintronic devices. However, a major challenge for these devices is the requirement of an auxiliary magnetic field to achieve spin‐orbit torque (SOT)‐driven magnetization switching, along with the extremely small electric read‐out signal from the adjacent heavy metal layer. In this work, an approach by introducing an ultrathin Co layer primarily with in‐plane magnetization at the interface of the Tm3Fe5O12/Pt bilayers, which enables field‐free deterministic switching of the perpendicular Tm3Fe5O12 layer is demonstrated. Meanwhile, it is observed that a large anomalous Hall resistance readout signal from the coupling‐induced perpendicular component of the interfacial Co, which is nearly two orders of magnitude larger than that observed in Tm3Fe5O12/Pt bilayers. The crucial role played by the Co layer in modifying the SOT is elucidated. This research represents a significant step toward the practical implementation of ferrimagnetic insulator devices.

Secondary Spin Current Driven Efficient THz Spintronic Emitters

AbstractFemtosecond laser‐induced photoexcitation of ferromagnet (FM)/heavy metal (HM) heterostructures has attracted attention by emitting broadband terahertz frequencies. The phenomenon relies on the formation of an ultrafast spin current, which is primarily attributed to the direct photoexcitation of the FM layer. However, during the process, the FM layer also experiences a secondary excitation led by the hot electrons from the HM layer that travel across the FM/HM interface and transfer additional energy in the FM. Thus, the generated secondary spins enhance the total spin current formation and lead to amplified spintronic terahertz emission. These results emphasize the significance of the secondary spin current, which even exceeds the primary spin currents when FM/HM heterostructures with thicker HM are used. An analytical model is developed to provide deeper insights into the microscopic processes within the individual layers, underlining the generalized ultrafast superdiffusive spin‐transport mechanism.

Nonvolatile magnetization switching in a single-layer magnetic topological insulator

Magnetization in a ferromagnetic layer could be manipulated by the spin-orbit torque whose generation commonly relies on the spin-orbit coupling from the adjacent heavy-metal layer within the bilayer. The fact that the magnetic topological insulator possesses both the ferromagnetic order with perpendicular anisotropy and inherent spin-orbit coupling inspires to realize such a torque-induced magnetization switching without forming any heterostructure with other materials. Here, only using a single layer of magnetically-doped topological insulator Cr:(Bi,Sb)2Te3, we realize a magnetization switching only by applying a large dc current. Assisted by the magnetic history, such a switching behaves nonvolatile under zero field but becomes volatile otherwise, as consistently shown by magnetoelectric transports and magneto-optical Kerr effect measurements. Static and quasistatic current are found to be equivalent for the switching. We propose that this switching may associate with the torque resulted from the spin-orbit coupling and the compositional asymmetry in the Cr-profile of the single layer.

Power efficient ReLU design for neuromorphic computing using spin Hall effect

We demonstrate that a magnetic tunnel junction injected with a spin Hall current can exhibit linear rotation of the magnetization of the free-ferromagnet using only the spin current. Using the linear resistance change of the magnetic tunnel junction (MTJ), we devise a circuit for the rectified linear activation (ReLU) function of the artificial neuron. We explore the role of different spin Hall effect (SHE) heavy metal (HM) layers on the power consumption of the ReLU circuit. We benchmark the power consumption of the ReLU circuit with different SHE layers by defining a new parameter called the spin Hall power factor. It combines the spin Hall angle, resistivity, and thickness of the HM layer, which translates to the power consumption of the different SHE layers during spin-orbit switching/rotation of the free FM. We employ a hybrid spintronics-CMOS simulation framework that couples Keldysh non-equilibrium Green’s function formalism with Landau–Lifshitz–Gilbert–Slonzewski equations and the HSPICE circuit simulator to account for the diverse physics of spin-transport and the CMOS elements in our proposed ReLU design. We also demonstrate the robustness of the proposed ReLU circuit against thermal noise and a non-trivial power-error trade-off that enables the use of an unstable free-ferromagnet for energy-efficient design. Using the proposed circuit, we evaluate the performance of the convolutional neural network for MNIST datasets and demonstrate comparable classification accuracies to the ideal ReLU with an energy consumption of 75 pJ per sample.

Nucleation and manipulation of skyrmions by spin currents mediated by an antiferromagnetic layer

Magnetic skyrmions can be used as information carriers in advanced memories, logic gates and computing devices in the future. How to generate skyrmions rapidly in zero field becomes an urgent problem. The application of spin currents will play a major role in promoting the applications of skyrmions in spintronic devices. In this paper, we propose a mechanism based on spin currents to generate skyrmions. A multilayer structure (Pt/Cr2O3/Co/Pt) is designed, where an antiferromagnetic layer (Cr2O3) is inserted into the traditional trilayer structure (Pt/Co/Pt). A spin current generated on the heavy metal layer makes the magnetizations of the antiferromagnetic layer oscillate, and then the magnetizations in the ferromagnetic layer are driven to oscillate through the interlayer exchange interaction, and magnetic skyrmions can finally be nucleated in the Co layer. Through this unique combination of spin currents and antiferromagnetic layers, we effectively reduce the current density and increase operability, and this application provides more possibilities for future information transmission.

Towards control of the chirality sign at ultrathin metal films: Bi at 2Ni/Co

Proximity effects can be used to introduce spin–orbit interactions in magnetic metallic layers in contact to a heavy metal (HM). This well known phenomenon has been exploited to induce chiral spin textures at Co ultrathin films, where the left- or right- handedness can be tuned by the HM layer position, based on the broken inversion symmetry of the film and the existence of an additive interface effect. Here we show that structural and chemical features introducing further symmetry reductions can be added to this scenario ultimately enabling control over the definition of a unique winding sense. We focus on 2Ni/Co heterostructures and Bi, a scarcely explored HM metal of large size, to combine a chemically inhomogeneous ferromagnetic stack along the normal to the surface with in-plane asymmetries. Our results are contrasted to 2Co layers combined with Ir.

Area-Efficient 1T-2D-2MTJ SOT-MRAM Cell for High Read Performance

This brief proposes a compact spin-orbit torque magnetic random-access memory (SOT-MRAM) cell structure with high read performance for embedded nonvolatile memory (eNVM) applications. The memory cell is composed of one transistor, two Schottky diodes, two magnetic tunnel junctions (MTJs) and one shared heavy metal (HM) layer, which can be abbreviated as 1T-2D-2MTJ cell. The two MTJs are stacked on the same HM layer for smaller area overhead and configured to be complementary with the magnetization of their pinned layers in the opposite direction to double the read sense margin. The write operation is also simplified since a current through the HM layer changes the resistances of both MTJs simultaneously. The evaluations include the micromagnetic simulation of the SOT device, the analysis of PVT variations for the read operation on bit-cell level, the estimation of bit-cell area and the simulations of performance and power consumption on array level. The results show that the proposed SOT-MRAM improves the performance and reliability of the read operation and achieves 75% (50%) reduction in bit-cell area compared to 4T-2MTJ SOT-MRAM (2T-2MTJ STT-MRAM), 83.0% reduction in write power consumption and 91.1% reduction in write latency compared to 2T-2MTJ STT-MRAM.

Search for large topological gaps in atomic spin chains on proximitized superconducting heavy-metal layers

One-dimensional systems comprising s-wave superconductivity with meticulously tuned magnetism realize topological superconductors hosting Majorana modes whose stability is determined by the gap size. However, for atomic spin chains on superconductors, the effect of the substrate’s spin-orbit coupling on the topological gap is largely unexplored. Here, we introduce an atomic layer of the heavy metal gold on a niobium surface combining strong spin-orbit coupling and a large superconducting gap with a high crystallographic quality, enabling the assembly of defect-free iron chains using a scanning tunneling microscope tip. Scanning tunneling spectroscopy experiments and density functional theory calculations reveal ungapped Yu–Shiba–Rusinov bands in the ferromagnetic chain despite the heavy substrate. By artificially imposing a spin spiral state, the calculations indicate minigap opening and zero-energy edge state formation. The methodology enables a material screening of heavy-metal layers on elemental superconductors for ideal systems hosting Majorana edge modes protected by large topological gaps.

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.

Modeling spintronic terahertz emitters as a function of spin generation and diffusion geometry

Spintronic THz emitters (STE) are efficient THz sources constructed using thin heavy-metal (HM) and ferromagnetic-metal (FM) layers. To improve the performance of the STE, different structuring methods (trilayers, stacked bilayers) have been experimentally applied. A theoretical description of the overall THz emission process is necessary to optimize the efficiency of STE. In particular, geometry, composition, pump laser frequency, and spin diffusion will be significant in understanding the pathways for further research developments. This work will apply a generalized model based on a modified Transfer Matrix Method (TMM). We will consider the spin generation and diffusion in the FM and HM layers and explain the spintronic THz emission process. This model is suitable for calculating emitted THz signal as a function of FM and HM thicknesses for different geometrical configurations. We will investigate a bilayer geometry as a test case, but the extension to a multi-layer configuration is straightforward. We will show how the different configurations of the sample will influence the THz emission amplitude.

Emission of coherent THz magnons in an antiferromagnetic insulator triggered by ultrafast spin–phonon interactions

Antiferromagnetic materials have been proposed as new types of narrowband THz spintronic devices owing to their ultrafast spin dynamics. Manipulating coherently their spin dynamics, however, remains a key challenge that is envisioned to be accomplished by spin-orbit torques or direct optical excitations. Here, we demonstrate the combined generation of broadband THz (incoherent) magnons and narrowband (coherent) magnons at 1 THz in low damping thin films of NiO/Pt. We evidence, experimentally and through modeling, two excitation processes of spin dynamics in NiO: an off-resonant instantaneous optical spin torque in (111) oriented films and a strain-wave-induced THz torque induced by ultrafast Pt excitation in (001) oriented films. Both phenomena lead to the emission of a THz signal through the inverse spin Hall effect in the adjacent heavy metal layer. We unravel the characteristic timescales of the two excitation processes found to be < 50 fs and > 300 fs, respectively, and thus open new routes towards the development of fast opto-spintronic devices based on antiferromagnetic materials.

Spintronic Heterostructures for Artificial Intelligence: A Materials Perspective

With the advent of the Big Data era, neuromorphic computing (NC) (also known as brain‐inspired computing) has gained a lot of research interest. Spintronic devices are the emerging candidates for implementing the NC due to their intrinsic nonvolatility, extremely high endurance, low‐power consumption, and complementary metal‐oxide compatibility. Many research groups have proposed various NC architectures based on spintronic devices. Herein, a collective survey of different spintronic‐based approaches is given for NC. The reviewed approaches include the progress of stochastic magnetic tunnel junction (MTJ) devices, spin‐torque nano‐oscillator, spin‐Hall nano‐oscillator, domain walls, and skyrmion devices. In all of these approaches, spin–orbit torque (SOT)‐based magnetization control, which is achieved via spintronics heterostructures, plays a significant role. Various heterostructures of heavy metal and ferromagnetic layers that have been proposed are reviewed for generating SOT. In addition, the phenomena and materials involved in the generation of orbital torque are summarized due to the orbital Hall effect (OHE), which has recently gained researchers’ attention. Finally, an outlook on the opportunities and challenges for spintronic‐based NC hardware is provided, shedding light on its great potential for artificial intelligence (AI) applications.

Perpendicular magnetic anisotropy, tunneling magnetoresistance and spin-transfer torque effect in magnetic tunnel junctions with Nb layers

Nb and its compounds are widely used in quantum computing due to their high superconducting transition temperatures and high critical fields. Devices that combine superconducting performance and spintronic non-volatility could deliver unique functionality. Here we report the study of magnetic tunnel junctions with Nb as the heavy metal layers. An interfacial perpendicular magnetic anisotropy energy density of 1.85 mJ/m2 was obtained in Nb/CoFeB/MgO heterostructures. The tunneling magnetoresistance was evaluated in junctions with different thickness combinations and different annealing conditions. An optimized magnetoresistance of 120% was obtained at room temperature, with a damping parameter of 0.011 determined by ferromagnetic resonance. In addition, spin-transfer torque switching has also been successfully observed in these junctions with a quasistatic switching current density of 7.3 times ;10^{5} A/cm2.

Study on the safety characteristics of molten corium in-vessel retention of annular fuel core

In the process of reactor severe accidents, maintaining the integrity of reactor pressure vessel to achieve IVR (In-Vessel Retention) of molten corium plays an important role in accidents mitigation and suppression. The ERVC (External Reactor Vessel Cooling) is an important technology to realize IVR. As a new economical annular fuel, the increase of zirconium content and power density in the core may threaten the integrity of lower head under severe accidents. In this paper, the IVRASA (In-Vessel Retention Analysis code in Severe Accident) is used to analyze the heat transfer characteristics of lower head molten pool corresponding to annular fuel core. The components of two-layer and three-layer configurations of molten pool are determined based on the geometric structures of annular fuel and verified heavy metal layer model. In terms of the integrity of lower head, the power density of the annular fuel core can be increased to about 110%.

Engineering Co/MgO interface with heavy metals for voltage-controlled magnetic anisotropy effect

We have investigated the voltage-controlled magnetic anisotropy (VCMA) effect in Co/MgO junctions by inserting a sub-atomic layer of heavy metals at the interface. We evaluated an electrical control of the magnetic anisotropy by using a micro-magneto-optical Kerr effect magnetometer. The VCMA coefficient of the Co/MgO junctions increases with the insertion of an Ir or Pt layer and decreases with the insertion of an Os layer. These results imply that the VCMA coefficient can be engineered by doping heavy metals at the interface. The interface engineering using a heavy metal layer provides us with a method to control the VCMA coefficient over a wide range.

Pulsed far-infrared radiation of ferromagnetic heterojunction and its photothermal regulation

Under illumination of a femtosecond laser pulse on the Pt/CoFe/Ta trilayer heterostructure, an impulsive spin current can be generated in the ferromagnetic layer due to the ultrafast demagnetization. The spin current is super-diffusively transported and injected into the neighboring heavy metal layers, and is converted into the transversal charge current due to the spin-orbit coupling, which is named inverse spin Hall effect. The transient charge current on a time scale of sub-picosecond gives rise to the electromagnetic radiation in the far-infrared range to the free space. In this work, we demonstrate two kinds of experiments to investigate the modulation of far-infrared emission by photo-thermal effect, which is due to the thermal energy deposed by light pulses on a short timescales. First, the amplitude of the emitted far-infrared pulse as a function of an applied magnetic field is measured, which shows a far-infrared hysteresis behavior. The coercive field of the sample obtained by far-infrared hysteresis is smaller than that obtained by the &lt;i&gt;M&lt;/i&gt;-&lt;i&gt;H&lt;/i&gt; hysteresis through vibrating sample magnetometer. In addition, the coercive field decreases with pump laser fluence increasing. Second, the control of spin polarization on an ultrafast timescale in the presence of a small magnetic field applied oppositely to that of the magnetization of the ferromagnetic sample. The amplitude of far-infrared time-domain signal reaches a maximum value at a pump fluence of 1.43 mJ/cm&lt;sup&gt;2&lt;/sup&gt;. For the pump fluence larger than 1.43 mJ/cm&lt;sup&gt;2&lt;/sup&gt;, the far-infrared pulse experiences a phase reversal. After the reversal, a decrease of the laser pump fluence cannot restore the original phase of the far-infrared pulse. The above two experimental results not only elucidate the photothermal effect of femtosecond laser pulses, but also provide a new method for controlling the far-infrared radiation pulses based on ultrafast spintronics. These results demonstrate that far-infrared emission spectroscopy can be used as an ultrafast optical method to investigate magnetic properties, such as the coercive field and anisotropy field of the samples.

Enhanced Spin–Orbit Torque Performances in PtRu–Co–PtRu–Ni–Ru-Based Spintronic Devices

The Pt80Ru20 alloy has been testified to be a satisfying heavy metal layer (HM) in spin–orbit torque (SOT)-induced spintronic devices with a relatively high-spin Hall angle (θDL) and a low critical current density. In this work, the Pt80Ru20–Co–Ni (xNi nm)–Ru multilayers are first prepared to find the field-free SOT-induced switching by the direct coupling of out-of-plane and in-plane ferromagnetic (FM) layers. A low in-plane assist field of −20 Oe and some optimized SOT performances are observed in the devices with xNi = 0.4 and 0.8 nm, respectively. To further enhance the performances of the devices, a 0.6 nm thin Pt80Ru20 layer is inserted between two FM layers. In the Pt80Ru20–Co–Pt80Ru20–Ni (tNi nm)–Ru multilayers, the energy consumption decreases dramatically and a higher θDL of 0.156 is obtained in the device with tNi = 0.8 nm. More importantly, the field-free current-induced magnetization switching is realized in the device with tNi = 1.4 nm. Furthermore, the interfacial Dzyaloshinskii–Moyria Interaction (DMI) can be weakened by the thin insertion as well. Our results reveal that the PtRu-based spintronic devices with enhanced SOT performances and field-free SOT-driven switching can be a promising candidate in the application of SOT-induced spintronic devices.

Process-induced magnetic tunnel junction damage and its recovery for the development of spin–orbit torque magnetic random access memory

The generation of torques in heterostructures of heavy metals and ferromagnets (HM / FM) in spin–orbit torque magnetic random access memory (SOT-MRAM) makes them interesting for the development of magnetic memory technology. Currently, the emerging SOT-MRAM technology faces major challenges in etching based on current manufacturing tools. In that respect, we experimentally investigate the two different etching recipes (namely, CF-Etch, & NH-Etch) of the reactive ion etching (RIE) process on the performance of a Ta/CoFeB/MgO/Ta (hard mask) magnetic film stacks to develop the SOT-MRAM technology. The experimental results demonstrate that CF-Etch etches Ta very fast, and MgO etches very slowly. In contrast, NH-Etch etches those two materials at a comparable rate. Both etches are terminated before MgO is removed. Based on the experimental results, we found that the CF-Etch reduces the interfacial anisotropy of CoFeB/MgO films. Moreover, the CF-Etch also destroys the magnetic moment of CoFeB film, which further deteriorates after the annealing process. In contrast, the magnetic properties of the stack are only slightly affected when the NH-Etch recipe is used. We also discuss the advantages and disadvantages of these two recipes based on the experimental study. Finally, we developed the SOT-MRAM devices based on these two etching recipes of the RIE process to improve the etching yield of this technology. The current SOT-MRAM fabrication process enables a uniform etch stop on the thin tunnel barrier layer and the Ta heavy metal layer (HM). A wafer-level statistical study of SOT switching characteristics, including tunnel magnetoresistance (TMR), SOT switching thresholds, and parallel/antiparallel state resistances (RP/RAP), has been performed.

Emergence of considerable thermoelectric effect due to the addition of an underlayer in Pt/Co/Pt stack and its application in detecting field free magnetization switching

Application of sufficient lateral current to a heavy metal (HM) can switch the perpendicular magnetization orientation of an adjacent ferromagnetic layer through spin–orbit torques (SOTs). The choice of the HM and its arrangement plays a major role for the SOT induced magnetization switching in magnetic heterostructures. Generally, thin Ta is used as an underlayer to the HM layer for better adhesion and smoothness of the HM layer. Here, we show that Ta addition to the asymmetric stack Pt/Co/Pt gives rise to several compelling effects, viz., thermoelectric effects [particularly, anomalous Nernst effect (ANE)], and enhanced perpendicular magnetic anisotropy which was negligible in a Pt/Co/Pt stack. For this Ta/Pt/Co/Pt stack, the antidamping-SOT values are evaluated after carefully removing the contribution from the ANE and it is found to match the AD-SOT of the Pt/Co/Pt stack. We have observed current-induced field-free magnetization switching Ta/Pt/Co/Pt stack with Co thickness gradient. Furthermore, we have utilized the thermoelectric effects to develop a technique to detect the field-free magnetization switching. This technique detects the second harmonic ANE signal as a reading mechanism. Using ANE symmetry with the applied current, the switching can be detected in a single current sweep which was corroborated to the conventional DC Hall method.

Large enhancement of magnetic damping in half-metallic Co2MnSi Heusler alloy thin films with heavy metal Pt layer observed by the all-optical method

Manipulation of magnetic moments through spin transport is determined by the effective spin mixing conductance g eff ↑↓ in the ferromagnetic metal/heavy metal heterostructure. Magnetization dynamics detection and evaluation of the Gilbert damping constant α is usually employed to estimate g eff ↑↓. An all-optical pump-probe method is a powerful method for investigating α with high accuracy because an extrinsic contribution on α can be suppressed at a high precession frequency with an increasing external magnetic field. A large voltage signal was recently observed in half-metallic Heusler alloy Co2MnSi thin film due to spin current injection; however, that voltage signal may also include contributions from thermal effect and spin Hall angle in the heavy metal layer. In this study, a large enhancement of α was observed in Co2MnSi/Pt heterostructure by using the all-optical method. g eff ↑↓ for Co2MnSi/Pt was 2 times larger than that for Fe/Pt. This large g eff ↑↓ is promising for spintronic device applications.