Switching Of Magnetic Tunnel Junctions Research Articles

An Energy and Quality Efficient MTJ/FinFET Approximate 5:2 Compressor for Image Processing Applications

Shrinking the transistor dimensions in complementary metal-oxide-semiconductor (CMOS) technology has led to many huge problems, like high power density. Various methods at different design levels of abstraction, such as approximate computing and spintronic devices based on magnetic tunnel junction (MTJ), have been studied to solve these problems. In this paper, we propose a novel hybrid MTJ/FinFET-based approximate 5:2 compressor. The proposed design employs the spin-transfer torque (STT) method assisted by the spin-Hall effect (SHE) to store inputs in MTJs. Due to the SHE assistance, the energy efficiency of the MTJ switching is improved considerably over the conventional STT method. Our design significantly improves the energy consumption compared to the previous compressors, thanks to the decrease in MTJ and transistor counts. The proposed circuit and previous designs are simulated using HSPICE with 7-nm FinFET and SHE perpendicular-anisotropy MTJ model. From the simulation results, we can see that the proposed design improves power consumption, write energy, read energy, number of transistors and MTJ count on average by 49%, 50%, 63%, 20% and 50%, respectively, in comparison with the existing counterparts. Furthermore, the accuracy of the approximate designs is evaluated through comprehensive MATLAB simulations. The results indicate that the proposed circuit outperforms the best previous energy-efficient designs in terms of accuracy despite having better hardware characteristic parameters.

Substantial reduction of write-error rate for voltage-controlled magnetoresistive random access memory by in-plane demagnetizing field and voltage-induced negative out-of-plane anisotropy field

Voltage-controlled magnetoresistive random access memory (VC-MRAM) based on voltage-induced dynamic switching in magnetic tunnel junctions (MTJs) is a promising ultimate non-volatile memory with ultralow power consumption. However, the dynamic switching in a conventional MTJ is accompanied by a relatively high write error rate (WER), hindering the reliable operation of VC-MRAM. Here, we propose a reliable writing scheme using the in-plane demagnetizing field (IDF) and voltage-induced negative out-of-plane anisotropy field (NOAF). Numerical simulations based on macrospin model demonstrate that the voltage induced NOAF modifies the switching dynamics and increases the torque due to the IDF, thereby reducing the switching time. The IDF and voltage-induced NOAF also reduce the mean energy difference between the magnetization direction at the end of the pulse and the equilibrium direction. As a result, an appropriate combination of the IDF and voltage-induced NOAF reduces the WER by one order of magnitude compared with that of the dynamic switching in a conventional MTJ.

Nonvolatile Magnetoelectric Switching of Magnetic Tunnel Junctions with Dipole Interaction

AbstractThe magnetoelectric effect is technologically appealing because of its ability to manipulate magnetism using an electric field rather than magnetic field or current, thus providing a promising solution for the development of energy‐efficient spintronics. Although 180° magnetization switching is vital to spintronic devices, the achievement of 180° magnetization switching via magnetoelectric coupling is still a fundamental challenge. Herein, voltage‐driven full resistance switching of a magnetic tunnel junction (MTJ) with dipole interaction on a ferroelectric substrate through switchable parallel/antiparallel magnetization alignment is demonstrated. Parallel magnetization alignment along the y direction is obtained under a bias magnetic field. By rotating the magnetic easy axis via strain‐mediated magnetoelectric coupling, the parallel magnetizations in the MTJ reorient to the x axis with opposite paths because of dipole interaction, thus resulting in antiparallel alignment. Moreover, this voltage switching of MTJs is nonvolatile owing to variations in dipole interaction and can be well understood via phase field simulations. The results provide an avenue to realize electrical switching of MTJs and are significant for exploring energy‐efficient spintronic devices.

A Low-Power, High-Speed Auto-Terminated Write Circuit Architecture With Hybrid MTJ/CMOS Circuits

For the energy waste of write operation in hybrid magnetic tunnel junction (MTJ)/CMOS circuits, an automatic termination write circuit based on 6T write circuit is proposed. The function of eliminating energy waste is achieved by detecting the state of the MTJ in time to detect its writing completion and turn off the writing current. The proposed write circuit reduces the average write power consumption by 47.92% and 76.93% compared to the conventional 4T and 6T write circuits, respectively, and its write speed increases by 20.63% compared to other auto-terminated write circuits. Moreover, Monte Carlo (MC) simulations were performed considering the process errors of CMOS transistors and the actual parameter variations of MTJs to ensure the correct and stable operation of the proposed writing circuit. The proposed writing circuit fully takes into account the randomness and asymmetry of MTJ switching, which is more practical and can work stably in hybrid MTJ/CMOS circuits. The simulation results show that the proposed write circuit has a better performance in terms of low power consumption and speed.

Control of interface anisotropy for spin transfer torque in perpendicular magnetic tunnel junctions for cryogenic temperature operation

The possibility of higher electrical efficiency in computing by operating at low temperatures raises the need for non-volatile memory cells optimized for cryogenic operation. We report a study on low temperature spin transfer torque switching of magnetic tunnel junctions with 20 to 100 nm in diameter with thermal stability adapted to low temperature operation. The evolution of magnetic and electrical properties are characterized for four different stacks from 300 to 10 K comprising insertions of Mg, Ru and permalloy (Py) in the storage layer to reduce its effective anisotropy. Two figures of merit are used to compare different devices and stacks, Δ/Ic and Δ/Esw, normalizing the thermal stability Δ by the critical current or switching energy. Devices with a Py insertion layer show a higher FOM (3.78 kBTop/μA) and switching energy Esw below 655 fJ for 100 ns pulses at Top = 10 K. A procedure to optimize the reference layer stray field was also implemented to achieve full compensation using a synthetic antiferromagnetic layer for 20 nm diameter devices.

Switching current reduction in magnetoresistive random access memories

We present an approach for minimizing the critical current for the magnetization switching in magnetic tunnel junctions by optimizing the spatial distribution of the current density. We show that such a minimization is possible because critical current is determined by the condition of making one of the magnetization eigenstates grow in time. The excitation of the eigenstates is enhanced when the spatial distributions of the eigenstates and current density overlap. Critical current can be viewed as a functional of the current density spatial distribution and it can be minimized by optimizing this distribution. Such an optimization results in a major reduction of the critical current and increase of the switching efficiency, viz. the ratio between the energy barrier and critical current. The minimized critical current increases approximately linearly with the magnetic tunnel junction size, which is much slower than critical current for the case of a uniform current density. The optimized efficiency can be approximately a constant with respect to the magnetic tunnel junction size, which is much higher than the efficiency for the uniform current density. Additional optimization can be achieved by spatially modulating the material parameters, e.g., the saturation magnetization. The presented approach and obtained scaling of the critical current and efficiency offers opportunities for the magnetic tunnel junction optimization.

Voltage-gated spin-orbit torque switching in IrMn-based perpendicular magnetic tunnel junctions

In this work, IrMn-based perpendicular magnetic tunnel junctions (MTJs) are investigated. By inserting a thin W layer at an antiferromagnet/ferromagnet (AFM/FM) interface, we enhance the annealing temperature to 355 °C and obtain a high tunnel magnetoresistance ratio of 127%. Subsequently, field-free spin–orbit torque (SOT) switching of perpendicular MTJ is realized thanks to the in-plane exchange bias generated at the AFM/FM interface. Moreover, by applying a gate voltage, a coercive field is effectively decreased due to the voltage-controlled magnetic anisotropy (VCMA) effect. Finally, through the interplay of the SOT and VCMA effects, the critical switching current density is dramatically reduced by 73% (to 2.4 MA/cm2) and the total writing power consumption is decreased by 84% when a gate voltage of 0.76 V is applied. These findings pave the way for the practical applications of the IrMn-based perpendicular MTJs in low-power magnetic random-access memory.

Reliable Sub-Nanosecond Switching in Magnetic Tunnel Junctions for MRAM Applications

We demonstrate reliable sub-nanosecond switching in two-terminal spin transfer torque magnetoresistive random access memory (STT-MRAM) devices by using double spin-magnetic tunnel junctions (DS-MTJs). A write-error-rate (WER) of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1{E} -6$ </tex-math></inline-formula> was achieved in 194 devices with 250-ps write pulses and tight distributions. The WER = <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1{E} -6$ </tex-math></inline-formula> was also demonstrated over a temperature range of −40 °C–85 °C in a single device with 225-ps write pulses. No degradation was observed after <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1{E}10$ </tex-math></inline-formula> write cycles in selected single devices, written with 250-ps write pulses. We compare the DS-MTJ device switching performance with the published results from the state-of-the-art three-terminal spin–orbit torque (SOT) MRAM devices and show a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10\times $ </tex-math></inline-formula> reduction in switching current density ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${J}_{\text {c}}$ </tex-math></inline-formula> ) and 3– <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10\times $ </tex-math></inline-formula> reduction in power consumption for devices with similar energy barriers ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${E}_{\text {b}}$ </tex-math></inline-formula> ).

Heavily Damped Precessional Switching with Very Low Write-Error Rate in Elliptical-Cylinder Magnetic Tunnel Junctions

Voltage-induced dynamic switching in magnetic tunnel junctions (MTJs) is a writing technique for voltage-controlled magnetoresistive random access memory (VCMRAM), which is expected to be an ultimate nonvolatile memory with ultralow power consumption. In conventional dynamic switching, the width of subnanosecond write voltage pulses must be precisely controlled to achieve a sufficiently low write-error rate (WER). This very narrow tolerance of pulse width is the biggest technical difficulty in developing VCMRAM. Heavily damped precessional switching is a writing scheme for VCMRAM with a substantially high tolerance of pulse width, although the minimum WER has been much higher than that of conventional dynamic switching with an optimum pulse width. In this study, we theoretically investigate the effect of MTJ shape and the direction of the applied magnetic field on the WER of heavily damped precessional switching. The results show that the WER in elliptical-cylinder MTJs can be several orders of magnitude smaller than that in usual circular-cylinder MTJs when the external magnetic field is applied parallel to the minor axis of the ellipse. The reduction in the WER is due to the fact that the demagnetization field narrows the component of the magnetization distribution perpendicular to the plane direction immediately before the voltage is applied.

Sub-volt switching of nanoscale voltage-controlled perpendicular magnetic tunnel junctions

Magnetic random-access memory (MRAM) based on voltage-controlled magnetic anisotropy in magnetic tunnel junctions (MTJs) is a promising candidate for high-performance computing applications, due to its lower power consumption, higher bit density, and the ability to reduce the access transistor size when compared to conventional current-controlled spin-transfer torque MRAM. The key to realizing these advantages is to have a low MTJ switching voltage. Here, we report a perpendicular MTJ structure with a high voltage-controlled magnetic anisotropy coefficient ~130 fJ/Vm and high tunnel magnetoresistance exceeding 150%. Owing to the high voltage-controlled magnetic anisotropy coefficient, we demonstrate sub-nanosecond precessional switching of nanoscale MTJs with diameters of 50 and 70 nm, using a voltage lower than 1 V. We also show scaling of this switching mechanism down to 30 nm MTJs, with voltages close to 2 V. The results pave the path for the future development and application of voltage-controlled MRAMs and spintronic devices in emerging computing systems.

Tailoring the Switching Efficiency of Magnetic Tunnel Junctions by the Fieldlike Spin-Orbit Torque

Current-induced spin-orbit torques provide a versatile tool for switching magnetic devices. In perpendicular magnets, the dampinglike component of the torque is the main driver of magnetization reversal. The degree to which the fieldlike torque assists the switching is a matter of debate. Here we study the switching of magnetic tunnel junctions with a CoFeB free layer and either W or Ta underlayers, which have a ratio of fieldlike to dampinglike torque of 0.3 and 1, respectively. We show that the fieldlike torque can either assist or hinder the switching of CoFeB when the static in-plane magnetic field required to define the polarity of spin-orbit torque switching has a component transverse to the current. In particular, the non-collinear alignment of the field and current can be exploited to increase the switching efficiency and reliability compared to the standard collinear alignment. By probing individual switching events in real-time, we also show that the combination of transverse magnetic field and fieldlike torque can accelerate or decelerate the reversal onset. We validate our observations using micromagnetic simulations and extrapolate the results to materials with different torque ratios. Finally, we propose device geometries that leverage the fieldlike torque for density increase in memory applications and synaptic weight generation.

High-Speed and Energy-Efficient Sense Amplifier

Sense amplifier is a key element of memory and a CMOS-based spin hybrid circuit. Magnetic tunnel junction (MTJ) is a key spintronic device used in spin-based memory and hybrid circuits. This paper presents a design of a voltage-controlled magnetic anisotropy MTJ-based sense amplifier (VCMA-SA) that offers better reliability, power efficiency and high-speed performance. MTJ switching between different resistive states in this paper takes place through the faster and energy-efficient voltage-controlled magnetic anisotropy (VCMA) scheme rather than the traditional spin-transfer torque (STT)-based magnetization switching. This paper demonstrates that SA based on VCMA-MTJ switching is faster, reliable and more energy-efficient compared with iPMA-based STT switching.

Bi-stable toggle switching in magnetic tunnel junctions using sub-nanosecond Joule heat pulses

In this study, bi-stable toggle magnetization switching in magnetic tunnel junctions induced by electrically injected sub-nanosecond unipolar heat pulses was demonstrated. The switching probability of magnetization between bi-stable states was estimated by applying 0.5 ns voltage pulses and measuring the perpendicular component of the magnetization direction. The maximum switching probability was approximately 70%, suggesting that bi-stable toggle magnetization switching was induced by the torque created by the Joule-heat-induced magnetic anisotropy change. Joule-heat-driven magnetization switching has the potential to become a fundamental technology for fast spin control.

Reducing the Effect of Carry Propagation on Spintronic Adders

Due to the exponential increase of transistor leakage currents, especially in the technologies lower than 90[Formula: see text]nm, the static power consumption has dramatically increased. So, it is regarded as one of the main problems of the CMOS circuits. The spintronic devices, such as magnetic tunnel junction (MTJ), have notable advantages including low-static power consumption, nonvolatility, high endurance, compatibility with the CMOS transistors and the possibility of fabrication in high scales. Hence, the hybrid MTJ/CMOS circuits are considerable options to mitigate the problem of high-static power consumption in CMOS circuits. In this paper, a nonvolatile and low-power hybrid MTJ/CMOS full adder is proposed for implementation of in-memory computing. The drawback of existing full adders is their low-speed and high-power consuming for MTJ switching, which terribly affect the performance of multi-bit adders using these full adders. In this work, a new full adder is designed by eliminating the MTJ and transistor related to the input carry bit and modifying the tree structure of the MTJ network associated with the other inputs. In the proposed full adder, the effect of carry propagation has drastically decreased and its power consumption is also improved.

Low-Energy Picosecond Magnetic Switching for Synthetic Ferrimagnetic Free Layer Utilizing the Electric-Field-Tuned RKKY Effect

The processional switching mechanism governs magnetic switching in magnetic tunnel junctions (MTJs) in the sub-nanosecond range, which limits the application of spin transfer torque magnetic random access memory (STT-MRAM) in the ultrafast region. In this paper, we propose a new picosecond magnetic switching mechanism in a synthetic antiferromagnetic (SAF) structure using the adjustable Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction controlled by an external electric field (E-field). It is shown that along with the sign change of the RKKY interaction in the SAF structure with an external E-field, the critical switching current density can be significantly reduced by one order of magnitude compared to that of a normal MTJ design at 100 ps; thus, this novel STT-MRAM can be written with a very low switching current density to avoid the MTJ breakdown problem and reduce the writing energy. To understand the physical origin of this abnormal phenomenon, a toy model is proposed in which the external-E-field-controlled sign change of the RKKY interaction in the SAF structure provides an extra contribution to the total energy that helps thespins overcome the energy barrier and break the processional switching mechanism.

Thermal Effects in Spin-Torque Switching of Perpendicular Magnetic Tunnel Junctions at Cryogenic Temperatures

Temperature plays an important role in spin-torque switching of magnetic tunnel junctions, causing magnetization fluctuations that decrease the switching voltage but also introduce switching errors. Here we present a systematic study of the temperature dependence of the spin-torque-switching probability of state-of-the-art perpendicular-magnetic-tunnel-junction nanopillars (40--60 nm in diameter) from room temperature down to 4 K, sampling up to a million switching events. The junction temperature at the switching voltage---obtained from the thermally assisted spin-torque-switching model---saturates at temperatures below about 75 K, showing that junction heating is significant below this temperature and that spin-torque switching remains highly stochastic down to 4 K. A model of heat flow in a nanopillar junction shows this effect is associated with the reduced thermal conductivity and heat capacity of the metals in the junction.

Field-free and sub-ns magnetization switching of magnetic tunnel junctions by combining spin-transfer torque and spin–orbit torque

We investigate the magnetization switching via a combination of spin-transfer torque (STT) and spin–orbit torque (SOT). STT and SOT are simultaneously induced by a pulsed current flowing through an in-plane easy-axis magnetic tunnel junction and an underneath Ta/W channel. SOT allows the magnetization to be switched with the sub-ns pulse down to 200 ps and STT eliminates the necessity of an external field. The switching current is much smaller than the case driven solely by STT in the short pulse regime. We also compare the threshold current between two structures having orthogonal (Type Y) and collinear (Type X) magnetic easy axes to the longitudinal direction of the channel and find that the Type X achieves smaller switching current by a factor of 1/4 at 200 ps.

High-Performance Radiation-Hardened Spintronic Retention Latch and Flip-Flop for Highly Reliable Processors

Radiation vulnerability and high power density are critical challenges in modern CMOS processors. Spin-based devices like magnetic tunnel junction (MTJ) are among the promising alternatives for addressing these challenges thanks to their fascinating properties such as radiation immunity, non-volatility, high endurance, and compatibility with the CMOS fabrication process. Utilizing the fascinating non-volatile property of the MTJ device, a high-performance, and energy-efficient soft error immune retention latch, and a high-reliable non-volatile flip-flop (FF) are proposed in this paper. Thanks to the single event upset (SEU) immunity, the proposed circuits are applicable in designing highly reliable processors, especially in applications like aerospace systems, where immunity to radiation induced soft errors is very critical. Because of the innovative design of the proposed approach, the MTJ switching delay does not affect the performance of the proposed latch. The simulation results indicate that the proposed latch offers lower delay, power consumption, and power delay product (PDP) than the state-of-the-art designs. The results of the comprehensive Monte-Carlo simulations also demonstrate that the proposed approach is more robust to process, voltage, and temperature variations as compared to their previous counterparts. This is because our designs do not use a pre-charge sense amplifier (PCSA) to read the data stored in the MTJs, and hence, the proposed designs have lower error rates. Furthermore, the transient impact of the particle strikes on the internal nodes is not transferred to the output node of the proposed circuits. The proposed design also has a significantly shorter output recovery time compared to its state-of-the-art counterparts.

A four-state magnetic tunnel junction switchable with spin–orbit torques

We present a magnetic tunnel junction (MTJ) where its two ferromagnetic layers are in the form of a single ellipse (SE) and two-crossing ellipses (TCEs). The MTJ exhibits four distinct resistance states corresponding to the four remanent states of the TCE structure. Flowing current in an underlying Ta layer generates in the adjacent TCE structure spin–orbit torques, which induce field-free switching of the four-state MTJ between all its resistance states. The demonstrated four-state MTJ is an important step toward fabricating multi-level MTJs with numerous resistance states, which could be important in various spintronics applications, such as multi-level magnetic random access or neuromorphic memory.

High Tunneling Magnetoresistance in Magnetic Tunnel Junctions with Subnanometer Thick Al2O3 Tunnel Barriers Fabricated Using Atomic Layer Deposition.

Pinhole-free and defect-free ultrathin dielectric tunnel barriers (TBs) are a key to obtaining high-tunneling magnetoresistance (TMR) and efficient switching in magnetic tunnel junctions (MTJs). Among others, atomic layer deposition (ALD) provides a unique approach for the fabrication of ultrathin TBs with several advantages including atomic-scale control over the TB thickness, conformal coating, and a low defect density. Motivated by this, this work explores the fabrication and characterization of spin-valve Fe/ALD-Al2O3/Fe MTJs with an ALD-Al2O3 TB thickness of 0.55 nm using in situ ALD. Remarkably, high TMR values of ∼77 and ∼90% have been obtained, respectively, at room temperature and at 100 K, which are comparable to the best reported values on MTJs having thermal AlOx TBs with optimized device structures. In situ scanning tunneling spectroscopy characterization of the ALD-Al2O3 TBs has revealed a higher TB height (Eb) of 1.33 ± 0.06 eV, in contrast to Eb ∼ 0.3-0.6 eV for their AlOx TB counterparts, indicative of significantly lower defect concentrations in the former. This first success of the MTJs with subnanometer thick ALD-Al2O3 TBs demonstrates the feasibility of in situ ALD for the fabrication of pinhole-free and low-defect ultrathin TBs for practical applications, and the performance could be further improved through device optimization.