Single Tunnel Junction Research Articles

High Curie temperature and perfect spin filtering effect in a single layer Ga2O3 magnetic tunnel junction

High Curie temperature and perfect spin filtering effect in a single layer Ga2O3 magnetic tunnel junction

Markov Chain Signal Generation Based on Single Magnetic Tunnel Junction

Markov Chain Signal Generation Based on Single Magnetic Tunnel Junction

Greatly enhanced tunneling electroresistance in ferroelectric tunnel junctions with a double barrier design

We propose that the double barrier effect is expected to enhance the tunneling electroresistance (TER) in the ferroelectric tunnel junctions (FTJs). To demonstrate the feasibility of this mechanism, we design a model structure of Pt/BaTiO3/LaAlO3/Pt/BaTiO3/LaAlO3/Pt double barrier ferroelectric tunnel junction (DB-FTJ), which can be considered as two identical Pt/BaTiO3/LaAlO3/Pt single barrier ferroelectric tunnel junctions (SB-FTJs) connected in series. Based on density functional calculation, we obtain the giant TER ratio of 2.210 × 108% in the DB-FTJ, which is at least three orders of magnitude larger than that of the SB-FTJs of Pt/BaTiO3/LaAlO3/Pt, together with an ultra-low resistance area product (0.093 KΩμm2) in the high conductance state of the DB-FTJ. Moreover, it is possible to control the direction of polarization of the two single ferroelectric barriers separately and thus four resistance states can be achieved, making DB-FTJs promising as multi-state memory devices.

The spin-polarized tunnel current and the spin-transfer torque in triple-barrier junctions with external electrodes and quantum wells made of ferromagnetic materials

The electron transport in triple-barrier junctions consisting of external ferromagnetic layers and three non-magnetic barriers separated by two inner ferromagnetic layers is analysed. It is assumed that the magnetic moments in the external electrodes are oriented in the same direction, whereas the magnetic moments of the inner layers can form any angle with this direction. The possibility of the gigantic enhancement of the normal torque component has been found in the junctions with a non-colinear orientation of magnetic moments in the inner layers. This enhancement may be much larger than in the double junctions. The current and the in-plane torque are most amplified when the quantum-well states in both inner layers have similar energies and can form the complex resonant states located slightly below the Fermi level. When the tunnelling by these complex resonant states occurs, the torque is greatest in the magnetic configuration other than that where the current is maximally enhanced. Additionally, the angular dependence of the torque differs significantly from that observed in single tunnel junctions.

Spintronic Hodgkin-Huxley-Analogue Neuron Implemented with a Single Magnetic Tunnel Junction

Spiking neural networks aim to emulate the brain's properties to achieve similar parallelism and high processing power. A caveat of these neural networks is the high computational cost for emulation, while current proposals for analogue implementations are energy inefficient and not scalable. We propose a device based on a single magnetic tunnel junction to perform neuron firing for spiking neural networks without the need for any resetting procedure. We leverage two areas of physics, magnetism and thermal effects, to obtain biorealistic spiking behavior analogous to the Hodgkin-Huxley model of the neuron. The device is also able to emulate the simpler leaky-integrate-and-fire model. Numerical simulations using experimental-based parameters demonstrate firing frequency in the megahertz to gigahertz range under constant input at room temperature. The compactness, scalability, low cost, CMOS compatibility, and power efficiency of magnetic tunnel junctions advocates for their broad use in hardware implementations of spiking neural networks.

Multiple increase in the radiative efficiency of 1060-nm laser diodes based on heterostructures with tunnel-coupled injection and a single waveguide mode.

An approach aimed at increasing the radiative efficiency in heterostructures operating in a single vertical mode at 1060 nm has been studied. Two types of heterostructures-the STJH (single tunnel junction heterostructure) and DTJH (double tunnel junction heterostructure)-have been developed to obtain the operation of a single waveguide mode of the first and second order, respectively. A multiple increase in the injection efficiency is realized by using tunnel junctions (TJs) embedded in the wide-gap barriers and placed between the active regions. An internal quantum efficiency of close to 100% and a transparency current density of 78 A/cm2 is demonstrated for both types of heterostructures, while the internal optical loss is 5.2 and 3.8 cm-1 for STJH and DTJH, respectively. The maximum radiative efficiency (for the initial linear part of the light-current curve) and the output optical power (for the current pulse of 1 µs and 9.6 A) reach 1.6 W/A and 7.8 W for STJH and 2.2 W/A and 15.8 W for DTJH.

A High Performance MRAM Cell Through Single Free-Layer Dual Fixed-Layer Magnetic Tunnel Junction

As technology size scales down, magnetic tunnel junctions (MTJs) as a promising technology are becoming more and more sensitive to process variation, especially in oxide barrier thickness. Process variation particularly affects the cell resistance and the critical switching current for the smaller dimensions. This article proposes an MTJ cell with one free and two pinned layers, which highly improves the process variation robustness. By employing the spin transfer torque (STT)-spin-Hall effect (SHE) switching method, our proposed MTJ cell improves the switching speed and lowers the switching power consumption. Per simulations, an MRAM cell built with the proposed MTJ cell offers up to 36% lower total power consumption, up to 32% higher write performance, and 35% higher read performance over the previous state-of-the-art MRAMs. Also, the error rate resulting from process variation is 1.4%, which is about one-tenth of the best examples reported in the literature.

Enhanced Spin Transfer Torque in Single Barrier Model Tunnel Junctions Containing Disordered Interface

We investigate the role of magnetic impurities in the barrier region in enhancing the torque transferred to the free ferromagnet layer in a non-collinear model tunnel junction. The tunnel junction is modeled using the non-equilibrium Green’s function formalism within the tight-binding approximation. The position of the impurity which is described by the single impurity Anderson model is found to have a profound impact on the trends observed in the spin transfer torque. The parallel ([Formula: see text]) torque is observed to have a switch on-off step-like bias behavior while the perpendicular ([Formula: see text]) torque exhibits a change in sign between the switch-on bias voltages. The spin transfer torque is found to be dramatically enhanced when the impurity is positioned at the interface of a five-layer barrier which is three orders of magnitude larger even at temperatures as high as 200[Formula: see text]K. With this formulation the effect of Coulomb repulsion at the impurity site on the torque could be deduced and the obtained results are explained in terms of the region-wise electronic density of states in the junction.

Spin Logic Operated by Unipolar Voltage Inputs

We propose and demonstrate a reconfigurable logic gate based on a single spin-orbit torque magnetic tunnel junction (SOT-MTJ) device operated by unipolar voltage inputs. The SOT-MTJ used in the logic can be switched between high and low resistance state when large and small amplitude voltage pulses inject to it. Based on this magnetic switching characteristic, the logic gates of XOR, XNOR and NOT are designed and realized by different amplitude of voltage with the same sign. Compared with spin logics with bipolar current signals as logic inputs, our logic implementation uses much less transistors and significantly simplifies the circuit complexity, which is advantageous for logic-in-memory from the perspective of energy and area efficiency.

Research progress of tunneling magnetoresistance sensor

Sensors play an important role in Internet of Things (IoT) industry and account for a rapidly growing market share. Among them, the magnetic sensor based on tunneling magnetoresistance (TMR) effect possesses great potential applications in the fields of biomedical, navigation, positioning, current detection, and non-destructive testing due to its extremely high sensitivity, small device size and low power consumption. In this paper, we focus on the development of TMR sensor technology routes, covering a series of research advances from a sensor transducer to three-dimensional magnetic field detection, and then to the applications. Firstly, we recall the development history of TMR sensors, explain its working principle, and discuss the method to improve the output linearity of single magnetic tunnel junction. Next, we state the Wheatstone-bridge structure, which can inhibit temperature drift in detail and review several methods of fabricating the full bridge of TMR sensors. Furthermore, for the market demand of three-dimensional magnetic field detection, we summarize the methods of designing and fabricating three-dimensional sensing structure of the TMR sensor. At the same time, we list several optimization schemes of TMR sensor performance in terms of sensitivity and noise level. Finally, we discuss two types of emerging applications of TMR sensors in recent years. The TMR sensors can also be used in intelligence healthcare due to their ultra-high sensitivity. In addition, devices from the combination of spin materials and MEMS structure have attracted wide attention, especially, because of the large commercial market of microphones, spin-MEMS microphones utilized TMR techniques will be the next research hotspot in this interdisciplinary field.

Exchange field determination in a quantum dot spin valve by the spin dynamics

We develop the theory of the electron transport through quantum dot weakly coupled to ferromagnetic leads with noncollinear magnetization directions, that has been studied in recent experiments. One can observe much richer transport behavior of the canted quantum dot spin valves, as compared to single magnetic tunnel junctions, that relies on the possibility to generate a nonequilibrium accumulated spin on the quantum dot and the presence of the exchange interaction between dot and electrodes, depending on system parameters such as gate and bias voltages, the charging energy, an asymmetry of the tunnel couplings, and the external magnetic field. We demonstrate that one can extract information about spin dynamics on quantum dot from the dc current–voltage characteristic even at the linear response, and detect the exchange field similarly to the FMR (ferromagnetic resonance) experiment. This exchange field can be widely used in nano-spinelectronics, as a local field controlled by the gate or bias voltages also at high temperatures.

Quaternary-Digital Data Storage Based on Magnetic Bubbles in Anisotropic Materials

The topologically non-trivial nano-whirls, called magnetic skyrmions, are often considered attractive for spintronic applications like the racetrack data storage device. However, skyrmions do not move parallel to applied currents and typically do not coexist with other nano-objects, making the realization of such storage concepts difficult. Herein we consider materials with an anisotropic DMI, like tetragonal Heusler materials, and show that four distinct types of topologically trivial bubbles can coexist. We show that each of them can be written by spin torques and that they can be distinguished using a single magnetic tunneling junction. Due to their trivial topology, the four types of bubbles move parallel to applied electrical currents and remain equidistant under motion. Still, the bubbles have a small size and high stability comparable to topologically non-trivial spin textures. This allows to construct a quaternary-digit-based racetrack storage device that overcomes the two mentioned drawbacks of skyrmion-based racetracks.

Quantitative Evaluation of Hardware Binary Stochastic Neurons

Recently there has been increasing activity to build dedicated Ising Machines to accelerate the solution of combinatorial optimization problems by expressing these problems as a ground-state search of the Ising model. A common theme of such Ising Machines is to tailor the physics of underlying hardware to the mathematics of the Ising model to improve some aspect of performance that is measured in speed to solution, energy consumption per solution or area footprint of the adopted hardware. One such approach to build an Ising spin, or a binary stochastic neuron (BSN), is a compact mixed-signal unit based on a low-barrier nanomagnet based design that uses a single magnetic tunnel junction (MTJ) and three transistors (3T-1MTJ) where the MTJ functions as a stochastic resistor (1SR). Such a compact unit can drastically reduce the area footprint of BSNs while promising massive scalability by leveraging the existing Magnetic RAM (MRAM) technology that has integrated 1T-1MTJ cells in ~Gbit densities. The 3T-1SR design however can be realized using different materials or devices that provide naturally fluctuating resistances. Extending previous work, we evaluate hardware BSNs from this general perspective by classifying necessary and sufficient conditions to design a fast and energy-efficient BSN that can be used in scaled Ising Machine implementations. We connect our device analysis to systems-level metrics by emphasizing hardware-independent figures-of-merit such as flips per second and dissipated energy per random bit that can be used to classify any Ising Machine.

Breaking of Coulomb blockade by macrospin-assisted tunneling

A magnet with precessing magnetization pumps a spin current into adjacent leads. As a special case of this spin pumping, a precessing macrospin (magnetization) can assist electrons in tunneling. In small systems, however, the Coulomb blockade effect can block the transport of electrons. Here, we investigate the competition between macrospin-assisted tunneling and the Coulomb blockade for the simplest system where both effects meet; namely, for a single tunnel junction between a normal metal and a metallic ferromagnet with precessing magnetization. By combining Fermi's golden rule with magnetization dynamics and charging effects, we show that the macrospin-assisted tunneling can soften or even break the Coulomb blockade. The details of these effects---softening and breaking of the Coulomb blockade---depend on the macrospin dynamics. This allows us, for example, to measure the macrospin dynamics via a system's current-voltage characteristics. It also allows us to control a spin current electrically. From a general perspective, our results provide a platform for the interplay between spintronics and electronics on the mesoscopic scale. We expect our work to provide a basis for the study of Coulomb blockade in more complicated spintronic systems.

A Cascade Electron Source Based on Series Horizontal Tunneling Junctions

On-chip electron sources have wide potential applications in miniature vacuum electronic devices and emission efficiency is one of their performance benchmarks. A cascade electron source based on series metal-insulator-metal horizontal tunneling junctions is proposed, where free electrons are additively extracted from each tunneling junction. A cascade electron source with n horizontal tunneling junctions shows a theoretical emission efficiency of approximately {\eta}(n)=1-(1-{\eta}_0 )^n, with {\eta}_0 being the efficiency of a single tunneling junction. Experimentally, a cascade electron source with three Si-SiOx-Si tunneling junctions is demonstrated, achieving an emission efficiency as high as 47.6%. This work provides a new way of realizing highly efficient on-chip electron sources.

Multi‐Level Switching and Reversible Current Driven Domain‐Wall Motion in Single CoFeB/MgO/CoFeB‐Based Perpendicular Magnetic Tunnel Junctions

AbstractOne of the critical issues in spintronics‐based technologies is to increase the data storage density. Current strategy is based on shrinking the devices size down to tens of nanometers, or several nanometers, which is reaching its limit. A new proposal is to use multi‐level cells (MLCs) to store more than two bits in each cell. In this work, the multi‐level switching is realized in CoFeB/MgO/CoFeB based nano‐scale single perpendicular magnetic tunnel junctions (p‐MTJs) with three or four stable resistance states. A large range of writing currents for each state is obtained, accompanying with a good repeatability of set‐reset operations between different states. The developed multi‐domain model perfectly matches the experimental results, reflecting the magnetic behaviors during multi‐level switching. Furthermore, current‐driven domain wall (DW) motion is revealed in the circular p‐MTJs, where the DW position can be reversibly manipulated by applied current. To design high‐performance multi‐level p‐MTJs, the parameter diagrams are calculated, suggesting various feasible strategies to improve the multi‐level switching through materials optimization and devices geometry. In summary, the demonstration of multi‐level switching in single p‐MTJ shows the high potential of realizing the new generation of p‐MTJ‐based multi‐level spintronic devices, such as multi‐level memories and spin‐neuron devices.

Electrode-molecule energy level offsets in a gold-benzene diamine-gold single molecule tunnel junction.

One means for describing electron transport across single molecule tunnel junctions (MTJs) is to use density functional theory (DFT) in conjunction with a nonequilibrium Green's function formalism. This description relies on interpreting solutions to the Kohn-Sham (KS) equations used to solve the DFT problem as quasiparticle (QP) states. Many practical DFT implementations suffer from electron self-interaction errors and an inability to treat charge image potentials for molecules near metal surfaces. For MTJs, the overall effect of these errors is typically manifested as an overestimation of electronic currents. Correcting KS energies for self-interaction and image potential errors results in MTJ current-voltage characteristics in close agreement with measured currents. An alternative transport approach foregoes a QP picture and solves for a many-electron wavefunction on the MTJ subject to open system boundary conditions. It is demonstrated that this many-electron method provides similar results to the corrected QP picture for electronic current. The analysis of these two distinct approaches is related through corrections to a junction's electronic structure beyond the KS energies for the case of a benzene diamine molecule bonded between two gold electrodes.

Reconfigurable spin orbit logic device using asymmetric Dzyaloshinskii–Moriya interaction

A spin–orbit torque-based logic device that combines the Dzyaloshinskii–Moriya interaction and asymmetric shape anisotropy is proposed in this paper. Through magnetization switching of the magnetic tunnel junction (MTJ), four distinct input cases are determined to perform all of the 16 basic Boolean functions by a single magnetic tunnel junction (MTJ). With various combinations of the tilted angle of the MTJ and the value of current density, a single MTJ can serve as a reconfigurable logic gate.

Realizing both short- and long-term memory within a single magnetic tunnel junction based synapse

Synaptic plasticity forms the basis of memory retention in the human brain. Whereas a low “rehearsal” rate causes short-term changes in the synaptic connections such that the synapse soon “forgets,” a high rehearsal rate ensures long-term retention of memory in the brain. In this paper, we propose an artificial short- and long-term memory magnetic tunnel junction (SALT-MTJ) synapse. Changes in the synaptic strength are mapped to the SALT-MTJ conductance, which is varied stochastically via spin-transfer torque resulting from input current stimuli. A meta-stable intermediate magnetic state of the SALT-MTJ synapse provides short-term synaptic plasticity and the associated forgetting behavior as in a biological synapse. Repeated spin-current stimulations, while the SALT-MTJ remains in the short-term state, then can cause a near-permanent change in the magnetic state and associated conductance to provide long-term potentiation. The synaptic weight sensitivity to the input stimulus and the forgetting behavior of these short- and long-term states can be controlled via shape engineering of the artificial synapse.

Tunneling magnetoresistance and light modulation in Fe4N(La2/3Sr1/3MnO3)/C60/Fe4N single molecule magnetic tunnel junctions

Spin-dependent transport properties and light modulation of Fe4N/C60/Fe4N and LSMO/C60/Fe4N single molecule magnetic tunnel junctions.