Voltage-controlled Magnetic Tunnel Junction Research Articles

Magnetic field-free stochastic computing based on the voltage-controlled magnetic tunnel junction

The stochastic computing (SC) has been proven to be an energy-efficient way to perform neural network. In this study, we propose a field-free voltage-controlled spintronics SC system based on the magnetic tunnel junction (MTJ). We observe a stochastic switching behavior of the MTJ and that it could be controlled by the voltage applied on the device. The voltage-controlled stochastic switching behavior is used to encode numbers ranging from 0 to 1 into a series of random bit-streams in the SC system. Furthermore, the handwritten recognition task is performed on the MTJ-based SC system, achieving a 95% maximum accuracy, which is comparable with the floating-point based neural network. Our work provides inspiration for the energy-efficient neural network systems.

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 Tunnel Junctions Enabled Low-Power Feature Extractor

In the letter, a stable magnetic tunnel junction (MTJ) at 300 K that is changed stochastically by spin Hall effect (SHE) and voltage controlled magnetic anisotropy (VCMA) is designed as a switching component of a neuron, which is used to implement restricted Boltzmann machine (RBM) as on-chip feature extractor. With the effect of VCMA, the naturally probability of the device can be regulated to be acted as the activation function of RBM. An online training scheme with VCMA-assisted SHE-MTJ and modified crossbar architecture is presented, which achieves high accuracy as well as neuron and synaptic sparsity. To meet the low power and low cost requirements, the training procedure and crossbar architecture is adjusted to be an offline training scheme, which achieves 72.50% energy consumption reduction of extraction and 2.84% accuracy promotion to 95.48%.

A Voltage-Controlled Gain Cell Magnetic Memory

This letter presents a magnetic gain cell structure that consists of a new kind of voltage-controlled magnetic tunnel junction (MTJ) and two transistors for a large separation in the output current for the two memory states while retaining the low power advantages of a voltage-controlled write mechanism. The voltage-controlled MTJ utilizes the interlayer exchange coupling and the resonant tunneling phenomena to enable a resonant-exchange-controlled (REC) magnetization switching, leading to a substantially low write energy and delay, in the order of 29.5 fJ/bit and 1.6 ns, respectively. Two transistors in a gain cell configuration can provide a 103 times change in the output current between the 0 and 1 states from a ~25% magnetoresistance of the REC MTJ with a high baseline resistance in the order of a few $\text{M}\Omega $ . Such a magnetic gain-cell also exhibits low read energy and delay, in the range of 7.6~39 fJ/bit and 0.6~1 ns, thus promising for low energy, fast, and high-density memory technologies.

Physically Unclonable Functions With Voltage-Controlled Magnetic Tunnel Junctions

We demonstrated for the first time physically unclonable functions (PUFs) based on voltage-controlled magnetic tunnel junctions (VC-MTJs). The proposed PUF completely extracts the intrinsic variation of magnetization switching in the VC-MTJ as a random bit string without a short pulse, which is required in the spin-transfer torque (STT) MTJ and spin-orbit torque (SOT) MTJ-based PUF. Moreover, it can be initialized by a bias voltage without an external magnetic field. Using 40 MTJs fabricated in the same design, we evaluated the PUF fundamental parameters: uniqueness, reliability, and uniformity as 57.5%, 87.1%, and 45.0%, respectively.

Voltage-controlled magnetic tunnel junctions with synthetic ferromagnet free layer sandwiched by asymmetric double MgO barriers

Memory and computing applications utilizing voltage-controlled magnetic random-access memory (MRAM) require perpendicular magnetic tunnel junctions (pMTJs) capable of high thermal stability and large write efficiency at advanced technology nodes. We first discuss the scaling requirements for voltage-controlled MRAM to replace various existing memory applications at an advanced CMOS technology node, including cell size, thermal stability, interfacial perpendicular magnetic anisotropy (PMA), and voltage-controlled magnetic anisotropy (VCMA). For replacing volatile memories including SRAM, eDRAM, and DRAM, we employ the retention relaxation technique along with a DRAM-style refresh scheme in the analysis. To enhance both PMA and VCMA at scaled nodes, we explore pMTJs with asymmetric double MgO barriers sandwiching a synthetic ferromagnet (SyF) free layer. In a thin MgO/SyF free layer/thick MgO/fixed layer MTJ stack, the SyF free layer is realized in various ways: single layer (X = Ta, W, Mo, Ir) and bilayers (X = Ta/W, Mo/Ir) of heavy metals insertions in CoFeB/X/CoFeB and Co2FeAl/X/Co2FeAl structures, and multiple insertions in [CoFeB/X]n/CoFeB structures for n = 1–4. A VCMA coefficient of 36 fJ (V m)−1 is achieved in a MgO/CoFeB/Ta/CoFeB/MgO-based MTJ which is comparable to that of the single MgO barrier MTJ. We find PMA enhancement, although with VCMA reduction for an increasing number n of [CoFeB/X]n repetitions. In addition, we demonstrate large TMR of 78%, large interfacial PMA of 1.45 mJ m−2, and VCMA of 65 fJ (V m)−1 in a MgO/Co2FeAl/W/Co2FeAl/MgO-based MTJ annealed above 400 °C.

Torque Optimization for Voltage-Controlled Magnetic Tunnel Junctions as Memory and Stochastic Signal Generators

We propose an optimization principle for the operation of voltage-controlled magnetic tunnel junctions (VC-MTJs): optimization via magnetic torque, in contrast to the conventional optimization via magnetization. For memory applications, we minimize the magnetic torque at the end of the write pulse, whereas for stochastic applications, we attempt to find a balance between damping and precessional torque at the beginning of the pulse. We simulate the method using a physics-based VC-MTJ model and show that, for memory applications, optimization for torque reduces ringing by 43% and improves pulse shape tolerance by a factor of 2.1, whereas for stochastic applications, it improves convergence time by 61% and energy by 37%.

Analysis and Compact Modeling of Magnetic Tunnel Junctions Utilizing Voltage-Controlled Magnetic Anisotropy

A macrospin compact model of a perpendicularly magnetized voltage-controlled magnetic tunnel junction (MTJ) is introduced, for the evaluation of electronic and magnetic characteristics associated with its switching behavior. The voltage-driven precessional switching has been shown to outperform current-driven switching in terms of energy, speed, and bit density. However, to realize the device in embedded system memory applications, developing a compact model for simulating hybrid MTJ/CMOS circuits is necessary. This framework also needs to be compatible with conventional computer-aided circuit design tools and flows. In this paper, the voltage-controlled magnetic anisotropy effect is included as a component of the effective magnetic field in a Landau–Lifshitz–Gilbert (LLG) equation-based model. The compact model is described by Verilog-A, providing a numerical solution for a 3-D magnetization trajectory and the corresponding conductance change of the MTJ. We also include the thermal noise effect in the LLG equation to allow the evaluation of stochastic switching behavior under a wide variety of bias conditions, which can be quantified in terms of write error rate (WER). It is shown that the model allows for optimization of write pulse shape and design considerations to achieve the lowest WER for given MTJ parameters.

Voltage-Controlled Magnetic Tunnel Junctions for Processing-In-Memory Implementation

The processing-in-memory (PIM) paradigm has been considered as a promising alternative to break the bottlenecks of conventional von-Neumann architecture by realizing the unity of data storage and processing in the same die. On the road toward implementing such an architecture, finding a novel memory that can support both dense data storage and efficient logic processing is the critical step. In this letter, we report a voltage-controlled magnetic tunnel junction (MTJ), which is a potential candidate for PIM implementation. Stateful Boolean logic functions can be realized with a single device through the memory-like write/read operations. The device was fabricated and characterized at room temperature. Afterwards, typical Boolean logic operations, e.g., “OR”, “AND”, and “NXOR”, were experimentally demonstrated with the fabricated MTJ device. The proposed approach opens up a new way for PIM implementation in spintronic memories.

Design of high-throughput and low-power true random number generator utilizing perpendicularly magnetized voltage-controlled magnetic tunnel junction

A true random number generator based on perpendicularly magnetized voltage-controlled magnetic tunnel junction devices (MRNG) is presented. Unlike MTJs used in memory applications where a stable bit is needed to store information, in this work, the MTJ is intentionally designed with small perpendicular magnetic anisotropy (PMA). This allows one to take advantage of the thermally activated fluctuations of its free layer as a stochastic noise source. Furthermore, we take advantage of the voltage dependence of anisotropy to temporarily change the MTJ state into an unstable state when a voltage is applied. Since the MTJ has two energetically stable states, the final state is randomly chosen by thermal fluctuation. The voltage controlled magnetic anisotropy (VCMA) effect is used to generate the metastable state of the MTJ by lowering its energy barrier. The proposed MRNG achieves a high throughput (32 Gbps) by implementing a 64×64 MTJ array into CMOS circuits and executing operations in a parallel manner. Furthermore, the circuit consumes very low energy to generate a random bit (31.5 fJ/bit) due to the high energy efficiency of the voltage-controlled MTJ switching.