Magnetic Random Access Memory Research Articles

Large Field-like Spin-Orbit Torque and Enhanced Magnetization Switching Efficiency Utilizing Amorphous Mo.

Spin-orbit torque magnetic random access memory (SOT-MRAM) has great promise in high write speed and low power consumption. Mo can play a vital role in constructing a CoFeB/MgO-based MRAM cell because of its ability to enhance the perpendicular magnetic anisotropy (PMA), thermal tolerance, and tunneling magnetoresistance. However, Mo is often considered as a less favorable candidate among SOT materials because of its weak spin-orbit coupling. In this study, we experimentally investigate the SOT efficiencies in Mo/CoFeB/MgO heterostructures over a wide range of Mo thicknesses and temperature. Decent damping-like SOT efficiency |ξDL| = 0.015 ± 0.001 and field-like SOT efficiency |ξFL| = 0.050 ± 0.001 are found in amorphous Mo. The ξFL/ξDL ratio is greater than 3. Furthermore, efficient current-induced magnetization switching is demonstrated with the critical current density comparable with heavy metal Ir and W. Our work reveals new understanding and possibilities for Mo as both an SOT source component and PMA buffer layer in the implementation of SOT-MRAMs.

Exploration and optimization of novel replacement and prefetching strategies for inefficiencies of advanced MRAM-based hybrid cache systems

Abstract With the emergence of cutting-edge hardware systems such as cloud computing, edge computing, and on-chip neural network accelerators, how to design advanced memory strategies to substitute the traditional ones for maximizing the potential performance of non-volatile memory (NVM) under the existing hardware conditions, has become an urgent research issue for both academia and industrial communities. It is promising and innovative to improve computer systems in the layer of data exchanging with the emerging advanced semiconductor devices. In the paper, to address the inefficiencies of write-intensive, high power consumption, low hit rate and so on, which exist in hybrid Magnetic Random Access Memory (MRAM) cache systems, three novel cache replacement strategies and two cache prefetching strategies are put forward. The proposed triple novel replacement strategies, including historical frequency and time judgments, duplicate data-aware deletion, and dynamic relevance factors computing, can be utilized to compensate for the shortcomings of the traditional Least Recently Used (LRU) replacement strategy, respectively. In the two novel prefetching strategies, region distribution parameters and Listnet ranking network are imported into the caching process, respectively, to achieve optimized hitting performance. The simulation results demonstrate that the proposed replacement strategies can achieve up to 61.76%, 84.91%, 56.49%, and 53.21% optimization of write count, hit rate, dynamic power, and IPC compared to the conventional one. The proposed prefetching strategy can achieve up to 91.27%, 49.25% hit rate and IPC optimization. Meanwhile, the synthetic evaluation of the replacement and prefetching strategies are elaborated in the paper, including multi-core characteristics, information entropy, interplays and the performance constraints between replacement and prefetching mechanism, which would facilitate more credible ideas for future memory inefficiencies management and strategy design.

Spin-Orbit Torque Vector Quantification in Nanoscale Magnetic Tunnel Junctions.

Spin-orbit torques (SOT) allow ultrafast, energy-efficient toggling of magnetization state by an in-plane charge current for applications such as magnetic random-access memory (SOT-MRAM). Tailoring the SOT vector comprising of antidamping (TAD) and fieldlike (TFL) torques could lead to faster, more reliable, and low-power SOT-MRAM. Here, we establish a method to quantify the longitudinal (TAD) and transverse (TFL) components of the SOT vector and its efficiency χAD and χFL, respectively, in nanoscale three-terminal SOT magnetic tunnel junctions (SOT-MTJ). Modulation of nucleation or switching field (BSF) for magnetization reversal by SOT effective fields (BSOT) leads to the modification of SOT-MTJ hysteresis loop behavior from which χAD and χFL are quantified. Surprisingly, in nanoscale W/CoFeB SOT-MTJ, we find χFL to be (i) twice as large as χAD and (ii) 6 times as large as χFL in micrometer-sized W/CoFeB Hall-bar devices. Our quantification is supported by micromagnetic and macrospin simulations which reproduce experimental SOT-MTJ Stoner-Wohlfarth astroid behavior only for χFL > χAD. Additionally, from the threshold current for current-induced magnetization switching with a transverse magnetic field, we show that in SOT-MTJ, TFL plays a more prominent role in magnetization dynamics than TAD. Due to SOT-MRAM geometry and nanodimensionality, the potential role of nonlocal spin Hall spin current accumulated adjacent to the SOT-MTJ in the mediation of TFL and χFL amplification merits to be explored.

Highly Reliable Magnetic Memory-Based Physical Unclonable Functions.

Magnetic random-access memory (MRAM), which stores information through control of the magnetization direction, offers promising features as a viable nonvolatile memory alternative, including high endurance and successful large-scale commercialization. Recently, MRAM applications have extended beyond traditional memories, finding utility in emerging computing architectures such as in-memory computing and probabilistic bits. In this work, we report highly reliable MRAM-based security devices, known as physical unclonable functions (PUFs), achieved by exploiting nanoscale perpendicular magnetic tunnel junctions (MTJs). By intentionally randomizing the magnetization direction of the antiferromagnetically coupled reference layer of the MTJs, we successfully create an MRAM-PUF. The proposed PUF shows ideal uniformity and uniqueness and, in particular, maintains performance over a wide temperature range from -40 to +150 °C. Moreover, rigorous testing with more than 1584 challenge-response pairs of 64 bits each confirms resilience against machine learning attacks. These results, combined with the merits of commercialized MRAM technology, would facilitate the implementation of MRAM-PUFs.

Magnetization switching driven by spin current in a T-type ferromagnetic trilayer

The T-type CoFeB/spacer/CoFeB structure is a promising candidate for the development of perpendicular spin–orbit torque (SOT) magnetic random-access memory and other SOT devices. It consists of an in-plane magnetized layer, a perpendicularly magnetized layer, and a non-magnetic metal spacer that induces interlayer exchange coupling. By engineering the W spacer, this system achieves field-free SOT switching with a nearly 100% switching ratio. Furthermore, it realizes a high exchange coupling field of 255 Oe using a relatively thinner spacer thickness, enhancing the reliability and energy efficiency of SOT switching. Measurement of current switching probability suggests that this perpendicular magnetic anisotropy system may enable the implementation of probability-adjustable true random number generators in future applications. The T-type structures with strong interlayer coupling exhibit great potential for spintronic device applications.

Manufacturability Evaluation of Magnetic Tunnel Junction-Based Computational Random Access Memory

Manufacturability Evaluation of Magnetic Tunnel Junction-Based Computational Random Access Memory

The fabrication of spin transfer torque-based magnetoresistive random access memory cell with ultra-low switching power

We conducted research on the integration technology of a spin transfer torque-based magnetoresistive random access memory (STT-MRAM) cell (one transistor, one magnetic tunnel junction – 1T-1MTJ) on an 8-inch process platform. By combining a 0.5 μm CMOS front end of line process and STT-MTJ back end of line process, a 1T-1MTJ device with an MTJ of approximately 80 nm is realized. The Resistance(R)-magnetic field(H) measurement reveals a coercive force of approximately 450 Oe for the MTJ. Nevertheless, the 1T-1MTJ device exhibits a critical switching current of 4.7 μA (from high to low resistance state) and 17.9 μA (from low to high resistance state), accompanied by corresponding critical switching voltages of 0.05 V and 0.16 V, respectively, which is extremely low in an STT-MRAM with dual CoFeB/MgO interface free layer. Our findings demonstrate the potential for application in ultra-low-power portable mobile devices and embedded systems.

Current-induced magnetization switching behaviors in perpendicular magnetized L10-MnAl/B2-CoGa bilayer

Abstract Rare-earth-free Mn-based binary alloy L10-MnAl with bulk perpendicular magnetic anisotropy (PMA) holds promise for high-performance magnetic random-access memory (MRAM) devices driven by spin-orbit torque (SOT). However, the lattice-mismatch issue makes it challenging to place the conventional spin current sources, such as heavy metals, between the L10-MnAl layer and substrate. In this work, we propose a solution by using the B2-CoGa alloy as the spin current source. The lattice-matching enables high-quality epitaxial growth of 2-nm-thick L10-MnAl on B2-CoGa, and the L10-MnAl exhibits a large PMA constant of 1.04×106 J/m3. Subsequently, the considerable spin Hall effect in B2-CoGa enables the achievement of SOT-induced deterministic magnetization switching. Moreover, we have quantitatively determined the SOT efficiency in the bilayer. Furthermore, we have designed an L10-MnAl/B2-CoGa/Co2MnGa structure to achieve field-free magnetic switching. Our results offer valuable insights for achieving high-performance SOT-MRAM devices based on L10-MnAl alloy

Memory-compatible perpendicular magnetic tunnel junctions under bi-directional strobe write pulses: A method for generating true random number bits at high speed

We propose an operation method of generating true random number bits with a perpendicular magnetic tunnel junction (pMTJ) already in use as a memory element in spin-transfer-torque driven magnetic random access memory technology. This method uses a set of regularly spaced (or intentionally irregularly spaced), minimum width, and bi-directional fast strobe-write pulses, with read operations after each write. The resulting bit-stream’s statistical properties are analyzed, and a few digital logic operation following the read is described that could significantly improve the resulting bit-streams variance and stability, insulating those from the raw variations of pMTJs.

Advanced Modeling and Simulation of Multilayer Spin-Transfer Torque Magnetoresistive Random Access Memory with Interface Exchange Coupling.

In advancing the study of magnetization dynamics in STT-MRAM devices, we employ the spin drift-diffusion model to address the back-hopping effect. This issue manifests as unwanted switching either in the composite free layer or in the reference layer in synthetic antiferromagnets-a challenge that becomes more pronounced with device miniaturization. Although this miniaturization aims to enhance memory density, it inadvertently compromises data integrity. Parallel to this examination, our investigation of the interface exchange coupling within multilayer structures unveils critical insights into the efficacy and dependability of spintronic devices. We particularly scrutinize how exchange coupling, mediated by non-magnetic layers, influences the magnetic interplay between adjacent ferromagnetic layers, thereby affecting their magnetic stability and domain wall movements. This investigation is crucial for understanding the switching behavior in multi-layered structures. Our integrated methodology, which uses both charge and spin currents, demonstrates a comprehensive understanding of MRAM dynamics. It emphasizes the strategic optimization of exchange coupling to improve the performance of multi-layered spintronic devices. Such enhancements are anticipated to encourage improvements in data retention and the write/read speeds of memory devices. This research, thus, marks a significant leap forward in the refinement of high-capacity, high-performance memory technologies.

Toward Energy-efficient STT-MRAM-based Near Memory Computing Architecture for Embedded Systems

Convolutional Neural Networks (CNNs) have significantly impacted embedded system applications across various domains. However, this exacerbates the real-time processing and hardware resource-constrained challenges of embedded systems. To tackle these issues, we propose spin-transfer torque magnetic random-access memory (STT-MRAM)-based near memory computing (NMC) design for embedded systems. We optimize this design from three aspects: Fast-pipelined STT-MRAM readout scheme provides higher memory bandwidth for NMC design, enhancing real-time processing capability with a non-trivial area overhead. Direct index compression format in conjunction with digital sparse matrix-vector multiplication (SpMV) accelerator supports various matrices of practical applications that alleviate computing resource requirements. Custom NMC instructions and stream converter for NMC systems dynamically adjust available hardware resources for better utilization. Experimental results demonstrate that the memory bandwidth of STT-MRAM achieves 26.7 GB/s. Energy consumption and latency improvement of digital SpMV accelerator are up to 64× and 1,120× across sparsity matrices spanning from 10% to 99.8%. Single-precision and double-precision elements transmission increased up to 8× and 9.6×, respectively. Furthermore, our design achieves a throughput of up to 15.9× over state-of-the-art designs.

Field-Free Spin-Orbit Torque Magnetization Switching in a Perpendicularly Magnetized Semiconductor (Ga,Mn)As Single Layer.

Current-induced spin-orbit torque (SOT) in a perpendicularly magnetized single layer has a strong potential to switch the magnetization using an extremely low current density, which is generally 2-3 orders of magnitude smaller than that required for conventional metal bilayer systems. However, an in-plane external magnetic field has to be applied to break the symmetry and achieve deterministic switching. To further enhance the high-density integration and accelerate the practical application of highly efficient SOT magnetic random-access memory (SOT-MRAM) devices, field-free SOT magnetization switching in a ferromagnetic single layer is strongly needed. In a spin-orbit ferromagnet (a ferromagnet with strong spin-orbit interaction) with crystal inversion asymmetry and a multi-domain structure, the internal Dzyaloshinskii-Moriya effective fields are considered to induce field-free switching. Here, combined with strong spin-orbit coupling and a tilted anisotropy axis induced by a nonuniform Mn distribution and a possible magnetocrystalline anisotropy resulting from a slight substrate tilting, we successfully achieve magnetization switching in a spin-orbit ferromagnet (Ga,Mn)As single layer by utilizing SOT without applying any external magnetic field. Our findings help to deeply elucidate the SOT switching mechanism and can advance the development of a highly efficient MRAM with better scalability.

Energy-efficient superparamagnetic Ising machine and its application to traveling salesman problems

The growth of artificial intelligence leads to a computational burden in solving non-deterministic polynomial-time (NP)-hard problems. The Ising computer, which aims to solve NP-hard problems faces challenges such as high power consumption and limited scalability. Here, we experimentally present an Ising annealing computer based on 80 superparamagnetic tunnel junctions (SMTJs) with all-to-all connections, which solves a 70-city traveling salesman problem (TSP, 4761-node Ising problem). By taking advantage of the intrinsic randomness of SMTJs, implementing global annealing scheme, and using efficient algorithm, our SMTJ-based Ising annealer outperforms other Ising schemes in terms of power consumption and energy efficiency. Additionally, our approach provides a promising way to solve complex problems with limited hardware resources. Moreover, we propose a cross-bar array architecture for scalable integration using conventional magnetic random-access memories. Our results demonstrate that the SMTJ-based Ising computer with high energy efficiency, speed, and scalability is a strong candidate for future unconventional computing schemes.

Large enhanced thermal stability of perpendicular magnetic anisotropy films in magnetic tunnel junctions

For magnetic sensor and magnetic random access memory, the thermally tolerant need to be improved at a relatively high operation temperature. In this work, the correlations between the thermal stability and perpendicular magnetic anisotropy films with different heavy metal layers, structures and CoFeB compositions were investigated in details. It was found that the thin films adopting W or Mo heavy metal layer in the bottom structure, and the thin films adopting Mo heavy metal layer in the top structure exhibit a relatively good operation stability, no matter what the CoFeB composition is. Moreover, the thermally robust perpendicular magnetic anisotropy was obtained in the CoFeB with a relatively large Co atomic composition. According to the element distribution states analysis and oxidation conditions at CoFeB/MgO interface, the reasons are attributed to the structural- and composition-dependent Fe-O bonding. This research helps to design and optimize the ultra-low-power and thermally stable magnetic devices based on the suitable heavy metal layers, structures, and CoFeB compositions.

Write error rate analysis of field-free spin-orbit torque switching in conically magnetized free layer nanomagnet

Enhancing the performance of magnetic random access memories (MRAMs) is crucial, considering speed, energy efficiency, and endurance. Spin-orbit torque-based MRAMs offer ultrafast operation and enhanced reliability. Still, the energy efficiency and external magnetic field requirement for deterministic switching of nanomagnets with perpendicular magnetic anisotropy (PMA) are a significant hurdle. To address these issues, one of the proposed solutions employs a conically magnetized free layer magnetic tunnel junction device featuring second-order PMA. This approach can minimize the required switching current density by leveraging the interplay between first- and second-order PMA energies. Additionally, coupling with exchange bias from the antiferromagnet can eliminate the need for an external field entirely. Our analysis investigates the impact of current density (magnitude and rise/fall time), first- and second-order PMA fields, exchange bias, and field-like torque on the device's switching performance. By optimizing the perpendicular magnetic anisotropies, we report improvement in the write error rate from 10−4 to 10−7. Our findings hold promise for magnetic memory applications.

First-principle study of CrO2-BNNT-CrO2 based MTJ device using EHTB model and its application in a MRAM circuit

In this work we investigated the performance of Single-Walled Boron Nitride Nanotube (SWBNNT) sandwiched between CrO2 based Magnetic Tunnel Junction (MTJ) structures with atomistic simulations. MTJ based device-level simulation was performed using the Extended Huckel based Tight Binding (EHTB) model. The SWBNNT as a dielectric layer and CrO2 as Ferromagnetic (FM) layers based Magnetic Tunnel Junction (MTJ) heterostructure was computed. The proposed MTJ device operates at the nano-Ampere (nA) range depicting its potential application in a low-powered nanoelectronics device. It also shows a promising Tunnel Magnetoresistance (TMR) effect. The practical circuit-level implementation of the proposed MTJ device in Magnetic Random-Access Memory (MRAM) was validated using LTspice. Hence, the calculated static power consumption and writing energy per bit was minimal for S1 and S2 based MRAM circuit. The perfect switching operation was depicted by the proposed MTJ device based MRAM.

Existence of Long-Range Magnetic Order in Heisenberg Spin Nanoribbon with Edge Modification

Abstract A long-range magnetic order appears on side decorated Heisenberg spin nanoribbon at nonzero temperature, although no spontaneous magnetization exists in a one- or two-dimensional isotropic Heisenberg model at any nonzero temperature according to Mermin-Wagner theorem. By use of the spin Green’s function method, we calculated the magnetizations of Heisenberg nanoribbon decorated by side spins with single-ion anisotropy and found that the system exhibits a nonzero transition temperature, whether the decorated edge spins of the system link together or separate from each other. When the width of the nanoribbon achieves infinity limit, the transition temperatures of the system tend to a same finite constant eventually whether one edge or double edges are decorated by side spins in the nanoribbon. The results reveal that the magnetism of a low-dimensional spin system is different from that of a three-dimensional spin system. When the single-ion anisotropy of edge spins in Heisenberg spin nanoribbon can be modulated by an electric field experimentally, various useful long-range magnetic orders of the system can be obtained. The works can provide a detailed theoretical basis for designing and fabricating the next-generation low-dimensional magnetic random-access memory.

Controlling the helicity of light by electrical magnetization switching.

Controlling the intensity of emitted light and charge current is the basis of transferring and processing information1. By contrast, robust information storage and magnetic random-access memories are implemented using the spin of the carrier and the associated magnetization in ferromagnets2. The missing link between the respective disciplines of photonics, electronics and spintronics is to modulate the circular polarization of the emitted light, rather than its intensity, by electrically controlled magnetization. Here we demonstrate that this missing link is established at room temperature and zero applied magnetic field in light-emitting diodes2-7, through the transfer of angular momentum between photons, electrons and ferromagnets. With spin-orbit torque8-11, a charge current generates also a spin current to electrically switch the magnetization. This switching determines the spin orientation of injected carriers into semiconductors, in which the transfer of angular momentum from the electron spin to photon controls the circular polarization of the emitted light2. The spin-photon conversion with the nonvolatile control of magnetization opens paths to seamlessly integrate information transfer, processing and storage. Our results provide substantial advances towards electrically controlled ultrafast modulation of circular polarization and spin injection with magnetization dynamics for the next-generation information and communication technology12, including space-light data transfer. The same operating principle in scaled-down structures or using two-dimensional materials will enable transformative opportunities for quantum information processing with spin-controlled single-photon sources, as well as for implementing spin-dependent time-resolved spectroscopies.

Effect of metal insertion on the magnetic properties and anomalous Hall effect in MgO/CoFeB/Ta/MgO films

CoFeB-based nano-magnetic multilayers can be applied in magnetic sensors, magnetic random access memory and other logic devices, which have attracted wide attention. The magnetic properties and interface structure of the magnetic multilayers still need further research for future applications. The effects of Pt insertion on the perpendicular magnetic anisotropy (PMA) and the anomalous Hall effect (AHE) of MgO/CoFeB/Pt/Ta/MgO multilayers were studied. It is found that the sample with 0.2 nm Pt insertion at the CoFeB/Ta interface and annealed at 250 °C can transform the films from in-plane magnetic anisotropy to PMA. The effective magnetic anisotropy K eff can reach 1.82 × 106 erg cm−3. The Pt insertion and annealing heat treatment can promote the migration of oxygen from MgO at the top layer to CoFeB/Ta interface and combine with Fe to generate iron oxide, and improve the orbital hybridization of Fe 3d and O 2p, thus successfully inducing the PMA of the film samples. In addition, Pt insertion and annealing treatment can improve the side-jump mechanism and the skew scattering mechanism which contribute to the AHE. This work provides guidance for the application of PMA materials in magnetic sensor and memory devices.

Multi-bit MRAM based high performance neuromorphic accelerator for image classification

Binary neural networks (BNNs) are the most efficient solution to bridge the design gap of the hardware implementation of neural networks in a resource-constrained environment. Spintronics is a prominent technology among emerging fields for next-generation on-chip non-volatile memory. Spin transfer torque (STT) and spin-orbit torque (SOT) based magnetic random-access memory (MRAM) offer non-volatility and negligible static power. Over the last few years, STT and SOT-based multilevel spintronic memories have emerged as a promising solution to attain high storage density. This paper presents the operation principle and performance evaluation of spintronics-based single-bit STT and SOT MRAM, dual-level cells, three-level cells (TLCs), and four-level cells. Further, multi-layer perceptron architectures have been utilized to perform MNIST image classification with these multilevel devices. The performance of the complete system level consisting of crossbar arrays with various MRAM bit cells in terms of area, energy, and latency is evaluated. The throughput efficiency of the BNN accelerator using TLCs is 26.6X, and 3.61X higher than conventional single-bit STT-MRAM, and SOT-MRAM respectively.