Spin Transfer Torque Switching Research Articles

Broadband spin-filtered minimalistic magnetic tunnel junction

The tri-layer magnetic tunnel junction (MTJ) has surfaced as a building block for engineering next-generation integrated circuits while combining the attributes of non-volatility and meager energy consumption. Nevertheless, the perceptible switching energy (≈20–50 fJ/bit) and sub-optimal tunnelmagnetoresistance (TMR) (≈200%–300%) have acted as major hindrances, concealing its potential to supersede the capabilities of static and dynamic random access memories. In this work, we introduce a novel device that features a minimalistic non-uniform heterostructure/superlattice instead of the oxide layer in a conventional MTJ and analyze it in the premise of the self-consistent coupling of the Non-Equilibrium-Green’s Function (NEGF) and the Landau-Liftshitz-Gilbert-Slonczewski (LLGS) equation. We ascertain that the coupling of the electrodes to the proposed heterostructure renders a highly spin-selective broadband transmittance, thereby enabling a towering TMR (%) of 3.7 × 104% along with a significant reduction in the spin transfer torque (STT) switching energy (≈1.96 fJ). Furthermore, the sizable slonczewski term (Is‖) originating from the heterostructure facilitates a swift STT-switching within the scale of a few hundred picoseconds (≈400 ps).

Sub-10 nm magnetic sensor for gigahertz operation using lateral spin valve structure

We present a new magnetic sensor design using a lateral spin valve (LSV) structure that features a sensing end less than 10 nm thick, making it suitable for integration with nanoscale devices. Shot noise reduction and signal-to-noise ratio (SNR) improvement under gigahertz operation are expected for our design. The spin detection part of the LSV forms a magnetic tunnel junction (MTJ) beneath the aluminum channel. We propose to replace the single layer ferromagnet in the spin detection part of the LSV with a synthetic antiferromagnetic structure: this facilitates spin-transfer torque (STT) induced magnetization switching. Macrospin simulation shows that interlayer magnetic dipole coupling helps STT switching while interlayer exchange coupling hurts, owing to their different precession symmetries in the out-of-plane direction. It also shows that SNR > 20 dB can be achieved for a pseudorandom binary sequence with a minimum bit length 0.5 ns.

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.

Current induced magnetization switching in ferrimagnetic insulators with perpendicular magnetic anisotropy: A macrospin modeling

Spin-orbit torque (SOT) assisted magnetization switching by in-plane current in heavy-metal (HM)/ferrimagnetic insulators (FMIs) heterostructure with perpendicular magnetic anisotropy (PMA) has attracted more attention than in HM/FM (ferromagnetic) heterostructure due to it’s low magnetic losses and electric insulation. Here, macrospin modeling was employed to study the current-induced magnetization switching in Bi doped Y3Fe5O12 (BiYIG), Tm3Fe5O12 (TmIG) and Eu3Fe5O12 (EuIG) FMIs layers with heavy metal on to explore promising FMIs materials for SOT-based fast and efficient devices. The magnetization switching phase diagram (MSPD) of BiYIG, TmIG, EuIG1 and EuIG2 was calculated. It is found that the distribution of MSPD is strongly dependent both on the PMA field Hk and Gilbert damping parameter α. The larger the PMA field Hk is, the greater the damping-like effective field HDL (proportional to the current density) and in-plane magnetic field Hx required to realize magnetization switching in HM/FMIs. Furthermore, switching trajectories and diagrams of BiYIG, TmIG, EuIG1 and EuIG2 with variable Hx and HDL were calculated. The results show that compared with spin transfer torque (STT) and SOT-assisted magnetization switching in all-metallic heterostructures, the switching speed in HM/FMIs system has great advantages. Besides, presence of an external field-like effective field (HFL) can improve switching reliability for low Gilbert damping heterostructure like HM/BiYIG. It is noteworthy that although small damping coefficient is not conducive to the stability of magnetization switching, a huge advantage of smaller damping coefficient is that it can reduce the critical switching current density, which is of great significance to the application of SOT devices. In the end, the SOT-assisted magnetization switching loops of four kinds of FMIs were calculated. This study provides a new perspective for further exploring the current-induced magnetization switching in HM/FMIs system.

Optically induced ultrafast magnetization switching in ferromagnetic spin valves.

The discovery of spin-transfer torque (STT) enabled the control of the magnetization direction in magnetic devices in nanoseconds using an electrical current. Ultrashort optical pulses have also been used to manipulate the magnetization of ferrimagnets at picosecond timescales by bringing the system out of equilibrium. So far, these methods of magnetization manipulation have mostly been developed independently within the fields of spintronics and ultrafast magnetism. Here we show optically induced ultrafast magnetization reversal taking place within less than a picosecond in rare-earth-free archetypal spin valves of [Pt/Co]/Cu/[Co/Pt] commonly used for current-induced STT switching. We find that the magnetization of the free layer can be switched from a parallel to an antiparallel alignment, as in STT, indicating the presence of an unexpected, intense and ultrafast source of opposite angular momentum in our structures. Our findings provide a route to ultrafast magnetization control by bridging concepts from spintronics and ultrafast magnetism.

Perpendicular magnetic anisotropy, tunneling magnetoresistance and spin-transfer torque effect in magnetic tunnel junctions with Nb layers

Nb and its compounds are widely used in quantum computing due to their high superconducting transition temperatures and high critical fields. Devices that combine superconducting performance and spintronic non-volatility could deliver unique functionality. Here we report the study of magnetic tunnel junctions with Nb as the heavy metal layers. An interfacial perpendicular magnetic anisotropy energy density of 1.85 mJ/m2 was obtained in Nb/CoFeB/MgO heterostructures. The tunneling magnetoresistance was evaluated in junctions with different thickness combinations and different annealing conditions. An optimized magnetoresistance of 120% was obtained at room temperature, with a damping parameter of 0.011 determined by ferromagnetic resonance. In addition, spin-transfer torque switching has also been successfully observed in these junctions with a quasistatic switching current density of 7.3 times ;10^{5} A/cm2.

Non-uniform superlattice magnetic tunnel junctions

We propose a new class of non-uniform superlattice magnetic tunnel junctions (Nu-SLTJs) with the linear, Gaussian, Lorentzian, and Pöschl–Teller width and height based profiles manifesting a sizable enhancement in the TMR (≈104 − 106%) with a significant suppression in the switching bias (≈9 folds) owing to the physics of broad-band spin filtering. By exploring the negative differential resistance region in the current–voltage characteristics of the various Nu-SLTJs, we predict the Nu-SLTJs offer fastest spin transfer torque switching in the order of a few hundred picoseconds. We self-consistently employ the atomistic non-equilibrium Green’s function formalism coupled with the Landau–Lifshitz–Gilbert–Slonczewski equation to evaluate the device performance of the various Nu-SLTJs. We also present the design of minimal three-barrier Nu-SLTJs having significant TMR (≈104%) and large spin current for the ease of device fabrication. We hope that the class of Nu-SLTJs proposed in this work may lay the bedrock to embark on the exhilarating voyage of exploring various non-uniform superlattices for the next generation of spintronic devices.

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.

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.

Metrology and metrics for spin-transfer-torque switched magnetic tunnel junctions in memory applications

A spin-transfer torque (STT) switched magnetic tunnel junction (MTJ) enables modern magnetic random access memory (MRAM). Optimization of perpendicularly magnetized MTJ (pMTJ) for MRAM is a comprehensive effort encompassing many materials and device properties characterization techniques, some mature, others novel and innovative. These measurements and characterization methods in combination reveal physical processes of spin-transfer torque switching in a realistic pMTJ, and relationships between materials properties and resulting device performance. An effective set of such metrics made it possible to efficiently optimize STT-switched pMTJ for MRAM. Here we review some of these measurements and metrics, the methodology they brought together, and some lessons learnt in the process.

Local bifurcation with spin-transfer torque in superparamagnetic tunnel junctions

Modulation of the energy landscape by external perturbations governs various thermally-activated phenomena, described by the Arrhenius law. Thermal fluctuation of nanoscale magnetic tunnel junctions with spin-transfer torque (STT) shows promise for unconventional computing, whereas its rigorous representation, based on the Néel-Arrhenius law, has been controversial. In particular, the exponents for thermally-activated switching rate therein, have been inaccessible with conventional thermally-stable nanomagnets with decade-long retention time. Here we approach the Néel-Arrhenius law with STT utilising superparamagnetic tunnel junctions that have high sensitivity to external perturbations and determine the exponents through several independent measurements including homodyne-detected ferromagnetic resonance, nanosecond STT switching, and random telegraph noise. Furthermore, we show that the results are comprehensively described by a concept of local bifurcation observed in various physical systems. The findings demonstrate the capability of superparamagnetic tunnel junction as a useful tester for statistical physics as well as sophisticated engineering of probabilistic computing hardware with a rigorous mathematical foundation.

A Novel Auto-Write-Stopping Circuit for SHE + STT-MTJ/CMOS Hybrid ALU

A novel auto-write-stopping (AWS) circuit is proposed for magnetic tunnel junctions (MTJs) that use spin-Hall-effect-assisted spin transfer torque (SHE + STT) switching mechanism to store information when used in hybrid MTJ/CMOS circuits developed based on logic-in-memory (LIM) architecture. Continuous monitoring and AWS process of the proposed AWS circuit prevent redundant MTJ writing and eliminate unnecessary write current flow, and thereby save write energy/bit of 96.72%. Using this AWS write circuit, we have developed an AWS full adder (AWSFA), which in turn is used to develop a 1-bit hybrid arithmetic logical unit (ALU). AWSFA is better than its conventional equivalent by dissipating a lower power of 75.08%, whereas 1-bit hybrid ALU is superior to its CMOS counterpart with 39.99% and 41.61% lower power dissipation, and 24.1% and 34.28% lower power delay product (PDP) for arithmetic addition and subtraction, respectively. Furthermore, we have extended the working of hybrid ALU to perform 4-bit arithmetic addition and subtraction to demonstrate the feasibility of AWS hybrid circuits in higher bit operations. We have also conducted Monte Carlo simulations on AWS write and hybrid circuits to study its power dissipation behavior at an early design stage.

Interface imperfection effects on spin transfer torque switching: an atomistic approach

The further commercialization of spintronic memory devices depends on the development of methods by which to assess performance. This paper presents an approach to the atomistic investigation of switching performance in spin transfer torque magneto-resistive random access memory (MRAM) devices with the use of interface imperfection model. Switching simulation in the nanosecond regime was made possible under this model, and we first time demonstrate that switching time is inversely proportional to interface imperfection (i.e. roughness). In investigating the damping of CoFeB/MgO films, we analyzed the effective damping constant α eff, which cannot be accurately predicted for ferromagnetic layers of less than 2 nm using existing micromagnetic models. The proposed model includes a roughness parameter, which has nearly no effect on the effective damping constant in films of >2 nm, but a profound effect in films of <2 nm, reaching a 27% decrease in a 1.0 nm CoFeB film. Our finding is supported by the experimental data of classic references. We expect that these results will prove valuable in magnetic simulation and research on MRAM with ultrathin films.

High-performance voltage controlled multilevel MRAM cell

In the recent past, spin-transfer torque (STT) and spin-orbit torque (SOT) based magnetic random access memories (MRAMs) have been studied for future energy efficient and non-volatile memory applications. Multilevel cell (MLC) design has emerged as one of the promising solutions to enhance the storage density of these MRAMs. However, the conventional MLC design adds a larger magnetic tunnel junction (MTJ) stack that makes it difficult to maintain low switching current and high speed. Moreover, it becomes very difficult to reduce the driver transistor size. This paper describes the application of voltage controlled magnetic anisotropy effect to design energy efficient and fast MLC MRAM cell. So far, this approach has been reported only in single-bit MTJ devices. In the proposed MLC the voltage control is able to reduce both SOT and STT switching currents. The results show that the voltage control in MLC enhances energy efficiency and switching speed by more than 80 times and 3 times, respectively, in comparison to conventional SOT based MLCs. The reduction in switching currents also achieves smaller transistor size and enhances area efficiency by 3.5% as compared to conventional SOT-MLC. The effect of different channel materials on SOT switching current has also been explored. Furthermore, the system level evaluation shows that voltage controlled MLC outperforms STT-MRAM and SOT-MRAM for designing cache memory.

Spin-transfer torque switching probability of CoFeB/MgO/CoFeB magnetic tunnel junctions beyond macrospin

We present an empirical description of experimental spin-torque switching probability for the CoFeB/MgO/CoFeB type of magnetic tunnel junctions beyond macrospin limit, parametrizing measurement data for direct comparison with the corresponding macrospin asymptote expression. We show that, near 35 nm in diameter, spin-torque switching speed in these tunnel devices is faster than macrospin-limit predictions. These devices have a faster reduction of switching error rate versus spin-torque drive amplitude than macrospin. While the functional form similar to macrospin can still describe experimental data satisfactorily, the parameters no longer correspond to materials values. Instead they reflect the nonuniform nature of the switching process. Further, the parameters depend on the resistance-area product ${r}_{A}$ of the junction, with higher ${r}_{A}$ causing a steeper slope of switching error versus switching current. This ${r}_{A}$ dependence could not originate from low-bias spin-dependent tunneling. These observations suggest that, in addition to nonuniform nonlinear dynamics during switching, it is also important to consider higher-order dynamic processes, including a high-bias tunnel electron's spin-flip scattering, voltage-induced change to interface magnetism, and possibly Joule heating.

Phase-change-assisted spin-transfer torque switching in perpendicular magnetic tunnel junctions

Magnetic anisotropy modulation is an effective method to simultaneously reduce the switching current and extend the data retention of magnetic tunnel junction (MTJ), which is promising to be used in the next-generation spin transfer torque (STT) magnetic random-access memory. However, to meet the requirements of high storage life and harsh environments, the improved perpendicular magnetic anisotropy of MTJ makes the conventional modulation methods suffer from high breakdown risk owing to the relatively low efficiency. In this paper, a method of phase-change controlled magnetic anisotropy (PCMA) is introduced to a physical model of VO2/CoFeB/MgO/CoFeB perpendicular MTJ with superior modulation capability proved by systematical simulation. The time sequence of phase change pulse and STT pulse is studied, proving that there exists a specific interval to achieve both rapid and low-power switching. With the joint effect of PCMA and STT, low-energy (68.2 fJ), low-error-rate (0.08), and fast (2 ns) write operation can be achieved in the MTJ accompanied by a high thermal stability factor (78). The results demonstrate that the PCMA-STT switching strategy is most suitable for MTJ with large perpendicular magnetic anisotropy, paving a promising way to replace NOR flash memories.

Design and analysis of SHE-assisted STT MTJ/CMOS logic gates

We have investigated the spin-Hall effect (SHE)-assisted spin transfer torque (STT) switching mechanism in a three-terminal MTJ device developed using p-MTJ (perpendicular magnetic tunnel junction) and heavy metal materials of high atomic number, which possesses large spin–orbit interaction. Using p-MTJ schematic and complementary-metal-oxide-semiconductor (CMOS) logic, we have designed three basic hybrid logic-in-memory structure-based logic gates NOR/OR, NAND/AND, and XNOR /XOR. Then the performances of these hybrid gates are evaluated and the results are compared with the conventional CMOS-based gates in terms of power, delay, power delay product, and device count. From the analysis, it is concluded that SHE-assisted STT MTJ/CMOS logic gates are nonvolatile, consume less power, and occupy a smaller die area as compared to conventional CMOS only logic gates.

Critical switching current of a perpendicular magnetic tunnel junction owing to the interplay of spin-transfer torque and spin-orbit torque

Using the linearized Landau–Lifshitz-Gilbert (LLG) equation in the rotation coordinate, we calculate the critical switching current density of a perpendicular magnetic tunnel junction (MTJ), where magnetization switching is achieved by the interplay of spin-transfer and spin-orbit torques. In terms of the critical current density, we find that as the current density inducing the spin-orbit torque increases, the current density inducing the spin-transfer torque decreases nonlinearly. In the presence of the spin-orbit field-like torque (β), the critical switching current density by the spin-transfer torque is proportional to the damping constant (α) as the conventional spin-transfer torque switching, while the critical switching current density by the spin-orbit torque increases with when . We also investigated the β dependence of the critical switching current density. For a given , the critical switching current density for the spin-transfer torque, decreases linearly with increasing β and nonlinearly decreases with increasing for a given β. Further, we discuss the β dependence of the critical switching current density on energy.

High-Performance Computing-in-Memory Architecture Using STT-/SOT-Based Series Triple-Level Cell MRAM

Spin-torque-based magnetic random access memories (MRAMs) have emerged as a promising option for next-generation data-centric computing systems. Multi-level cell (MLC) configuration is an efficient method to increase storage density. In this article, we propose a series triple-level cell (sTLC) architecture based on spin-transfer torque (STT) and spin-orbit torque (SOT) switching mechanisms. The proposed hybrid STT/SOT sTLC MRAM architecture is capable of storing 3 bits of data using a maximum of two writing steps. However, most of the switching transitions (72%) use only single-step writing. The simulation results of the sTLC MRAM showed 82% and 68% saving in the write energy compared with previously published STT and STT-/SOT-based TLC structures, respectively. One-step parallel read operation for sTLC is presented in this work that enables ultra-fast reading of 3 bits of data. Furthermore, a novel sTLC-based computing-in memory (CiM) architecture is proposed, and high-performance AND/OR/XOR and magnetic full-adder (MFA) logic circuits have been implemented. The proposed sTLC-based CiM MFA shows 33% lesser transistor counts with nearly equivalent energy performance in comparison to the recently published spin-Hall effect (SHE)-based CiM MFA.

Acoustic control of magnetism toward energy-efficient applications

The control of magnetism by acoustically induced strain has driven significant research activities, with the ultimate goal of pursuing novel, ultrafast, compact, and energy-efficient electronic and spintronic applications. Here, we aim to present for the first time a comprehensive review of this field, which has seen a surge of interest in recent years. We review fundamental understanding of magnetoelastic coupling phenomena and mechanisms, diverse experimental configurations, recent advances in modeling and microscopic tools to intuitively describe them, and the experimental and theoretical exploration of devices and technological innovations. These include acoustic spintronics, surface acoustic wave (SAW)-assisted spin transfer torque (STT) switching, SAW-assisted all-optical switching (AOS), SAW-driven spin textures (e.g., Skyrmions and domain walls), acoustic Terahertz emitters, SAW magnetic field sensors, magnetoelastic antenna, on-demand magnonic crystals, and so on. Focusing on the translation of many fundamental research breakthroughs into potential technological applications, we identify the key challenges and opportunities in the field, which we hope may motivate further research efforts of moving scientific discoveries toward real applications.