Funan Tan

We demonstrate a fabrication strategy for multilayered CoFeB structures that overcomes the intrinsic trade‐off between magnetic volume and perpendicular magnetic anisotropy, enabling a substantial enhancement in thermal stability. In conventional CoFeB/MgO heterostructures, interfacial anisotropy restricts the ferromagnetic layer thickness, severely limiting the magnetic volume and thermal robustness. This limitation is eliminated using a continuous vacuum process combining thin‐film deposition, annealing, and an intermediate reverse‐sputtering step to remove temporary MgO layers. The resulting multilayer stacks consist of ferromagnetically coupled CoFeB layers separated by thin Ta interlayers, yielding a significantly increased effective magnetic volume. Compared with a single‐layer reference, the estimated thermal stability factor, calculated from the experimentally determined K _eff and magnetic volume, is enhanced by 400%. The multilayer structure is intrinsically asymmetric, allowing direct integration with adjacent spin‐source layers for efficient net spin orbit torque‐driven magnetization switching. These results establish a viable route toward thermally robust perpendicular magnetic devices suitable for next‐generation spintronic applications.

Topological insulators offer unique properties for generating high spin–orbit torque (SOT), promising to revolutionize magnetoresistive random-access memory with a low power consumption. In this work, BiSb is integrated into perpendicular magnetic tunnel junctions (pMTJs) to enable efficient SOT switching. By optimizing the BiSb thickness and introducing a Ta buffer layer, a threefold enhancement in damping-like SOT efficiency and a 60% reduction in switching current are achieved compared to the BiSb-free sample. X-ray diffraction measurements confirm the improved crystalline quality with increasing BiSb thickness, contributing to the enhanced spin current generation. The fabricated BiSb-pMTJs exhibit key neuromorphic functionalities, including gradual long-term potentiation/depression and sigmoidal resistance modulation under pulsed current. Utilizing these features, a three-layer artificial neural network is implemented based on experimentally extracted device behavior, achieving over 90% accuracy in handwritten digit recognition.

All‐solid‐state inorganic hydrogen‐based three‐terminal memristors (H‐3TMs) suffer from poor retention, susceptibility to humidity and temperature, and the reliance on wet chemistry during fabrication, hindering their manufacturability within existing foundry processes. To address these, this study presents a CMOS‐compatible H‐3TM based on reversible intercalation and extraction of protons between the SiN _x electrolyte and WO _x channel. The protons are introduced via a straightforward hydrogen plasma treatment, promoting a compatible fabrication process with back‐end‐of‐line integration. Experimental and simulation results indicate that the low proton transport tendency across the electrolyte/channel interface without an external electric field contributes to high retention performance. Furthermore, the device demonstrates linear potentiation and depression, 512 conductance states with a dynamic range of ≈40, low energy operation (≈73 fJ per write), and excellent overall device‐to‐device variation. Its analog properties are evaluated under the training and inference framework of MNIST and Fashion‐MNIST datasets. The device achieved training and inference accuracies only 0.4% and 0.3% below the ideal benchmark on the F‐MNIST dataset. This work offers a rational approach for future artificial synaptic device design and fabrication.

In this work, we report a magnonic device capable of dynamic control over magnon propagation. By leveraging voltage-controlled magnetic anisotropy on yttrium iron garnet waveguides, we have carried out simulations of an active demultiplexer and half-adder designed using inverse design principles. A high output intensity multiplexer was similarly developed via inverse design to mitigate the magnon re-emission issue in Y-shaped combiners. Trapezoid electrodes were also introduced to minimize magnon intensity losses due to the magnetic anisotropy gradients across the cascading magnon circuit. The magnonic half-adder, constructed using active demultiplexers and a multiplexer, showcases the potential of magnonic logic circuits for binary addition operations.

High efficiency and out-of-plane spin–orbit torque (OOP-SOT) driven magnetization switching is essential for developing spin-based memory and logic devices. In this study, we report the generation of a large charge-to-spin conversion and bidirectional OOP-SOT by engineering a vertical magnetization gradient within a Co/Ho multilayer system. Exploiting the antiferromagnetic coupling between Co and Ho, the magnetization gradient up to 16.8 (emu/cm−3)/nm was achieved by gradually varying the Ho layer thickness from 0.4 to 0.9 nm. The presence of the OOP-SOT was confirmed through Hall resistance-field loop shift measurements, which has been attributed to the broken symmetry in spin current reflection and transmission in the Co/Ho multilayer with vertical magnetic property gradients. Additionally, the effective field of the OOP-SOT is strongly correlated with the direction of the magnetization gradient, measured to be around +0.7 and –2.5 Oe/mA for the positive and negative magnetization gradients, respectively. Furthermore, the largest SOT in the Co/Ho multilayer with a negative gradient was observed compared to the positive gradient and uniform Co/Ho multilayer structures. The enhanced SOT is attributed to the bulk Rashba field generated by the gradient structure, along with increased spin polarization. The results demonstrated here provide a promising approach for utilizing magnetization gradients for the development of efficient, current-driven spin-based storage and logic devices.

Rare-earth ferromagnetic (RE–FM) heterostructures have attracted significant attention due to their intricate spin structures and physical phenomena. The antiferromagnetic coupled (AFC) interface formed by the distinctive interaction between the FM and RE elements has critical contributions to the magnetization reversal process. In this work, we investigate the enhancement of current-induced magnetization switching with the AFC interface at the Co/Ho heterostructure. The results shows that an increased spin–orbit torque (SOT) efficiency of up to 250% was achieved at a Ho thickness of 7 nm, with a critical switching current density of 2.7 × 1010 A/m2. When a Cu interlayer was introduced between the Co/Ho interface, a decreased SOT efficiency was observed, indicating that the SOT enhancement is primarily attributed to the AFC interfacial effect. At the AFC interface, the interaction between Co and Ho atoms generates an additional torque, enhancing the effective SOT efficiency.

One of the main challenges in the development of conductive bridging random access memory (CBRAM) is the large stochastic nature of ion movement that ultimately leads to large parameter variability. In this study, the resistive switching variability of CBRAM devices is significantly improved by employing Co–Cu alloy as the active electrode. By comparing with Pt/Ta2O5/Co devices, the Co70Cu30 alloy exhibited lower forming voltage (<2 V), lower SET voltage (<0.70 V), and faster response time (∼70 ns). The filament stability indicated by the distribution of SET/RESET voltage and high resistance state/low resistance state variation was significantly improved. Our experimental results suggest the formation of Co filaments, and the proposed mechanism is governed by the galvanic effect. In addition, a comparison between Co70Cu30 and Co30Cu70 alloys highlights that the relative proportion between Co and Cu plays an essential role in the device performance. A physical model based on different electrochemical activities of the alloys has been proposed to explain the filament formation and the improved switching uniformity in the Co70Cu30 alloy. This study not only develops a CBRAM with enhanced performance but also advances the implementation of suitable alloy systems for the application of such devices.

We study the W/Cu multilayers as a spin current source and the coherent spin–orbit torques in a Fe layer using the spin-torque ferromagnetic resonance (STFMR) technique. With increasing numbers of layers, the line shape of the STFMR signals changes from predominantly antisymmetric to predominantly symmetric. When using [W(0.5)/Cu(0.5)]5 as a spin current source, the symmetric amplitude increases by a factor of 5 compared to a single W layer. Simultaneously, the effective damping parameter also increases slightly due to enhanced spin pumping. Along with an increasing trend in the damping-like torque efficiency, this suggests that the extrinsic spin Hall effect is enhanced. Concurrently, the antisymmetric amplitude decreases significantly by a factor of 27, which indicates an increase in the field-like torque when multilayers are used as a spin current source.

Spin–orbit torque (SOT) induced magnetization switching and SOT modulation by interfacial coupling exhibit good potential in spintronic devices. In this work, we report the enhancement of damping-like field and SOT efficiency of up to 60% and 23%, respectively, in perpendicularly magnetized Pt/Co/HfO_x heterostructures over a Pt/Co system at an optimal thickness of 2 nm HfO_x. The SOT improvement is primarily attributed to the interfacial oxidization of the Co layer, and the strength is tunable via voltage-induced oxygen ion migration at the Co/HfO_x interface. Our measurement reveals that by controlling gate voltages, the Co oxidation can be increased, which leads to the SOT efficiency enhancement. Our work promotes the SOT enhancement and modulation by oxidation effects for energy-efficient spintronic devices.

Spin–orbit torque (SOT) characterization techniques generally require the Hall cross that generally demands lithography resources and time. It is highly desirable to characterize SOT efficiencies with minimal sample processing time. Here, we demonstrate a lithography-free technique to determine the spin–orbit torque efficiency in a perpendicular magnetic anisotropy ferromagnetic heterostructure. By utilizing a customized four-point probe in a rhombus geometry, harmonic Hall measurement was performed on continuous films of a Pt/Co/Ti structure to characterize the spin–orbit torque efficiency. A correction factor, which is due to the non-uniform current distribution across the continuous film, was experimentally evaluated by taking the ratio of the measured damping-like field of the continuous film to that of a fabricated Hall device. Additionally, this correction factor is analytically derived and experimentally shown to be determined by the configuration of the probes and is independent of the structure material. Our measurement reveals that by performing a single calibration process for the particular set of probes, the same correction factor was validated on a second ferromagnetic heterostructure, Ti/Pt/Co/Ta; hence, it can be applied to other SOT films' stack measurements. Our four-probe harmonic Hall technique provides an alternative and swift way for SOT investigations by eliminating multiple lithography processes necessary in conventional approaches.