Voltage-controlled Magnetic Anisotropy Research Articles

Micromagnetic simulations for local phase control of propagating spin waves through voltage-controlled magnetic anisotropy

Spin waves, known for their ability to propagate without the involvement of moving charges, hold immense promise for on-chip information transfer and processing, offering a path toward post-CMOS computing technologies. This study investigates the potential synergy between propagating Damon–Eshbach spin waves and voltage-controlled magnetization in the pursuit of environmentally sustainable computing solutions. Employing micromagnetic simulations, we assess the feasibility of utilizing spin waves in DE mode in conjunction with localized voltage-induced alterations in surface anisotropy to enable low-energy logic operations. Our findings underscore the critical importance of selecting an optimal excitation frequency and gate width, which significantly influence the efficiency of the phase shift induced in propagating spin waves. Notably, we demonstrate that a realistic phase shift of 2.5 [π mrad] can be achieved at a Co(5 nm)/MgO material system via the voltage-controlled magnetic anisotropy effect. Moreover, by tuning the excitation frequency, Co layer thickness, gate width, and carefully selecting the dielectric layer, we extrapolate the potential to enhance the phase shift by a factor of 200 when compared to MgO dielectrics. This research contributes valuable insights toward developing next-generation computing technologies with reduced energy consumption.

Electrical control of the switching layer in perpendicular magnetic tunnel junctions with atomically thin Ir dusting

The use of magnetic tunnel junction (MTJ)-based devices constitutes an important basis of modern spintronics. However, the switching layer of an MTJ is widely believed to be an unmodifiable setup, instead of a user-defined option, posing a restriction to the function of spintronic devices. In this study, we realized a reliable electrical control of the switching layer in perpendicular MTJs with 0.1 nm Ir dusting. Specifically, a voltage pulse with a higher amplitude drives the magnetization switching of the MTJ's bottom electrode, while a lower voltage amplitude switches its top electrode. We discussed the origin of this controllability and excluded the possibility of back-hopping. Given the established studies on enhancing the voltage-controlled magnetic anisotropy effect by adopting Ir, we attribute this switching behavior to the significant diffusion of Ir atoms into the top electrode, which is supported by scanning transmission electron microscopy with atomic resolution.

Prospects for Antiferromagnetic Spintronic Devices

This article examines recent advances in the field of antiferromagnetic spintronics from the perspective of potential device realization and applications. We discuss advances in the electrical control of antiferromagnetic order by current-induced spin–orbit torques, particularly in antiferromagnetic thin films interfaced with heavy metals. We also review possible scenarios for using voltage-controlled magnetic anisotropy as a more efficient mechanism to control antiferromagnetic order in thin films with perpendicular magnetic anisotropy. Next, we discuss the problem of electrical detection (i.e., readout) of antiferromagnetic order and highlight recent experimental advances in realizing anomalous Hall and tunneling magnetoresistance effects in thin films and tunnel junctions, respectively, which are based on noncollinear antiferromagnets. Understanding the domain structure and dynamics of antiferromagnetic materials is essential for engineering their properties for applications. For this reason, we then provide an overview of imaging techniques as well as micromagnetic simulation approaches for antiferromagnets. Finally, we present a perspective on potential applications of antiferromagnets for magnetic memory devices, terahertz sources, and detectors.

Spintronic virtual neural network by a voltage controlled ferromagnet for associative memory

Recently, an associative memory operation by a virtual oscillator network, consisting of a single spintronic oscillator, was examined to solve issues in conventional, real oscillators-based neural networks such as inhomogeneities between the oscillators. However, the spintronic oscillator still carries issues dissipating large amount of energy because it is driven by electric current. Here, we propose to use a single ferromagnet manipulated by voltage-controlled magnetic anisotropy (VCMA) effect as a fundamental element in a virtual neural network, which will contribute to significantly reducing the Joule heating caused by electric current. Instead of the oscillation in oscillator networks, magnetization relaxation dynamics were used for the associative memory operation. The associative memory operation for alphabet patterns is successfully demonstrated by giving correspondences between the colors in a pattern recognition task and the sign of a perpendicular magnetic anisotropy coefficient, which could be either positive or negative via the VCMA effect.

Electrical Detection and Nucleation of a Magnetic Skyrmion in a Magnetic Tunnel Junction Observed via Operando Magnetic Microscopy.

Magnetic skyrmions are topological spin textures which are envisioned as nanometer scale information carriers in magnetic memory and logic devices. The recent demonstrations of room temperature skyrmions and their current induced manipulation in ultrathin films were first steps toward the realization of such devices. However, important challenges remain regarding the electrical detection and the low-power nucleation of skyrmions, which are required for the read and write operations. Here, we demonstrate, using operando magnetic microscopy experiments, the electrical detection of a single magnetic skyrmion in a magnetic tunnel junction (MTJ) and its nucleation and annihilation by gate voltage via voltage control of magnetic anisotropy. The nucleated skyrmion can be manipulated by both gate voltages and external magnetic fields, leading to tunable intermediate resistance states. Our results unambiguously demonstrate the readout and voltage controlled write operations in a single MTJ device, which is a major milestone for low power skyrmion based technologies.

Bifurcation to complex dynamics in largely modulated voltage-controlled parametric oscillator

An experimental demonstration of a parametric oscillation of a magnetization in a ferromagnet was performed recently by applying a microwave voltage, indicating the potential to be applied in a switching method in non-volatile memories. In the previous works, the modulation of a perpendicular magnetic anisotropy field produced by the microwave voltage was small compared with an external magnetic field pointing in an in-plane direction. A recent trend is, however, opposite, where an efficiency of the voltage controlled magnetic anisotropy (VCMA) effect is increased significantly by material research and thus, the modulated magnetic anisotropy field can be larger than the external magnetic field. Here, we solved the Landau–Lifshitz–Gilbert equation numerically and investigated the magnetization dynamics driven under a wide range of the microwave VCMA effect. We evaluated bifurcation diagrams, which summarize local maxima of the magnetization dynamics. For low modulation amplitudes, the local maximum is a single point because the dynamics is the periodic parametric oscillation. The bifurcation diagrams show distributions of the local maxima when the microwave magnetic anisotropy field becomes larger than the external magnetic field. The appearance of this broadened distribution indicates complex dynamics such as chaotic and transient-chaotic behaviors, which were confirmed from an analysis of temporal dynamics.

Voltage-controlled magnetic anisotropy effect through a high-k MgO/ZrO2/MgO hybrid tunneling barrier

We investigated the voltage-controlled magnetic anisotropy (VCMA) effect in epitaxial magnetic tunnel junctions (MTJs) with a hybrid MgO/ZrO2/MgO tunnel barrier. A metastable cubic ZrO2(001) thin film was successfully grown on a MgO(001) layer, leading to the high dielectric constant of 26.5. Using the hybrid tunneling barrier, we achieved the large VCMA coefficient of −350 fJ V−1 m−1, which is 70% larger than that observed in the magnetic tunnel junction with the single MgO barrier. Introduction of crystalline high-k dielectric tunneling barrier can open up new pathways to improving the VCMA properties in MTJs for voltage-driven spintronic devices.

Reconfigurable Skyrmion-Based Logic Gates: Versatile Design and Full-Scale Implementation.

Herein, we investigate the behavior of skyrmions within a racetrack design incorporating voltage-controlled magnetic anisotropy (VCMA) gates. Our analysis encompassed multiple forces, including spin currents and anisotropy gradients induced by bias voltages. As a result, the efficient control of skyrmion dynamics was achieved across various VCMA gate configurations. Building upon these findings, we propose an efficient approach to reconfigurable skyrmion logic (RSL) in a thin antiferromagnetic (AFM) film through a versatile design. Our RSL harnesses the selective integration of VCMA, spin-polarized currents, and skyrmion-skyrmion (sky-sky) interactions to implement multiple logic gates, including AND, OR, XOR, NOT, NAND, XNOR, and NOR. The design brings a significant advantage with its simplified fabrication process, making the implementation of the RSL practical and accessible for various applications. Furthermore, the RSL enables seamless dynamic switching between logic gates, thereby enhancing its multifunctionality. Additionally, the strategic incorporation of sky-sky interactions and skyrmion-edge repulsion prominently facilitates the realization of complex gates, such as NAND, XNOR, and NOR gates, that typically require intricate design efforts. Hence, this streamlined integration of RSL, coupled with its adaptability to changing computational needs, underscores its potential as a practical solution for implementing high-functionality skyrmion-based logic gates.

Voltage-controlled magnetic anisotropy gradient-driven skyrmion-based half-adder and full-adder.

Spintronic devices have revolutionized the way we process or store information compared to dissipative charge-based electronics. Among various spin-based technologies, skyrmions - topologically protected nano-size spin textures - have emerged as the most promising alternative for future data processing. Here, we have proposed binary adder circuits - central to most digital logic circuits - based on skyrmions. Using micromagnetic simulations, we have demonstrated half-adder and full-adder logic functionalities by precisely driving the skyrmions through voltage-controlled magnetic anisotropy gradient, besides taking advantage of the physical effects such as the skyrmion Hall effect, skyrmion-skyrmion topological repulsion and skyrmion-edge repulsions. The proposed voltage-control-based method of driving the skyrmions is energy efficient compared to the electrical current-driven approach, and it also overcomes the issue of Joule heating. A reliable operation in a wide range of Dzyaloshinskii-Moriya interaction strengths, magnetic anisotropy gradient, and dimensional parameters has been shown, which offers robustness to the device design. The results pave the way for the skyrmion-based computational architecture, which is significant for next-generation non-volatile data processing.

Large voltage-controlled magnetic anisotropy effect in magnetic tunnel junctions prepared by deposition at cryogenic temperatures

We investigated the influence of the buffer material and a cryogenic temperature deposition process on the voltage-controlled magnetic anisotropy (VCMA) effect for an ultrathin CoFeB layer in bottom-free type MgO-based magnetic tunnel junctions prepared by a mass production sputtering process. We used Ta and TaB buffers and compared the differences between them. The TaB buffer enabled us to form a flat and less-contaminated CoFeB/MgO interface by suppressing the diffusion of Ta with maintaining a stable amorphous phase. Furthermore, the introduction of cryogenic temperature deposition for the ultrathin CoFeB layer on the TaB buffer improved the efficiency of the VCMA effect and its annealing tolerance. Combining this with interface engineering employing an Ir layer for doping and a CoFe termination layer, a large VCMA coefficient of −138 ± 3 fJ/Vm was achieved. The developed techniques for the growth of ultrathin ferromagnet and oxide thin films using cryogenic temperature deposition will contribute to the development of high-performance spintronic devices, such as voltage-controlled magnetoresistive random access memories.

Engineering of Voltage-Controlled Magnetic Anisotropy Magnetic Tunnel Junctions at Cryogenic Temperatures

We identified through numerical simulations the optimal condition to have a deterministic switching regime assisted by Voltage Controlled Magnetic Anisotropy (VCMA). To minimize the write energy required to reach this regime, we measure the VCMA coefficient ξ on perpendicular magnetic tunnel junctions (pMTJ) with high resistance-area product (RA) and varying thicknesses of the FeCoB storage layer and the naturally oxidized tunnel barrier. The VCMA coefficient is higher as the effective anisotropy decreases, which is the case for larger Mg and FeCoB thicknesses. The temperature dependence of ξ was shown to increase from room temperature to 5K, showing values up to 35 fJ/Vm at 10K.

Voltage-controlled magnetic solitons motion in an anisotropic ferromagnetic nanowire

The precise manipulation of magnetic solitons remains a challenge and is considered a crucial process in magnetic storage. In this paper, we investigate the control of velocity and spatial manipulation of magnetic solitons using the voltage-controlled magnetic anisotropy effect. A long-wave model, known as the generalized derivative nonlinear Schrödinger (GDNLS) equation, is developed to describe the dynamics of magnetic solitons in an anisotropic ferromagnetic nanowire. By constructing the Lax pair for the GDNLS equation, we obtain the exact solutions including magnetic dark solitons, anti-dark solitons, and periodic solutions. Moreover, we propose two approaches to manipulate magnetic solitons: direct voltage application and inhomogeneous insulation layer design. Numerically results show the direct modulation of soliton velocity by a constant voltage, while time-varying voltage induces periodic oscillations. Investigation of Gaussian-type defects reveals soliton being trapped beyond a critical defect depth. These results provide a theoretical basis for future applications in magnetic soliton-based memory devices.

Strain-mediated voltage controlled magnetic anisotropy based switching for magnetic memory applications

Reliability and packing density concerns are the two major shortcomings of spin transfer torque and spin orbit torque based magnetic memory, respectively. Voltage controlled magnetic anisotropy (VCMA) becomes energy efficient and fast, showing transcendence for the writing mechanism in the magnetic tunnel junction. Deterministic switching cannot be achieved by VCMA alone in the out of plane nanomagnet. It requires an external in-plane magnetic field, but the use of an external field is inconvenient for on-chip applications. We exploit stress and exchange bias provided by an antiferromagnetic material to mitigate the external magnetic field requisite. We perform macro-spin simulations using the Landau–Lifshitz–Gilbert equation at room temperature. We use the VCMA effect cum stress effect to investigate field free switching performance, and this improves the write error rate (WER) to 5×10−5 against WER of 0.1 with the VCMA effect alone. We studied the effects of applied voltage (amplitude and pulse width), exchange bias field, and VCMA coefficient on the switching performance in detail. This proposed two-terminal device can be helpful in achieving high cell density to implement nonvolatile magnetic memory.

Probabilistic computing with voltage-controlled dynamics in magnetic tunnel junctions

Probabilistic (p-) computing is a physics-based approach to addressing computational problems which are difficult to solve by conventional von Neumann computers. A key requirement for p-computing is the realization of fast, compact, and energy-efficient probabilistic bits. Stochastic magnetic tunnel junctions (MTJs) with low energy barriers, where the relative dwell time in each state is controlled by current, have been proposed as a candidate to implement p-bits. This approach presents challenges due to the need for precise control of a small energy barrier across large numbers of MTJs, and due to the need for an analog control signal. Here we demonstrate an alternative p-bit design based on perpendicular MTJs that uses the voltage-controlled magnetic anisotropy (VCMA) effect to create the random state of a p-bit on demand. The MTJs are stable (i.e. have large energy barriers) in the absence of voltage, and VCMA-induced dynamics are used to generate random numbers in less than 10 ns/bit. We then show a compact method of implementing p-bits by using VC-MTJs without a bias current. As a demonstration of the feasibility of the proposed p-bits and high quality of the generated random numbers, we solve up to 40 bit integer factorization problems using experimental bit-streams generated by VC-MTJs. Our proposal can impact the development of p-computers, both by supporting a fully spintronic implementation of a p-bit, and alternatively, by enabling true random number generation at low cost for ultralow-power and compact p-computers implemented in complementary metal-oxide semiconductor chips.

Reconfigurable skyrmion logic gates with auto-annihilating skyrmion function

Skyrmions are a spin texture with topological protection properties, which can be considered as an excellent candidate for the next generation of high-speed, nonvolatile, and low-power consumption spin devices due to their unique dynamic characteristics. In this work, we propose reconfigurable skyrmion-based logic gates with auto-annihilating skyrmion function, and six logical operations can be implemented in our model relying on the competition among the spin transfer torque (STT), skyrmion Hall effect (SkHE), skyrmion-edge repulsion, skyrmion-skyrmion topological repulsion, and voltage-controlled magnetic anisotropy (VCMA) effect. The influence of temperature on the logical operation processes indicates that the temperature range for achieving all types of logic operations and automatic annihilation function is 0–96 K. The function of automatically clearing skyrmions after the completion of logic operations can free up space for storing unnecessary skyrmions, and save energy consumption for applying additional current to eliminate them. Our results can be beneficial for the design and development of magnetic skyrmion logic gates.

Numerical analysis of voltage-controlled magnetization switching operation in magnetic-topological-insulator-based devices

We theoretically investigate influences of electronic circuit delay, noise, and temperature on write-error-rate (WER) in voltage-controlled magnetization switching operation of a magnetic-topological-insulator-based device by means of the micromagnetic simulation. This device realizes magnetization switching via spin–orbit torque (SOT) and voltage-controlled magnetic anisotropy (VCMA), which originate from the 2D-Dirac electronic structure. We reveal that the device operation is extremely robust against circuit delay and signal-to-noise ratio. We demonstrate that the WER on the order of ∼10−4 or below is achieved around room temperature due to steep change in VCMA. Also, we show that the larger SOT improves thermal stability factor. This study provides a next perspective for developing voltage-driven spintronic devices with ultra-low power consumption.

Progress and Application Perspectives of Voltage‐Controlled Magnetic Tunnel Junctions

AbstractThis article discusses the current state of development, open research opportunities, and application perspectives of electric‐field‐controlled magnetic tunnel junctions that use the voltage‐controlled magnetic anisotropy effect to control their magnetization. The integration of embedded magnetic random‐access memory (MRAM) into mainstream semiconductor foundry manufacturing opens new possibilities for the development of energy‐efficient, high‐performance, and intelligent computing systems. The current generation of MRAM, which uses the current‐controlled spin‐transfer torque (STT) effect to write information, has gained traction due to its nonvolatile data retention and lower integration cost compared to embedded Flash. However, scaling MRAM to high bit densities will likely require a transition from current‐controlled to voltage‐controlled operation. In this perspective, an overview of voltage‐controlled magnetic anisotropy (VCMA) as a promising beyond‐STT write mechanism for MRAM devices is provided and recent advancements in developing VCMA‐MRAM devices with perpendicular magnetization are highlighted. Starting from the fundamental mechanisms, the key remaining challenges of VCMA‐MRAM, such as increasing the VCMA coefficient, controlling the write error rate, and achieving field‐free VCMA switching are discussed. Then potential solutions are discussed and open research questions are highlighted. Lastly, prospective applications of voltage‐controlled magnetic tunnel junctions (VC‐MTJs) in security applications, extending beyond their traditional role as memory devices are explored.

Voltage-controlled magnetic anisotropy based physical unclonable function

We design a spintronic physical unclonable function (PUF) based on sub-100 nm voltage-controlled magnetic anisotropy hybrid magnetic tunnel junctions (VCMA-MTJs). This complementary metal-oxide-semiconductor VCMA-MTJ (CMOS/VCMA-MTJ) PUF architecture was evaluated by combining micromagnetic simulations, Verilog-A modeling, and circuit-level simulations. The PUF architecture, comprising four 16 rows × 16 columns arrays, demonstrates effective read and write operations using conventional voltage sensing that are orders of magnitudes lower than previous spintronic-based PUFs. This study proves the potential of the proposed solution in security applications based on hardware authentication.

A magnetic skyrmion diode based on potential well inducting effect

Magnetic skyrmions have great potential in the application of spintronic devices due to their stable topologically protected spin configuration. To meet the needs of spintronic device design, it is necessary to manipulate the movement of the magnetic skyrmions. Here we propose a skyrmion diode based on potential well induced skyrmion motion through theoretical calculations. The potential well is generated by the voltage-controlled magnetic anisotropy (VCMA) gradient. By utilizing the induction of the potential well as well as the skyrmion Hall effect (SkHE), the velocity and trajectory of the skyrmions can be controlled and the forward pass and reverse cutoff functions of diode-like devices have been realized. Furthermore, we report the dynamics of current-driven skyrmions in a racetrack with locally applied VCMA. Under the influence of the SkHE, the difference in dynamic behavior between forward and reverse motion of the skyrmions is obvious, and the potential well can produce different pinning, depinning and annihilating effects on forward and reverse moving skyrmions. Our results can be beneficial for the design and development of magnetic skyrmion diodes.

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.