Magnetic Tunnel Junction Structure Research Articles

High-Frequency Microwave Emission of a Trilayer Magnetic Tunnel Junction in the Absence of External Magnetic Bias Field

We perform an experimental study of DC current induced microwave emission in a magnetic tunnel junction (MTJ) consisting of three active magnetic layers. For this tri-layer structure, in addition to a conventional bilayer orthogonal MTJ containing a perpendicular free layer and an in-plane fixed layer, a second perpendicular layer has been introduced. We found that the microwave emission frequency induced by spin-transfer torque (STT) can reach as high as 6 GHz in the absence of any applied magnetic field. Moreover, microwave emission is observed for both current polarizations where a redshift is seen with increase in magnitude of current. We discuss spin-dynamics of the observed bi-directional high-frequency emission and the physical origin of the red-shift. The distinct microwave emission properties exhibited in this tri-layer MTJ structure could potentially be useful for future applications in nanoscale spintronics devices such as microwave communication and neuromorphic computing.

RETRACTED ARTICLE: Modeling and design of Magnetic Tunneling Junction using MoS2/graphene quantum dots/MoS2 approach

Spintronics is a novel field of research technique used to control the electron spin on the spin-dependent current. From that the Magnetic Tunneling Junction (MTJ) based spintronics plays a significant role because of its extensive application, namely, great scalability, high write speed, and huge relative magnetoresistance. Nevertheless, there is a problem in the fabrication of spintronics-based MTJ devices. Hence, in this paper, a novel efficient approach of MTJ structure is designed to meet the objective of the devices. Therefore, molybdenum disulfide (MoS2)/graphene quantum dots (GQDs)/MoS2 based MTJ structure design is developed for better efficiency with low power consumption. The two-dimensional MoS2 acts as ferromagnetic electrodes in MTJ due to its magnetic properties, and GQD acts as a barrier. Furthermore, the current vs. voltage characteristics for both parallel (P) and anti-parallel (AP) MTJ junctions are estimated; as a result, the MTJ tunnel magnetoresistance (TMR) design is achieved at 1450 % at zero bias voltage. Besides, the effects of spin relaxation and magnetization relaxation are also investigated for expanding the lifetime of the spin. Thus, the stimulated setup outcome shows the effectiveness of the proposed MTJ structure for spintronics, and the results are compared with existing MTJ designs.

Thermally consistent SPICE model for magnetic tunnel junction with multi-layered structures via interlayer interaction

With the recent commercialization of large-scale integration products, which include embedded magnetoresistive random-access memory, the importance of simulation models for MRAM circuits (MRAM compact models) has been continuously increasing. To accurately reproduce the characteristics of MRAM devices, the physical mechanism of the MRAM device including temperature dependency should be precisely described. In this study, we propose an MRAM compact model based on the Landau–Lifshitz–Bloch equation according to a previously developed model, which was based on the Landau–Lifshitz–Gilbert equation. Furthermore, we extend the model by considering interlayer interactions for handling multi-layered magnetic tunnel junction (MTJ) structures. We infer that circuits including MRAM devices, which consist of multi-layered MTJ structures having unique thermal characteristics, can be designed through SPICE simulations.

NVLCFF: An Energy-Efficient Magnetic Nonvolatile Level Converter Flip-Flop for Ultra-Low-Power Design

Power dissipation has become a major concern in nanoscale integrated circuits. The power gating and dual-supply design methods are among the most effective approaches for reducing the static and dynamic power consumptions. However, these methods require efficient data retention and voltage level conversion. In this paper, a nonvolatile level converter flip-flop (NVLCFF) is proposed to be used in ultra-low-power integrated circuits. Our proposed NVLCFF employs the magnetic tunnel junction (MTJ) for data retention. Spin transfer torque along with the spin Hall effect is used for reconfiguring the MTJs. The peripheral circuitry is designed using 7-nm FinFET as one of the leading industrial technologies. Furthermore, to facilitate the use of the dual-supply approach, voltage level conversion is performed in the structure of the proposed NVLCFF. According to the HSPICE simulations, the power consumption, backup energy and restore energy of the proposed circuit are on average 59%, 48% and 92% lower than the other NVLCFFs based on the previous nonvolatile flip-flops with different MTJ structures. Furthermore, the comprehensive Monte Carlo simulations indicate the robustness of the proposed design in the presence of major MTJ and FinFET process variations as compared to the previous designs.

Flexible CoFeB/MgO-based magnetic tunnel junctions annealed at high temperature (≥350 °C)

This study investigates the effect of high-temperature (350–500 °C) annealing on CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) directly formed on a flexible polyimide substrate, which has superior thermal tolerance. As the annealing temperature increases, the tunnel magnetoresistance (TMR) ratio enhances and reaches up to ∼200% at an annealing temperature of 450 °C. The annealing temperature dependence is similar to that of MTJs fabricated in the same way on a thermally oxidized silicon substrate. Images taken by a scanning transmission electron microscope confirm the improvement of the crystallization of the CoFeB and MgO layers, which can be an important factor in enhancing the TMR ratio. Furthermore, the endurance of the flexible MTJ against repeated stretching of its substrate is investigated. The TMR ratio shows no change during and after a 1000-cycle application of a tensile strain larger than 1%. The high TMR ratio and strain endurance demonstrated in this study suggest that the flexible MTJ structure is a promising candidate for a future strain-sensing device.

Large voltage control of magnetic anisotropy in CoFeB/MgO/OX structures at room temperature

Voltage control of magnetic anisotropy (VCMA) provides an energy-efficient approach to manipulate spintronic devices. Currently, VCMA only shows a weak effect in magnetic tunnel junctions (MTJs) composed of CoFeB/MgO/CoFeB that are the core structure of spintronic memories and logic devices. Multiple approaches have been proposed and studied by researchers to increase the VCMA effect. Here, we demonstrate a large VCMA effect in the CoFeB/MgO/SiO2 double-oxide structure, which can be potentially modified to be compatible with the MTJ cell. The VCMA coefficient as high as 174 fJ/Vm is achieved in this structure at room temperature, with its magnitude comparable to the reported ion-driven VCMA with a high ion-conductive oxide at an elevated temperature. Theoretical analysis indicates that the large VCMA is a magnetoionic effect, which is dominated by ion migration and can be explained by a nanograin cluster model. This double-oxide structure is promising to be extended to an MTJ structure to reduce switching energy in spintronic devices.

Large spin Hall effect and tunneling magnetoresistance in iridium-based magnetic tunnel junctions

Magnetic tunnel junctions (MTJs) switched by spin-orbit torque (SOT) have attracted substantial interest owing to their advantages of ultrahigh speed and prolonged endurance.Both field-free magnetization switching and high tunneling magnetoresistance (TMR) are critical for the practical application of SOT magnetic random access memory (MRAM).In this work, we propose an MTJ structure based on an iridium (Ir) bottom layer. Ir metal is a desirable candidate for field-free SOT switching owing to its strong intrinsic spin Hall conductivity (SHC), which can be enhanced via doping. Herein, we study TMR in Ir-based MTJs with symmetric and asymmetric structures.Ir-based MTJs exhibit large TMR, which can be further enhanced by heavy metal symmetry owing to the resonant tunneling effect. Our comprehensive investigations illustrate that Ir-based MTJs are promising candidates for realizing SOT switching and high TMR.

High frequency voltage-induced ferromagnetic resonance in magnetic tunnel junctions

Voltage-induced ferromagnetic resonance (V-FMR) in magnetic tunnel junctions (MTJs) with a W buffer is investigated. Perpendicular magnetic anisotropy (PMA) energy is controlled by both the thickness of a CoFeB free layer deposited directly on the W buffer and a post-annealing process at different temperatures. The PMA energy and the magnetization damping are determined by analyzing field-dependent FMR signals in different field geometries. An optimized MTJ structure enabled the excitation of V-FMR at frequencies exceeding 30 GHz. The macrospin modeling is used to analyze the field- and angular-dependence of the V-FMR signal and to support experimental magnetization damping extraction.

Stability Diagrams of a Tunnel Nanoheterostructure in the Free-Electron Approximation

One of important characteristics of a magnetic tunneling junction (MTJ) nanoheterostructure that is used as magnetic random access memory (MRAM) cell and is switched by the spin transfer torque (STT) effect is its stability diagram, which determines the range of values of the external magnetic field and applied voltage under which a cell is in one of two stable states. To numerically construct such diagrams, one usually uses the solution of the Landau–Lifshitz dynamic equation in the single-domain approximation along with phenomenological constants that determine the values of spin torque in the material. In the present paper, the problem of spin-dependent electron transfer in an MTJ structure is considered, whose solution in the free-electron approximation allows one to calculate the values of spin torque for a given material in any magnetic configuration of the system and apply them to the integration of the Landau–Lifshitz equation. The method used allows one to more precisely reproduce the shape of the stability diagram and predict the critical values of the magnetic field and voltage necessary to switch an MTJ-based MRAM cell.

Perpendicular Spin-Transfer-Torque Magnetic Tunnel Junction Multi-Level Cell with Dual Free Layers and MgO Tunnel Barriers

Perpendicular spin-transfer-torque magnetic random access memory (p-STT-MRAM) is a promising candidate memory device for replacing dynamic random access memory (DRAM) facing with 10-nm sub scaling down. In addition to scaling down, p-STT MRAM has many advantages like fast write/read speed (~10ns) as DRAM, low power consumption (<1pJ), high endurance (>1014), and non-volatility (retention time > 10 years). But to replace DRAM, there are several challenges to be solved. First, high tunneling magneto-resistance (TMR) ratio for read sensing margin. Second, low critical current density(J C) for low power consumption. Third, thermal stability(Δ) for long retention time to save data. And also these parameters should be achieved at 350oC annealing temperature of the back-end-of line (BEOL) to be commercialized. Among them, thermal stability is closely related with cell size. As a cell size of MTJ decreases to achieve a high density memory device, thermal stability decreases with retention time, thus it is not easy to implement both high density and non-volatile memory device simultaneously by decreasing the cell size of MTJ. In our study to solve this issue, we propose the p-STT multi-level cell (MLC) magnetic tunnel junction (MTJ) because MLC MTJ can satisfy both high density and non-volatile memory device at the same time. In particular, we design the MTJ structure with dual free layers and MgO tunnel barriers, so that the MTJ can demonstrate four resistance states. In addition, we investigated the effect of the thickness of two free layers and MgO tunnel barriers on two magneto-resistance (MR) and switching voltage. In our experiments, the p-STT MTJ spin-valves were fabricated on 12-inch-diameter Si/ SiO2/W/TiN wafers by utilizing a 12-in. multi-chamber-cluster magnetron-sputtering system under a high vacuum (less than 110-8 Torr) without breaking the vacuum. Then, all samples were subject to ex-situ annealing at 350℃ at a back end of line (BEOL) for 30 minutes under a magnetic field of 3 tesla. In our presentation, we present how to optimize the thicknesses of two free layers and MgO tunnel barriers. In addition, we repot how the magnetic behavior and the resistance of MTJ is affected by magnetic and how the magneto-resistance of MTJ is closely related to the crystallinity of two MgO tunnel barriers. Acknowledgements This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2017R1A2A1A05001285)

Over 100% magnetoresistance ratio at room temperature in magnetic tunnel junctions with CuGaSe2 spacer layer

We investigated the structure and magneto-transport properties of magnetic tunnel junctions (MTJs) consisting of half-metallic Co2FeGa0.5Ge0.5 ferromagnetic electrodes and a semiconductive CuGaSe2 interlayer. The magnetoresistance (MR) ratio reaches more than 100% at room temperature and nearly 250% at 30 K with the resistance area product (RA) smaller than 1 Ω μm2. Microstructural analysis clarified that Co and Fe atoms diffuse into the CuGaSe2 layer, which can be a possible reason for the scattering of RA values but poorly affect the MR ratio itself. Our findings stress the potential of semiconductor barriers in MTJs for spintronics application that requires low RA.

Simulation of Strain-Assisted Switching in Synthetic Antiferromagnetic Free Layer-Based Magnetic Tunnel Junction

We perform macrospin simulations on a magnetic tunnel junction structure with perpendicular magnetic anisotropy, in which the conventional free layer is substituted with synthetic antiferromagnetic free layers to improve the thermal stability. In order to reduce the switching current Pb(Zr0.2Ti0.8)O3 material with (001) surface orientation is adopted, which is adjacent to the second free layer so that it applies biaxial stress to the interface under external voltage. We solve the Landau–Lifshitz–Gilbert–Slonczewski equation incorporating thermal and strain fields to describe the transient dynamics of magnetizations in both the free layers. The simulation results show a substantial reduction in switching current and significantly enhanced switching probability by modulating current and strain pulses.

Micromagnetic simulation of spin torque ferromagnetic resonance in nano-ring-shape confined magnetic tunnel junctions

Spin-torque excited ferromagnetic resonance (ST-FMR) performed in nanoscale devices can be used to characterize material properties and optimize spintronic devices for applications. In this paper, micromagnetic simulation is employed to study the FMR property in a nano-ring shaped magnetic tunnel junction. The simulation predictions are confirmed by experiments, revealing that two resonance states (mode-1 and mode-2) with different resonance frequencies are excited by a microwave frequency current or magnetic field. The relatively weak response signal of FMR at the negative field can be attributed to the intrinsic asymmetric structure of magnetic tunnel junctions, which leads to a bias-dependent asymmetric spin transfer torque. Besides, high-order harmonics with two and three times the fundamental frequencies of the mode-1 and mode-2 are also observed.

Superparamagnetic ground state of CoFeB/MgO magnetic tunnel junction with dual-barrier

CoFeB/MgO-based magnetic tunnel junctions (MTJs) have considerable potential in magnetic random access memory (MRAM), thanks to their tunable perpendicular magnetic anisotropy (PMA). We found significant reduction of dead-layer by inserting additional MgO into the MTJ structure. Interface, electronic and transport characterizations were utilized to approach the modified magnetic properties driven by the dual-MgO structure in this work. The dual-MgO structure appeared to hinder boron (B) diffusion into the metallic layer and prevent capping-layer (Ta) penetration across the interface. This suppressed the dead-layer effect and promoted overall magnetization despite PMA degradation. A robust BOx phase that formed within the dual-MgO structure presented a superparamagnetic ground state. In the single-MgO structure, any reduction in the thickness of the CoFeB promoted PMA, albeit at the cost of spin-polarization. The dual-MgO structure could restore spin-polarization by preferentially populating spin electrons into Fe/Co minority states. X-ray magnetic spectroscopy and anomalous Hall effect suggest that, the dual-MgO differs from the single-MgO with a favorable longitudinal polarized spin-channel. This makes the dual-MgO structure applicable to applications requiring in-plane rather than out-of-plane sensing.

Band-pass Fabry-Pèrot magnetic tunnel junctions

We propose a high-performance magnetic tunnel junction by making electronic analogs of optical phenomena such as anti-reflections and Fabry-Pèrot resonances. The devices we propose feature anti-reflection enabled superlattice heterostructures sandwiched between the fixed and the free ferromagnets of the magnetic tunnel junction structure. Our predictions are based on non-equilibrium Green's function spin transport formalism coupled self-consistently with the Landau-Lifshitz-Gilbert-Slonczewski equation. Owing to the physics of bandpass spin filtering in the bandpass Fabry-Pèrot magnetic tunnel junction device, we demonstrate an ultra-high boost in the tunnel magneto-resistance (≈5 × 104%) and nearly 1200% suppression of spin transfer torque switching bias in comparison to a traditional trilayer magnetic tunnel junction device. The proof of concepts presented here can lead to next-generation spintronic device design harvesting the rich physics of superlattice heterostructures and exploiting spintronic analogs of optical phenomena.

Magnetic tunnel junction thermocouple for thermoelectric power harvesting

The thermoelectric power generated in magnetic tunnel junctions (MTJs) is determined as a function of the tunnel barrier thickness for a matched electric circuit. This study suggests that lower resistance area product and higher tunnel magnetoresistance will maximize the thermoelectric power output of the MTJ structures. Further, the thermoelectric behavior of a series of two MTJs, a MTJ thermocouple, is investigated as a function of its magnetic configurations. In an alternating magnetic configurations the thermovoltages cancel each other, while the magnetic contribution remains. A large array of MTJ thermocouples could amplify the magnetic thermovoltage signal significantly.

Methods to induce perpendicular magnetic anisotropy in full-Heusler Co2FeSi thin layers in a magnetic tunnel junction structure

In this study, to obtain perpendicular magnetic tunnel junctions (p-MTJs) using half-metallic ferromagnets (HMFs), several methods were developed to induce perpendicular magnetic anisotropy (PMA) in full-Heusler Co2FeSi (CFS) alloy thin layers in an MTJ multilayer composed of a layered CFS/MgO/CFS structure. Oxygen exposure at 2.0 Pa for 10 min after deposition of the bottom CFS layer was effective for obtaining PMA in the CFS layer. One of the reasons for the PMA is the formation of nearly ideal CFS/MgO interfaces due to oxygen exposure before the deposition of the MgO layer. The annealing process was effective for obtaining PMA in the top CFS layer capped with a Pd layer. PMA was clearly observed in the top CFS layer of a Cr(40 nm)/Pd(50 nm)/bottom CFS(0.6 nm)/MgO(2.0 nm)/top CFS(0.6 nm)/ Pd(10 nm) multilayer, where the top CFS and Pd thin films were deposited at RT and subsequently annealed at 300°C. In addition to the continuous layer growth of the films, the crystalline orientation alignment at the top CFS/Pd interface probably attributes to the origin of PMA at the top CFS layer.

Role of phase breaking processes on resonant spin transfer torque nano-oscillators

Spin transfer torque nano-oscillators (STNOs) based on magnetoresistance and spin transfer torque effects find potential applications in miniaturized wireless communication devices. Using the non-coherent non-equilibrium Green’s function spin transport formalism self-consistently coupled with the stochastic Landau-Lifshitz-Gilbert-Slonczewski’s equation and the Poisson’s equation, we elucidate the role of elastic phase breaking on the proposed STNO design featuring double barrier resonant tunneling. Demonstrating the immunity of our proposed design, we predict that despite the presence of elastic dephasing, the resonant tunneling magnetic tunnel junction structures facilitate oscillator designs featuring a large enhancement in microwave power up to 8μW delivered to a 50Ω load.

A Comparative Study Between Spin-Transfer-Torque and Spin-Hall-Effect Switching Mechanisms in PMTJ Using SPICE

The spin transfer torque magnetoresistive random access memory (STT-MRAM) is the leading candidate for spin-based memories. Nevertheless, the high write energy and read disturbance of the STT-MRAM motivated researchers to find other solutions. The spin Hall effect (SHE)-based MRAM is an alternative for the STT-MRAM, which also provides nonvolatility, zero leakage, and competitive area per bit, but with a lower write current. This paper focuses on a systematic performance analysis of these two proposed memory solutions. The SHE requires an external field to deterministically switch perpendicular magnetic anisotropy magnetic tunnel junction (MTJ). A previous experiment showed that the SHE can switch composite MTJ containing an in-plane layer without any field. In this paper, both traditional and composite MTJ structures are modeled in SPICE, which can reproduce realistic MTJ characteristics with user-defined input parameters. This self-contained model is used to compare the write energy and delay of the STT-MRAM and the SHE magnetoresistive random access memory (SHE-MRAM) for various write schemes including thermal fluctuation. Our simulations show, compared with the STT-MRAM, that the SHE-MRAM improves the write delay and the energy by eight times and seven times, respectively. Based on our extensive analysis incorporating the latest advances in magnetic materials and device technology, we predict that the SHE-MRAM is a feasible low-energy memory solution for future computing systems.

Numerical Study on a Novel Curie Temperature Controlled Hybrid Thermo-Magnetic Structure for Magnetic Random Access Memories

High thermal stability, a fast switching time, and a low switching current are key characteristics of merit for realizing gigabit-class magnetic random access memory (MRAM). Especially, the switching current needs to be reduced significantly for creating high-density MRAM. In order to realize these necessary features, a number of magnetic tunnel junction (MTJ) structures have been proposed. A previous study of ours led to the proposal of a novel Curie-temperature-controlled hybrid thermo-magnetic structure for MRAM, for the purpose of improving these advantageous properties. This paper presents an analysis of the switching time and switching current by using micromagnetic simulation techniques for comparing our structures with various MTJ structures that have been proposed recently. We confirm that our novel structure can improve the switching characteristics of MRAM. The results of our analysis revealed that a 44% lower switching current and a 32% faster switching time can be achieved compared with the conventional structure.