Magnetic Random Access Memory Research Articles

APPcache+: An STT-MRAM-Based Approximate Cache System With Low Power and Long Lifetime

Due to high static power and low scalability, the traditional SRAM-based cache is not a good solution for image processing applications. Emerging Spin Transfer Torque Magnetic RAM (STT-MRAM) is a promising candidate for cache due to its low leakage power and high density. However, STT-MRAM suffers from high write energy. Therefore, by making use of the ability of tolerating minor errors in image processing applications, this work presents an STT-MRAM based APProximate cache architecture (APPcache+) to write/read approximate data, which can largely reduce the cache energy and improve the STT-MRAM lifetime. APPcache+ includes three main designs. Firstly, we find that there are many similar elements (e.g., pixels in images) in cache lines. Therefore, APPcache+ presents several lightweight similarity-based encoding techniques to remove redundant elements, thus shortening the data size and reducing the energy of STT-MRAM cache. Secondly, we design a partial read scheme to reduce the read energy of the STT-MRAM cache. In the traditional decompression process, the whole line is fetched into the decompressor, leading to unnecessary read energy. The partial read scheme can largely reduce read energy while keeping the overhead low. Thirdly, we observe the encoding schemes may lead to bit write imbalance. Therefore, we propose a lightweight Ping-Pong intra-line wear-leveling scheme to improve the lifetime. Compared with the baseline, extensive evaluation results show that our APPcache+ can largely reduce the overall energy by 32.58%, improve lifetime by 40.7% with only 2.2% performance degradation and 1.86% output quality loss.

Effect of Defects on the Characteristics of CoFeB–MgO-Based MRAM Structure

Magnetic random-access memory (MRAM) is one of the most promising next-generation purpose memory devices for nonvolatile storage and in-memory computing. However, material defects can affect the performance of the MRAM device. Here, the relationship between the material defects and the properties of CoFeB–MgO-based MRAM have been studied with micromagnetic simulations. The results show that the coercivity and the switching speed are strongly influenced by the material defects. This work provides a useful guideline for the fabrication of the MARM devices.

Spin-based magnetic random-access memory for high-performance computing.

Spin-based memory technology is now available as embedded magnetic random access memory (eMRAM) for fast, high-density and non-volatile memory products, which can significantly boost computing performance and ignite the development of new computing architectures.

Room-temperature orbit-transfer torque enabling van der Waals magnetoresistive memories

The non-volatile magnetoresistive random access memory (MRAM) is believed to facilitate emerging applications, such as in-memory computing, neuromorphic computing and stochastic computing. Two-dimensional (2D) materials and their van der Waals heterostructures promote the development of MRAM technology, due to their atomically smooth interfaces and tunable physical properties. Here we report the all-2D magnetoresistive memories featuring all-electrical data reading and writing at room temperature based on WTe2/Fe3GaTe2/BN/Fe3GaTe2 heterostructures. The data reading process relies on the tunnel magnetoresistance of Fe3GaTe2/BN/Fe3GaTe2. The data writing is achieved through current induced polarization of orbital magnetic moments in WTe2, which exert torques on Fe3GaTe2, known as the orbit-transfer torque (OTT) effect. In contrast to the conventional reliance on spin moments in spin-transfer torque and spin–orbit torque, the OTT effect leverages the natural out-of-plane orbital moments, facilitating field-free perpendicular magnetization switching through interface currents. Our results indicate that the emerging OTT-MRAM is promising for low-power, high-performance memory applications.

Ultra High-Density SOT-MRAM Design for Last-Level On-Chip Cache Application

This paper presents ultra high-density spin-orbit torque magnetic random-access memory (SOT-MRAM) for last-level data cache application. Although SOT-MRAM has many appealing attributes of low write energy, nonvolatility, and high reliability, it poses challenges to ultra-high-density memory implementation. Due to using two access transistors per cell, the vertical dimension of SOT-MRAM is >40% longer than that of the spin-transfer torque magnetic random-access memory (STT-MRAM), a single transistor-based design. Moreover, the horizontal dimension cannot be reduced below two metal pitches due to the two vertical metal stacks per cell. This paper proposes an ultra-high-density SOT-MRAM design by reducing the vertical and horizontal dimensions. The proposed SOT-MRAM is designed by a single transistor with a Schottky diode to achieve lesser vertical dimension than the two-transistor-based design of conventional SOT-MRAM. Moreover, the horizontal dimension is also reduced by sharing a vertical metal between two consecutive bit-cells in the same row. The comparison of the proposed designs with the conventional SOT-MRAM reveals a 63% area reduction. Compared with STT-MRAM, the proposed high-density memory design achieves 48% higher integration density, 68% lower write power, 29% lower read power, and 1.9× higher read-disturb margin.

Methodology of backside preparation applied on a MRAM to lead a logical investigation with a near-field probe

This paper presents a method for the preparation of a magnetic random access memory (MRAM) whose data are stored in magnetic tunnel junctions (MTJ) as resistance states, in order to read them by near-field probing.The goal is to be able to visualize a difference of resistance between bits at ‘0’ and at ‘1’ thanks to the current passing through several MTJs. To do so, the MRAM needs to be prepared to create an electrical access to both sides of the MTJs. The main issue is the polishing of both sides as the stack of metallization being less than 10 μm thick. A chemical etch would in that case be encouraged by literature but we take a different approach since we choose to open further than the transistors. The preferred method is a backside preparation technique that creates a bevel allowing us to access the bottom side of the MTJs through vias and the top of them thanks to the bitlines. Since the resulting chip no longer has electrical connections, we also create a dedicated electrical path thanks to a focused ion beam (FIB) operation. At the end, it is then possible to collect the current flowing through the MTJs with a near-field probe.To probe the MTJ resistance, two near-field techniques are investigated: conductive atomic force microscopy (C-AFM) and scanning spreading resistance microscopy (SSRM). C-AFM provides a quite high resistivity probably due to its sensitivity to contact resistance. Using SSRM, a resistance of 12–16 kΩ and 17–22 kΩ were determined for “0” and “1” bits, which is in agreement with literature.

MTJ-based random number generation and its application in SNN handwritten digits recognition

Spiking Neural Networks (SNNs) that require synapse weight initialization using random numbers have been widely used in the neural morphological system. However, the random numbers generated by traditional digital circuits have certain repeatability, and the entire computing architecture has issues such as high resource consumption and low integration. In this letter, a hardware system for true random number generation is realized through integrating a magnetic tunnel junction, a memory cell of MRAM (magnetic random access memory) chips, with an interface circuit and using the same mechanism as writing data in spin transfer torque MRAM. The generated true random numbers are evaluated using NIST SP800-22 standard and are used for synapse weight initialization in an SNN system. The recognition rate of the system initialized by the generated true random numbers is about 84% for an MNIST handwritten digit dataset, which is 2%–3% higher than that using a traditional linear feedback shift register. The reported work provides a new approach for better SNN performance.

Recent progress in diluted ferromagnetism for spintronic application

With the continuous in-depth research of spintronics, the manufacture of high-performance magnetic random access memory devices and electronic devices that are more energy-efficient and generate less heat has received extensive attention. The traditional ferromagnet TbMnO3 is basically Tc at room temperature, which seriously limits its application. Since the discovery of diluted magnetic semiconductor materials at room temperature, such as AlNTiO2, ZnO, SnO2, etc., they have received increasing attention. Although these dopants can form ferromagnetism above-room temperature, the ferromagnetic mechanism and ferromagnetic properties are different. In this regard, we reviewed the current progress in the research on above room temperature dilute magnetic semiconductor materials; discussed the ferromagnetic mechanism of dilute magnetic semiconductors; summarized the problems and challenges, and advantages and disadvantages of different kinds of dilute magnetic semiconductor materials used in new memory devices; and prospected the application potential of spintronic devices.

Experimental and Theoretical Investigation of Intracell Magnetic Coupling-Induced Variability of Spin-Transfer Torque Magnetic RAMs

Experimental and Theoretical Investigation of Intracell Magnetic Coupling-Induced Variability of Spin-Transfer Torque Magnetic RAMs

Multiferroics in Magneto Electric – Spin Orbital Devices

The development of new computing technologies has given a new stimulus in the study of multiferroics. The use of multiferroics allows the realization of competitive energy efficient scalable logic and storage devices. The low - power consumption in Magneto Electric – Spin Orbital logics [1] and Magnetic Random Access Memory components is provided by magnetoelectric switching in multiferroic – based systems using a low-energy electric field. Our work concerns the modelling of the Magneto Electric – Spin Orbital elements with an emphasis on the magnetoelectric component and simulation of magnetization reversal processes in a model system. The use of the proposed approach makes it possible to analyze the influence of dimensional factors (film thicknesses, transverse dimensions, sample shape) affecting the magnetic states of multiferroic nanoelements; taking into account interfacial interactions (magnetic anisotropy and interlayer exchange); energy-efficient external influences that allow switching magnetic states using magnetic and electric fields.

Numerical study of two-terminal SOT-MRAM

A fully three-dimensional model coupling charge, spin, magnetization, and temperature dynamics is employed to study the switching behavior of the two-terminal spin–orbit torque magnetoresistive random access memory (2T-SOT-MRAM). We demonstrated that free layer (FL) spin-transfer torque (STT) switching is strongly affected by the inclusion of the SOT. The nucleation of the FL magnetization reversal is similar to the conventional three-terminal (3T)-SOT-MRAM. In comparison to the traditional STT-MRAM, the switching times and critical switching voltages are considerably reduced. Furthermore, the full thermal simulations show the significance of the heating process and temperature modeling in MRAM devices as they demonstrate a significantly lower switching time than the one in a constant temperature model.

Electric field controlled perpendicular exchange bias in Ta/Pt/Co/IrMn/Pt heterostructure

Perpendicular exchange bias (PEB) is an essential effect of antiferromagnetic materials. It has the advantages over the in-plane exchange bias in application of magnetic random access memory (MRAM) technology, particularly in terms of higher packing density. The influence of the strain effect on PEB of Ta/Pt/Co/Ir20Mn80/Pt multilayers deposited on the piezoelectric substrate is studied in this work. It is found that the exchange bias field (Hex) can be regularly regulated by the strain effect introduced by the piezoelectric substrate. It indicates that the uncompensated spins at the interface of Ir20Mn80/Co can be reoriented under the strain effect, resulting in the variation of Hex. Furthermore, we propose a MRAM device structure based on the strain-mediated PEB, which enables field-free magnetization switching driven by the pure strain. Our study demonstrates the controllability of PEB and its application in spintronics, providing a method for the development of antiferromagnetic material-based next generation MRAM.

Electrically controllable spin filtering in zigzag phosphorene nanoribbon based normal–antiferromagnet–normal junctions

We investigated the electric controllable spin-filtering effect in a zigzag phosphorene nanoribbon (ZPNR) based normal–antiferromagnet–normal junction. Two ferromagnets are closely coupled to the edges of the nanoribbon and form the edge-to-edge antiferromagnetism. Under an in-plane electric field, the two degenerate edge bands of the edge-to-edge antiferromagnet split into four spin-polarized sub-bands and a 100% spin-polarized current can be easily induced with the maximal conductance 2e 2/h. The spin polarization changes with the strength of the electric field and the exchange field, and changes sign at opposite electric fields. The spin-polarized current switches from one edge to the other by reversing the direction of the electric field. The edge current can also be controlled spatially by changing the electric potential of the scattering region. The manipulation of edge current is useful in spin-transfer-torque magnetic random-access memory and provides a practical way to develop controllable spintronic devices.

Picosecond spin-orbit torque-induced coherent magnetization switching in a ferromagnet.

Electrically controllable nonvolatile magnetic memories show great potential for the replacement of conventional semiconductor-based memory technologies. Here, we experimentally demonstrate ultrafast spin-orbit torque (SOT)-induced coherent magnetization switching dynamics in a ferromagnet. We use an ultrafast photoconducting switch and a coplanar strip line to generate and guide a ~9-picosecond electrical pulse into a heavy metal/ferromagnet multilayer to induce ultrafast SOT. We then use magneto-optical probing to investigate the magnetization dynamics with sub-picosecond resolution. Ultrafast heating by the approximately 9 picosecond current pulse induces a thermal anisotropy torque which, in combination with the damping-like torque, coherently rotates the magnetization to obtain zero-crossing of magnetization in ~70 picoseconds. A macro-magnetic simulation coupled with an ultrafast heating model agrees well with the experiment and suggests coherent magnetization switching without any incubation delay on an unprecedented time scale. Our work proposes a unique magnetization switching mechanism toward markedly increasing the writing speed of SOT magnetic random-access memory devices.

FeCrypto: Instruction Set Architecture for Cryptographic Algorithms Based on FeFET-Based In-Memory Computing

Recently, computing-in-memory (CiM) becomes a promising technology for alleviating the memory wall bottleneck. CiM is suitable for data-intensive applications, especially cryptographic algorithms. Most current cryptographic accelerators are specific to a single function. It is expensive to accelerate different cryptographic algorithms with different accelerators. In this work, we first introduce a CiM architecture FeMIC that supports multi-operand CiM operations, by exploring advantages of state-of-the-art ferroelectric field-effect transistors. Based on that, we propose a novel instruction set together with an accelerator architecture named FeCrypto which supports the acceleration of various cryptographic algorithms. Evaluation results show that FeCrypto has better performance and energy efficiency than software implementations. The energy-delay product (EDP) of FeCrypto is 118.4× and 1.93× lower than that of the dedicated AES accelerator AIM that is built based on phase-change memories (PCMs) and magnetic random-access memories (MRAMs), respectively. EDP is reduced by 44.7× compared with PCM-based EIM, a recent AES accelerator. Compared with MRAM-based EIM, the EDP overhead of FeCrypto for supporting multiple functions is 23.2%.

Area-Selective Atomic Layer Deposition for Resistive Random-Access Memory Devices.

Resistive random-access memory (RRAM) is a promising technology for data storage and neuromorphic computing; however, cycle-to-cycle and device-to-device variability limits its widespread adoption and high-volume manufacturability. Improving the structural accuracy of RRAM devices during fabrication can reduce these variabilities by minimizing the filamentary randomness within a device. Here, we studied area-selective atomic layer deposition (AS-ALD) of the HfO2 dielectric for the fabrication of RRAM devices with higher reliability and accuracy. Without requiring photolithography, first we demonstrated ALD of HfO2 patterns uniformly and selectively on Pt bottom electrodes for RRAM but not on the underlying SiO2/Si substrate. RRAM devices fabricated using AS-ALD showed significantly narrower operating voltage range (2.6 × improvement) and resistance states than control devices without AS-ALD, improving the overall reliability of RRAM. Irrespective of device size (1 × 1, 2 × 2, and 5 × 5 μm2), we observed similar improvement, which is an inherent outcome of the AS-ALD technique. Our demonstration of AS-ALD for improved RRAM devices could further encourage the adoption of such techniques for other data storage technologies, including phase-change, magnetic, and ferroelectric RAM.

Accurate Torque Evaluation in Elongated Ultra-Scaled STT-MRAM Devices

Emerging spin-transfer torque magnetoresistive random access memory (STT-MRAM) is attractive for stand-alone and embedded applications. STT-MRAM is nonvolatile, operates fast, and possesses excellent endurance. STT-MRAM is compatible with CMOS technology and can be integrated during the back-end-of-line process. This makes STT-MRAM promising for automotive and IoT applications, microcontrollers, frame buffer memory, and last level cache. The core of modern STT-MRAM cells consists of a magnetic tunnel junction (MTJ), which is a sandwich of two ferromagnetic (FM) layers separated by a tunnel barrier (TB). The FM layers are referred to as reference layer (RL) and free layer (FL). The magnetization of the RL is fixed, while the magnetization of the FL can be reversed. The use of MTJs with perpendicular magnetic anisotropy (PMA) permits to obtain better scalability and lower switching currents. When the magnetization vectors of the FL and RL are parallel (P), the electrical resistance is lower than when the vectors are anti-parallel (AP), which provides a way to store binary information. Importantly, the switching between these two stable configurations is achieved by purely electrical means.To reduce the footprint and benefit from shape-induced anisotropy, an ultra-scaled MRAM cell with a composite FL made of several elongated ferromagnetic pieces, separated by multiple MgO barriers, was proposed and demonstrated [1]. In elongated FLs, shown in Fig.1a, domain walls or magnetization textures can be generated during switching. In this case, additional spin torques created by the presence of magnetization gradients inside the ferromagnetic parts must be considered. This type of torque is typically modeled by the Zhang-Li expression [2]. If the spin dephasing length correctly describes the coupled charge and spin transport through the device, the expression for bulk torque must be generalized [3].In MTJs, however, there exists another torque acting on a FL close to the interface with MgO. This torque, predicted by Slonczewski [4] and Berger [5], was utilized to flip the magnetization in traditional MTJs with a FL fabricated as a flat and thin disk. It is due to the spin-polarized current coming across the TB from the RL. Electrons tunneling from the RL become spin-polarized. When entering the FL, the spin polarized electrons interact with the magnetization via the exchange interaction. When the magnetization vectors in the RL and FL are not aligned, the spin current components normal to the FL magnetization are quickly absorbed. Due to conservation of the angular momentum, the absorption of the normal component of the spin current results in a torque acting on the FL magnetization in the opposite sense. If the current is sufficiently strong, the magnetization of the FL can be switched between the two stable P and AP configurations relative to the RL.As the Slonczewski torque is localized at the interface and Zhang-Li (ZL) torque is acting in the bulk, the two torques are usually assumed to be independent. However, this is not correct [5]. Although the spin component normal to the FL magnetization is indeed quickly absorbed, the interface-polarization induced spin current parallel to the magnetization propagates into the FL at a longer distance determined by the spin-flipping length. This length is about 10nm in typical FL materials. As the interface-polarization induced spin current hits the spin texture, it modifies the ZL torque value determined by the spin polarization in the bulk. The FL in ultra-scaled MRAM cells contains a TB on both interfaces. Here, we compare the ZL torque acting on the domain wall (Fig.1b) for the case, when the interface polarization at the second TB is lower than the spin polarization in the bulk (Fig.1c) or higher (Fig.1d). It is demonstrated that the torque acting on the domain wall can be enhanced compared to the ZL value for the case of larger interface polarization than spin polarization in the bulk. B. Jinnai, J. Igarashi, K. Watanabe et al. High-performance shape-anisotropy magnetic tunnel junctions down to 2.3 nm, in Proc. IEDM Conf., 2020, pp. 24.6.1-24.6.4, doi: 10.1109/IEDM13553.2020.9371972S. Zhang and Z. Li. Roles of nonequilibrium conduction electrons on the magnetization dynamics of ferromagnets. Phys. Rev. Lett., vol. 93, 127204, 2004 doi: 10.1103/PhysRevLett.93.127204J. C. Slonczewski. Current-driven excitation of magnetic multilayers, J. Magn.Magn. Mater., vol. 159, no. 1, pp. L1 - L7, 1996, doi: 10.1016/0304-8853(96)00062-5L. Berger. Emission of spin waves by a magnetic multilayer traversed by a current, Phys. Rev. B, vol. 54, pp. 9353-9358, 1996, doi: 10.1103/PhysRevB.54.9353S. Fiorentini, M. Bendra, J. Ender et al. Spin and charge drift-diffusion in ultra-scaled MRAM cells, Sci. Rep., vol. 12, 20958, pp.1-13, 2022, doi: 10.1038/s41598-022-25586-4 Figure 1

A Comprehensive Study of Temperature and Its Effects in SOT-MRAM Devices.

We employ a fully three-dimensional model coupling magnetization, charge, spin, and temperature dynamics to study temperature effects in spin-orbit torque (SOT) magnetoresistive random access memory (MRAM). SOTs are included by considering spin currents generated through the spin Hall effect. We scale the magnetization parameters with the temperature. Numerical experiments show several time scales for temperature dynamics. The relatively slow temperature increase, after a rapid initial temperature rise, introduces an incubation time to the switching. Such a behavior cannot be reproduced with a constant temperature model. Furthermore, the critical SOT switching voltage is significantly reduced by the increased temperature. We demonstrate this phenomenon for switching of field-free SOT-MRAM. In addition, with an external-field-assisted switching, the critical SOT voltage shows a parabolic decrease with respect to the voltage applied across the magnetic tunnel junction (MTJ) of the SOT-MRAM cell, in agreement with recent experimental data.

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.

High room temperature magnetoresistance in La0.9Sr0.1Mn1-yZnyO3 epitaxial films

Developing large room temperature magnetoresistance materials has been a major hotspot for a few last decades due to its importance in the practical applications in magneto-resistive random access memory, and it remains a challenge. Here, La0.9Sr0.1Mn1-yZnyO3 (LSMZO) films have been grown on SrTiO3 substrates by magnetron sputtering, showing excellent quality of epitaxy. Upon increasing Zn content, the reduction of Curie temperature (TC) and metal-insulator transition temperature (TMI) is observed, accompanied by the increase of coercive field and room-temperature resistivity. Analysis on the fraction of Mn3+ and Mn4+ indicates that Zn2+ substitution at Mn site results in a higher Mn4+ fraction departing from the optimum value and hence a weaker double-exchange interaction by replacing more Mn3+. Besides that, ionic radius mismatch induced internal strain and substrate induced tensile strain also play a part. Most significantly, the enhanced magnetic disorder gives rise to an increase of intrinsic magnetoresistance around room temperature in LSMZO (y = 10%) film, and a large magnetoresistance of 18.4% is obtained under 0.3 T at 280 K in this sample. Our research provides a simple and effective approach to achieve a large room temperature magnetoresistance in Mn site doped manganite epitaxial films.