Antiparallel State Research Articles

Rationally Designed High-Performance Spin Filter Based on Two-Dimensional Half-Metal Cr2NO2

Summary Half-metals are promising candidates for designing efficient spin filters owing to their unique electronic structures, which show the electrical conductivity for spin-up states and a band gap for spin-down states. Herein, designing excellent ultrathin spin filters by using half-metal two-dimensional Cr2NO2, which has a Curie temperature of 566 K, is demonstrated based on first-principles calculations. Our results reveal that the required energy to overturn the spin arrangement of bilayer Cr2NO2, from parallel to anti-parallel states, is quite low. The bilayer Cr2NO2 maintains its half-metal behavior while sandwiched within the Au/Cr2NO2/Au heterojunction. Owing to the half-metal characteristic, the total current of Au/Cr2NO2/Au can reach a value of 15 nA per primitive cell under a low voltage of 10 mV in parallel states. The magnetoresistance ratio is 9,333% at low voltage. The robust half-metal behavior, high Curie temperature, and two-dimensional structure make Cr2NO2 an ideal ultrathin spin-filtering material with high switching ratio and low energy consumption.

Maximal tunnel magnetoresistance in magnetic nanoparticle arrays with perpendicular anisotropy

We study tunnel magnetoresistance (TMR) ratio of self-assembled $2d$ magnetic nanoparticle (MNP) arrays by modeling them as interacting dipoles on a triangular lattice, which is representative of experimentally obtained assemblies. Low-temperature Monte Carlo simulations are performed to understand the effects of dipolar interactions and uniaxial anisotropy on TMR behavior. In magnetic tunnel junctions with a perfect antiparallel state, the TMR amplitude reaches the ideal value $2{P}^{2}/(1\ensuremath{-}{P}^{2})$, where $P$ is the spin polarization. We show that, for MNPs with their anisotropy axes perpendicular to the array, the TMR amplitude can reach a value as large as $2400%$, which represents an order of magnitude improvement over previous studies. We evaluate the conditions for which these large amplitudes could be obtained experimentally at room temperature and find that spherical MNPs with large magnetocrystalline anisotropy or nanorods are suitable candidates. Our theoretical results should stimulate experimental groups into elaborating the samples proposed in this work, which could display large TMR amplitudes.

Spin-Orbit Currents, Spin-Transfer Torque and Anomalous Tunneling in III–V Heterostructures Probed by Advanced 30- and 40-Bands ${k}\cdot{p}$ Tunneling Methods

On the basis of tunneling magnetoresistance (TMR) spin-transfer torque (STT) experiments in (Ga,Mn)As/GaAs/(Ga,Mn)As submicronic magnetic tunnel junctions, we have analyzed the anatomy of the spin-current profiles within the heterostructures, location of strong spin–orbit interactions. Beyond the TMR, our robust 30- and 40-band $k\cdot p$ numerical methods reveal the strong peculiarity of the spin-currents in the antiparallel state with the evidence of a tunneling anomalous Hall effect. Using the boundary conditions corresponding to heavy-hole (HH)-to-light-hole (LH) mixing of the relevant $\text{C}_{2v}$ symmetry at III–V interfaces of the junctions, we demonstrate that the efficiency of the transverse spin-current necessary to STT is surprisingly enhanced by the HH-to-LH mixing taking benefit of the lighter mass of the LH state.

Real-time observation of fast and highly reliable magnetization switching in Voltage-Control Spintronics Memory (VoCSM)

Magnetization switching was observed in real time in voltage-control spintronics memory (VoCSM), which utilizes a hybrid writing method that combines spin–orbit torque and voltage-controlled magnetic anisotropy (VCMA). VCMA lowers the switching energy barrier between parallel and antiparallel states, leading to a reduction in the write current and acceleration of magnetization switching. Real-time observation revealed that this acceleration of magnetization switching was accomplished by reducing the incubation and switching times. Using this writing method, high write durability was realized over 1013 write cycles using 2-ns write pulses. These results indicate that VoCSM realizes high speed and highly reliable switching with low power consumption and has the potential to shape next-generation computing systems.

Increasing the signal-to-noise ratio of magnetic tunnel junctions by cryogenic preamplification

Cryogenic preamplification using silicon–germanium heterojunction bipolar transistors has proven to be effective in increasing the signal-to-noise ratio of the tunnel magnetoresistance of high resistance magnetic tunnel junctions at 8 K. The magnetic tunnel junctions used have resistances greater than 1 MΩ, and the cryogenic measurement system still has sufficient bandwidth for the 1/f noise to roll off. A noise model for the system has been proposed and evaluated experimentally. The noise temperature and minimum noise temperature of the transistor used in the experiment are calculated and compared. The signal-to-noise ratio of the junction alone and the transistor-junction system is derived from the sample and circuit parameters and compared. Experimental data show a signal-to-noise ratio increase by a factor of 6.62 after adding in the cryogenic preamplifier. An increase in 1/f noise in the antiparallel state of the tunneling junction as opposed to the parallel state is also observed giving evidence of 1/f noise dependence on the magnetic state of the junction.

Large Tunneling Magnetoresistance in VSe2/MoS2 Magnetic Tunnel Junction.

Two-dimensional (2D) van der Waals (vdW) materials provide the possibility of realizing heterostructures with coveted properties. Here, we report a theoretical investigation of the vdW magnetic tunnel junction (MTJ) based on VSe2/MoS2 heterojunction, where the VSe2 monolayer acts as a ferromagnet with room-temperature ferromagnetism. We propose the concept of spin-orbit torque (SOT) vdW MTJ with reliable reading and efficient writing operations. The nonequilibrium study reveals a large tunneling magnetoresistance of 846% at 300 K, identifying significantly its parallel and antiparallel states. Thanks to the strong spin Hall conductivity of MoS2, SOT is promising for the magnetization switching of VSe2 free layer. Quantum-well states come into being and resonances appear in MTJ, suggesting that the voltage control can adjust transport properties effectively. The SOT vdW MTJ based on VSe2/MoS2 provides desirable performance and experimental feasibility, offering new opportunities for 2D spintronics.

Asymmetric Magnetization Switching in Perpendicular Magnetic Tunnel Junctions: Role of the Synthetic Antiferromagnet’s Fringe Field

The field-and current-induced magnetization reversal of a Co-Fe-B layer in a perpendicular magnetic tunnel junction (pMTJ) is studied at room temperature. The magnetization switching probability from the parallel (P) state to the antiparallel (AP) state and from AP to P is found to be asymmetric. We observe that this asymmetry depends on the magnetic configuration of the synthetic antiferromagnetic (SAF). The state diagram and the energy landscape are compared for two SAF configurations. We conclude that the asymmetry is due to the inhomogeneity of the fringe field generated by the SAF. Our study highlights the role of the reference-layer fringe field on the free layer's energy barrier height, which has to be carefully controlled for memory applications.

Assessment of conduction mechanisms through MgO ultrathin barriers in CoFeB/MgO/CoFeB perpendicular magnetic tunnel junctions

Perpendicular magnetic tunnel junctions (p-MTJs) have been explored for spin transfer torque magnetic random access memory devices (STT-MRAMs). The current-induced switching (CIS) of the p-MTJs requires a relatively high current density (J); thereby, very thin insulating barriers are required, consequently increasing the risk of non-tunneling conduction mechanisms through the MgO film. In this work, we fabricated CoFeB/MgO/CoFeB p-MTJs and studied the CIS characteristics, with the obtained switching current densities of about 2 × 1010 A/m2. The filament conduction through the MgO film was induced by applying a high set current (Iset) until a significant decrease in the resistance (R) is observed. A decrease in R with increasing current (I) for parallel (P) and antiparallel (AP) states was observed. In contrast, an increase in R with the increasing I value was observed for filament p-MTJs. We used a two-channel model to extract the filament resistance (Rf) and filament current (If). The Rf dependence on the electrical power (Pf) was linearly fitted, and a heating coefficient β of about 6%/mW was obtained, which was much higher than 0.15%/mW obtained from the bulk metallic multilayers of the top electrode. The CIS for filament p-MTJs was modeled by considering the bias dependence of the tunneling and the thermal dependence of Rf, showing a significant change in the CIS curves and switching currents. Our study addresses the effect of filament conduction on the tunneling current of CoFeB/MgO/CoFeB p-MTJs, critical for the design and control of the p-MTJ based devices, such as STT-MRAMs.

Spin valve effect induced by spin-orbit torque switching

The spin valve effect driven by the magnetic field in sandwich structures stands out as a seminal phenomenon in the emerging field of spintronics, but it has not been realized in low-power-dissipation devices operated via electrical means. Here, we investigate spin-orbit torque switching in a perpendicularly magnetized CoFeB/Mo/CoFeB sandwich, where the spin current generated in the Mo layer flows upward and downward to the ferromagnetically coupled CoFeB layers. When scanning the current, two CoFeB layers can be switched from one antiparallel state to a parallel state and then to the other antiparallel state, producing an unprecedentedly current-dependent spin valve effect. Such a spin valve effect is sensitive to the additional magnetic field direction due to the small spin torque efficiency and efficiency difference of the two CoFeB layers. The experimental observations are supported by the dynamics simulation based on the Landau-Lifshitz-Gilbert equation. Besides the fundamental interest, our finding would add a different dimension to energy efficient memory devices and sensors.

Resonant Spin-Transfer Torque Magnetoresistive Memory

Spin transfer torque magnetoresistive random access memory (STT-MRAM) has shown a great promise to be a main stream novel nonvolatile memory technology. Reduction of state switching current still remains to be a continued research focus for scaling down the size of the addressing transistors. One of the switching characteristics attributing to the relatively high power consumption is the asymmetry of switching current threshold: switching from parallel (P) to antiparallel (AP) state requires more than twice as much current as switching from antiparallel to parallel state. In this paper, we present a design that can significant reduce the switching current threshold for switching from P to AP state to substantially reduce or eliminate the switching asymmetry. By incorporating an antiferromagnetic/ferromagnetic stack to provide a rotating spin transfer torque that matches with the ferromagnetic resonance frequency of the free layer, switching threshold can be significantly reduced.

Low-Temperature Performance of Nanoscale Perpendicular Magnetic Tunnel Junctions With Double MgO-Interface Free Layer

The temperature dependence of magnetoresistance and switching characterization based on spin transfer torque (STT) effect of perpendicular magnetic tunnel junctions (p-MTJs) with MgO/CoFeB/W/CoFeB/MgO double-interface free layer was studied. The tunneling magnetoresistance (TMR) ratio increases from 95% to 176% for both 84 and 64 nm-diameter p-MTJs, upon decreasing the temperature from 400 to 20 K. This change of TMR is dominated by a steady increase in the resistance of antiparallel state while the parallel state conductance remains almost constant. Switching behavior at various temperatures is investigated for pulse voltage-dependent STT measurements; resistance versus voltage loops for both the switching directions become more symmetric with decreasing temperature. Furthermore, low-temperature measurements of magnetic properties suggest that the effect of stray field for STT becomes weaker as the temperature decreases, which suggests the p-MTJs design for cryogenic memories.

Quantum correlation swapping in parallel and antiparallel two-qubit mixed states

Quantum correlations (QCs) in some mixed states are thought as important resources in performing certain quantum communication tasks. Recently, it is found that (Azam et al. in Phys Rev A 91:012304, 2015) separable two-qubit states with maximally mixed marginals can be divided into two classes, i.e., parallel and antiparallel two-qubit mixed states. Moreover, a profound difference between their QCs quantified by more probing measures is revealed. In this paper, we study the swapping of QCs in these two classes of states. To be concrete, before the swapping, the two initial states are selected as two parallel two-qubit mixed states, or two antiparallel ones, or a parallel one and an antiparallel one. It is found that, according to two QC quantifiers, i.e., OQD (Ollivier and Zurek in Phys Rev Lett 88:017901, 2001) and LQU (Girolami et al. in Phys Rev Lett 110:240402, 2013), although QCs in the two parallel initial states are different from those in the two antiparallel initial states, the differences are eliminated after the QC swapping. However, if the initial states are selected as a parallel one and an antiparallel one, contrast to the case of two parallel ones as initial states, then after QC swapping, differences emerge.

Tailoring of magnetic properties of giant magnetoresistance spin valves via insertion of ultrathin non-magnetic spacers between pinned and pinning layers

The low-field sensitivity of a giant magnetoresistance (GMR) spin valve can be enhanced by tailoring the bias field of the free layer because this sensitivity and bias field are known to show a strong correlation. In this study, the free-layer bias field is reduced considerably to almost zero via the insertion of an ultrathin nonmagnetic spacer between the pinned layer and the pinning layer. The spacer promotes an increase in the density of Néel walls in the pinned layer. This increase, in turn, induces domain-wall-induced magnetostatic interactions of the free poles formed on the Néel walls inside the free and pinned layers. The magnetostatic interactions result in the formation of flux closures that act as pinning sites during the magnetization reversal process and stabilize the antiparallel magnetization state between the free layer and the pinned layer by suppressing the switching of the free layer from the antiparallel state to the parallel state. Furthermore, the spacer offers an additional advantage of increasing the GMR ratio by inducing a specular scattering effect at its top and bottom interfaces. A highly improved low-field sensitivity of 12.01 mV/mA·Oe is achieved in the sample with a Cu/Pt dual spacer.

The annealing effect on memory state stability and interlayer coupling in perpendicular magnetic tunnel junctions with ultrathin MgO barrier

Abstract Perpendicular magnetic tunnel junctions (p-MTJs) attract great interest because of their excellent performance in spin-transfer-torque magnetic random access memories (STT-MRAMs). The annealing process is critical to achieve the required structural and magnetic properties, therefore obtaining high perpendicular tunneling magnetoresistance (p-TMR) and lower switching current. In this work, the CoFeB/MgO/CoFeB-based p-TMR stack was prepared and a high p-TMR value of 155.9% was obtained from CIPT. The p-MTJs were patterned with pillar sizes ranging from 50 nm to 1000 nm and were annealed upon different annealing temperatures (TA) of 300 and 340 °C. The resistance at parallel (RP) and antiparallel (RAP) state in p-MTJs was obtained upon the application of a ±30 mT perpendicular magnetic field, where the angle of magnetization between storage and reference layer at memory state, θP and θAP, were derived from the P and AP resistance ratios with and without magnetic field. The condition θP = θAP of about 63.5° was found in as-deposited samples, indicating a ferromagnetic interlayer coupling between storage and reference layers, where θP = 0° and θAP = 180° at TA = 300 °C and 340 °C demonstrate a stable memory state. The stability of the resistance state was modeled by considering the evolution of effective perpendicular magnetic anisotropy (Keff) and interlayer coupling constant (J), showing a method to evaluate the ratio of Keff/J via the memory state resistance characterization, where the Keff/J of about −1.26 was obtained in our as-deposited samples. Our study assesses the effect of TA on the θP and θAP and supplies the method to enhance the stability through the optimization of perpendicular magnetic anisotropy and interlayer coupling, so to better control the performance of p-MTJ-based STT-MRAMs.

Anisotropic Magnetoresistance in NiFe-Based Polymer Spin Valves.

Precise evaluation of magnetoresistance (MR) with the identified transport mechanism is one of the key points in organic spin-valve (OSV) devices. To investigate the origin of the spin-valve-like signal in polymer spin valve with a vertical structure of NiFe/P3HT/AlO x/Co, the magnetic response measurements in the rotated magnetization direction were established in different well-designed device configurations. We identified the significant influence of anisotropic MR (AMR) and further induced tunneling AMR in NiFe electrodes contributing to the total MR signal. These findings suggest that the mechanisms responsible for the transport mode in polymer spin-valve devices also strongly depend on the interface between the ferromagnetic electrodes and organic semiconductor layer. Even for the spin-valve-resembled MR response with distinct parallel and antiparallel states, carefully controlled experiments such as temperature-dependent and angle-dependent measurements should always be performed to track down the injected spin-polarized carriers. Our thorough experiments and analyses may shed light on the effective MR signal evaluation in OSVs and spin-related parameters with transport mechanism identification.

Skyrmion-Induced Memristive Magnetic Tunnel Junction for Ternary Neural Network

Novel skyrmion-magnetic tunnel junction (SK-MTJ) devices were investigated for the first time to implement the ternary neural networks (TNN). In the SK-MTJ, an extra magnetoresistance state beyond binary parallel and anti-parallel MTJ states was achieved by forming a skyrmion vortex structure in the free layer. Based on the SK-MTJ, we propose a synaptic architecture with bit-cell design of +1, 0, and -1 to replace the full precision floating point arithmetic with equivalent bit-wise multiplication operation. To explore the feasibility of the SK-MTJ-based synaptic devices for TNN application, circuitlevel simulations for image recognition task were conducted. The recognition rate can reach up to 99% with 5% device variation and an average power consumption of 29.23 μW.

Electrical detection of multiple ferromagnetic resonance modes in interlayer exchange coupled Fe20Ni80/Ru/Fe20Ni80 multilayers

We report the electrical detection of multiple ferromagnetic (FM) resonance modes in multilayers of Pt/Fe20Ni80/Ru/Fe20Ni80 using microwave spin pumping and the inverse spin Hall effect (ISHE) as a function of Ru layer thickness tRu. The electrically detected voltage signals were analyzed for the anti-parallel magnetization state as well as the “spin-flop” magnetization state of the two Fe20Ni80 layers. When the two FM layers are antiferromagnetically (AFM) coupled (tRu = 1 nm), both the optic and acoustic modes were observed. The amplitude of the acoustic mode voltage signals obtained increases with frequency. However, when the two magnetic layers are ferromagnetically coupled (tRu = 1.4 nm), only acoustic mode was observed whose voltage signals decreases as frequency increases. We demonstrate the tunability of the sign of the ISHE voltage in the AFM coupled system by controlling the polarization of the spin currents.

Threshold Switch Augmented STT MRAM: Design Space Analysis and Device-Circuit Co-Design

We analyze the augmentation of spin-transfer torque (STT) MRAM with electrically driven selective phase transition of a threshold switch (TS) to enhance the read operation. This paper provides a comprehensive discussion on necessary design considerations for TS augmented (TSA) MRAMs. We deduce constraints for read and write biasing that yields improved read operation of TSA MRAMs. We explain the dependence of read/write performance metrics on read/write biases and the properties (resistance and critical currents for transitions) of the TS. With proper device-circuit optimization, TSA MRAM shows up to 70% larger sense margin, ~27% higher data stability with ~40% less power for read operation compared to STT MRAM (in nominal condition). We evaluate the impact of variation on TSA MRAM through Monte Carlo simulations. We report that even with variation induced spread in the distribution of bit-line voltages, TSA MRAM provides $\sim 1.7\times $ larger voltage differential between parallel and antiparallel states. For the write operation, the TSA MRAM consumes ~10% less average power and demands only ~5% more write time extension than the STT MRAM to achieve the same level of variation tolerance.

Singlet superconductivity in a single-crystal NiBi3 superconductor

Andreev reflection spectroscopy with unpolarized and highly spin-polarized currents has been utilized to study an intermetallic single-crystal superconductor NiBi3. Magnetoresistance at zero bias voltage of point contacts shows the occurrence and suppression of Andreev reflection by unpolarized and polarized current, respectively. The gap value, its symmetry and temperature dependence have been determined using an unpolarized current. The spin state in the NiBi3 sample is determined to be antiparallel using a highly spin-polarized current. The gap value 2Δ/kBT, gap symmetry and its temperature dependence, combined with the antiparallel spin state show that the bulk NiBi3 is a singlet s-wave superconductor.

Telegraphic switching signals by magnet tunnel junctions for neural spiking signals with high information capacity

Magnetic tunnel junctions (MTJs) operating in the superparamagnetic regime produce telegraphic signals that emulate neural spiking signals. Previous studies have characterized the random spiking signals produced by MTJs in terms of the percentage of time spent in the anti-parallel (AP) magnetization state (referred to as the “AP rate”) but ignore the switching rate of the MTJ. In this work, we demonstrate that with proper tuning of both an external bias field and a bias voltage, we can control the average dwell time in the AP-state and P-state pulses separately. Our data show that the AP rate can be tuned with bias voltages ranging from 310 mV to 460 mV and bias fields from −200 Oe to −230 Oe. The average dwell times in each state ranged from 225 ns to 285 μs and could be controlled separately. This suggests that neural spiking signals produced by MTJs can be decoded by both the spike rate and the spike count, which creates the possibility for increasing the information capacity in the rate coding scheme.