Synthetic Antiferromagnetic Free Layer Research Articles

Sub-terahertz excitations in a synthetic antiferromagnet with perpendicular anisotropy

In this paper, micromagnetic simulations are employed to investigate terahertz (THz) magnetic excitations in a spin torque nano-oscillator (STNO) with a perpendicularly magnetized synthetic antiferromagnetic (SAF) free layer. The magnetization precession of the free layer can be finely tuned into the sub-THz range without the necessity of external magnetic fields. The excited frequency exhibits two distinctive regions, namely region-I and region-II, depending on the applied current strength. In region-I, characterized by relatively small currents, the two ferromagnetic layers are stabilized at two separate precession orbits. The frequency in this region decreases with current strength, exhibiting similar features as the Néel vector change observed in antiferromagnets. In contrast, region-II is defined by currents where the two ferromagnetic layers synchronize into the same precession orbit. The frequency increases with current, correlating with the variation in the net magnetization of the SAF layer. An analytical model is developed through the canonical transformation of Lagrange’s equation, which can describe the frequency dependence on both the applied current and the antiferromagnetic interlayer coupling strengths. The simulations and the analytical model show good agreement, offering a more profound understanding of the magnetic excitation properties in STNOs with ultrathin SAF free layers. These insights are crucial for the design of advanced terahertz spintronic devices.

Binary frequency shift keying modulation in spin torque oscillators with synthetic antiferromagnetic layer

Antiferromagnets exhibit ultrafast magnetization precession, which has the potential to enable the development of terahertz spin torque nano-oscillators. By utilizing perpendicularly magnetized magnetic nanopillars with a synthetic antiferromagnetic (SAF) free layer, we have demonstrated through theoretical and numerical analysis that stable out-of-plane precession states can be achieved by applying current and an electric field. In the case of small current, the two magnetic layers of the SAF are in antiparallel alignment and rotate around the z-axis with the precession frequency decreasing as the current strength increases. When the current-induced spin torque is strong enough to fully overcome the antiferromagnetic coupling, the SAF free layer is driven into a scissor-like precession state around the z-axis with the frequency increasing with current. By selecting the appropriate combination of the current and electric field, the magnetization precession orbits and precession frequencies can be adjusted. These controllable procession orbits with tunable frequencies and fixed magnetization precession amplitude may be a promising candidate for implementing binary frequency shift keying modulation techniques.

External-Field-Robust Stochastic Magnetic Tunnel Junctions Using a Free Layer with Synthetic Antiferromagnetic Coupling

The stochastic magnetic tunnel junction (MTJ), the resistance of which fluctuates in time between low and high states under thermal fluctuation, is expected to serve as a key element in probabilistic computers. For reliable operation and versatile application, one needs to address the challenge that the relaxation time in each state should be virtually independent of external magnetic fields in the range of, for example, several mT. In this research, we fabricate in-plane easy-axis elliptical stochastic MTJs with a synthetic antiferromagnetic (SAF) free layer and examine the robustness of their properties against external fields while comparing them with stochastic MTJs with a conventional single ferromagnetic structure. We show that the MTJ with a SAF free layer shows more robust relaxation times against fields applied along the easy- and hard-axis directions. The results are reproduced with a macrospin simulation, from which design guidelines for robust stochastic MTJs targeting probabilistic computers are discussed.

Bipolar Electric-Field Switching of Perpendicular Magnetic Tunnel Junctions through Voltage-Controlled Exchange Coupling.

Perpendicular magnetic tunnel junctions (p-MTJs) switched utilizing bipolar electric fields have extensive applications in energy-efficient memory and logic devices. Voltage-controlled magnetic anisotropy linearly lowers the energy barrier of the ferromagnetic layer via the electric field effect and efficiently switches p-MTJs only with a unipolar behavior. Here, we demonstrate a bipolar electric field effect switching of 100 nm p-MTJs with a synthetic antiferromagnetic free layer through voltage-controlled exchange coupling (VCEC). The switching current density, ∼1.1 × 105 A/cm2, is 1 order of magnitude lower than that of the best-reported spin-transfer torque devices. Theoretical results suggest that the electric field induces a ferromagnetic-antiferromagnetic exchange coupling transition of the synthetic antiferromagnetic free layer and generates a fieldlike interlayer exchange coupling torque, which causes the bidirectional magnetization switching of p-MTJs. These results could eliminate the major obstacle in the development of spin memory devices beyond their embedded applications.

Ferromagnetic resonance and magnetization switching characteristics of perpendicular magnetic tunnel junctions with synthetic antiferromagnetic free layers

Perpendicular magnetic tunnel junctions (pMTJs) with synthetic antiferromagnetic (SAF) free layers have attracted much interest for applications on spintronic memory devices with ultrafast speed and ultralow energy. In this work, SAF free layer pMTJs (SAF-pMTJs) were designed and fabricated, in which a Ru/Ta bilayer spacer is used to form the SAF structure. We first characterized the magnetization dynamics of the SAF free layer by using ferromagnetic resonance and found that the Gilbert damping constant of the SAF free layer is around 0.019. Then, in device level studies that span from 900 nm down to 200 nm lateral size, we observed a transition of the SAF free layer from a preferred antiparallel to parallel magnetic configuration at small device sizes, which can be explained by the increased dipole interaction. The impact of the operating current was also investigated. We report an extraordinarily strong dependence, up to 144.1 kOe per A/μm2, of the offset field on the applied current, suggesting an electric-field modulation on the interlayer exchange coupling of the SAF free layer. These results will be instructive to improve the understanding of material properties and device performance of SAF-pMTJs for ultrafast, ultralow-power consumption spintronic devices.

Design parameters for field-free spin–orbit torque switching of perpendicular synthetic antiferromagnets

We report micromagnetic simulations of spin–orbit torque (SOT) induced magnetization switching of a ferromagnetic layer with perpendicular anisotropy in the absence of an external magnetic field. Field-free switching is achieved by antiferromagnetic interlayer exchange coupling (IEC) between two perpendicular ferromagnetic layers. At appropriate IEC values and an SOT current density exceeding the critical value (Jc), magnetization reversal can be achieved within sub-ns. The complete magnetization reversal of the synthetic antiferromagnetic free layer occurs upon removing the current pulse. Higher damping is preferred for the proposed switching scheme, as Jc decreases with the increase of damping. Remarkably, we also found that Jc has a parabolic dependence on the nanomagnet's diameter, with the vertex occurring at 90 nm.

Thermal stability of SOT-MTJ thin films tuning by multiple interlayer couplings

We investigated the thermal stability of spin-orbit torque magnetic tunnel junction (SOT-MTJ) thin films by tuning multiple interlayer couplings among synthetic anti-ferromagnetic layer (SAF), reference layer and free layer. We find that the bridge layer (here we use W) can influence the thermal stability of perpendicular magnetic anisotropy (PMA) in all the magnetic layers through the interlayer couplings. Increasing the thickness of bridge layer decouples the SAF and reference layer, resulting in higher PMA of SAF layer and lower PMA of reference layer. The reference layer further adjusts the PMA of free layer by orange-peel coupling via the tunneling barrier layer MgO such that we find another way to optimize the anisotropy without changing ferromagnetic layers. This work is helpful for engineering the interlayer coupling of SOT-MTJ thin films to achieve SOT devices with high thermal stability.

Highly-sensitive magnetic sensor for detecting magnetic nanoparticles based on magnetic tunnel junctions at a low static field

Magnetic sensors to detect magnetic nanoparticles (MNPs) towards biomedical applications require very high sensitivity at low magnetic fields. Here we report a magnetic sensor consisting of a magnetic tunnel junction (MTJ) with a synthetic antiferromagnetic free layer. This sensor exhibits a low magnetic anisotropy and sensitivities of over 18%/Oe at low fields in the range of 0 to 3 Oe. We employ superparamagnetic MNPs with a large diameter of 200 nm. The sensor’s transfer curves show the magnetoresistance (MR) variations as a function of MNP concentration. We demonstrate the detection capability of MNP amounts of below 500 ng and low MNP concentrations of 0.01, 0.1, and 1 mg/ml in solvents. This result suggests that the combination of high-sensitivity TMR sensors and large MNPs has a substantial potential for biomarker detection applications.

Magnetoresistive Random Access Memory: Present and Future

Magnetoresistive random access memory (MRAM) is regarded as a reliable persistent memory technology because of its long data retention and robust endurance. Initial MRAM products utilized toggle mode writing of a balanced synthetic antiferromagnet (SAF) free layer to overcome problems with half-selected bits that challenged traditional Stoner–Wohlfarth switching. With the development of spin transfer torque (STT) switching in perpendicular magnetic tunnel junctions, the capability for scaling MRAM products increased markedly, enabling a 1-Gb device in 2019. Ongoing research will allow scaling to even higher capacities. Compared to traditional memories, STT-MRAM can save power, improve performance, and enhance system data integrity, which supports the growing computing demands for everything from data centers to Internet of Things (IoT) devices. This article provides a review of the technology that enabled present toggle and STT-MRAM products, future STT-MRAM products, and new MRAM technologies beyond STT.

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.

A Novel STT-MRAM Design With Electric-Field-Assisted Synthetic Anti-Ferromagnetic Free Layer

Spin-transfer torque magnetic random-access memory (STT-MRAM) is one of the next-generation nonvolatile memories, which has the most promising industrial prospect and has generated lots of new ideas. One of the continuous challenges in STT-MRAM development is to reduce the critical write current. Recently, one published paper reported that under a small electric bias voltage, the synthetic anti-ferromagnetic (SAF) multilayer system could be switched between an anti-ferromagnetic coupling state and a ferromagnetic coupling state. Based on this phenomenon, we propose a new type of STT-MRAM of which the critical write current can be reduced by an assisting electric-field (E-field). Micromagnetic simulation has been performed to study the dynamic switching behavior of the magnetization of the SAF free layer with tunable Ruderman–Kittel–Kasuya–Yosida interaction under the impact of an E-field.

Micromagnetic analysis of Heusler alloy-based perpendicular double barrier synthetic antiferromagnetic free layer MTJs

We investigate spin transfer torque switching in a perpendicular double barrier synthetic antiferromagnetic free layer MTJ stack using micromagnetic simulations. For the material used in free layers, we use two different Cobalt-based Heusler alloys and compare their performance on the basis of switching speed, thermal stability and Tunnel magnetoresistance. We show that for Heusler alloys switching from one state to other is significantly faster but they suffer from the drawback of low thermal stability.

Micromagnetic analysis of a double-barrier synthetic antiferromagnetic MTJ stack

We investigate spin transfer torque switching in an in-plane double-barrier synthetic antiferromagnetic free layer MTJ stack using micromagnetic simulations. We consider a magnetic tunnel junction with two rigid fixed layers having antiparallel magnetization and a composite-free layer with two coupled ferromagnetic layers. We compare switching properties of the in-plane MTJ with perpendicular anisotropy MTJ. The dependence of switching time on the thicknesses of free layer and spacer and coupling strength is also explored.

Reduction of Offset Field in Top-Pinned MTJ With Synthetic Antiferromagnetic Free Layer

We found that the offset field (H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</sub> ) of a top-pinned magnetic tunnel junction (MTJ) with a CoPd/Ru/Ta/CoFeB-synthetic antiferromagnetic (SAF) free layer can be reduced by controlling the exchange coupling energy (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ex</sub> ) value. A micromagnetic simulation was used to derive the direct relation between the J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ex</sub> ) of a CoPd/Ru/Ta/CoFeB-SAF free layer and the H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</sub> ) of a top-pinned MTJ with the SAF free layer. In addition, we found that the J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ex</sub> ) value can be controlled by changing the thickness of the Ta spacer between the CoPd and CoFeB layers. As a result, we fabricated a top-pinned MTJ with a SAF free layer that eliminated the offset field and made it possible to improve the thermal stability.

Asymmetric Composite Free Layers With Compensated Magnetization for Ultrahigh Density Integration of STT-MRAM

This paper analyzes the magnetization switching behavior of an asymmetric synthetic antiferromagnetic (AF) free layer for spin-transfer torque random access memory and numerically demonstrates thermally assisted magnetization switching. Optimization of the free-layer thickness and the magnetic properties enables successful magnetization switching while retaining the AF structure during the switching process. The thermal stability was improved by increasing the lateral aspect ratio of the free layers while also maintaining writability with a reduced current density under the thermal assistance.

Enhanced Thermal Stability in Perpendicular Top-Pinned Magnetic Tunnel Junction With Synthetic Antiferromagnetic Free Layers

Synthetic antiferromagnetic (SAF) free layers consisting of [Co (0.3 nm)/Pd (0.7 nm)]n/Ru/Ta/CoFeB were investigated for use in the free layers of top-pinned magnetic tunnel junctions (MTJs). We found that interlayer exchange coupling properly provides thermal stability when the saturation magnetization of the CoPd layer is less than that of the CoFeB layer. Furthermore, we demonstrated that the thermal stability factor of the top-pinned MTJ with [Co (0.3 nm)/Pd (0.7 nm)]n=1/ Co (0.3 nm)/Ru/Ta/CoFeB- SAF free layers was improved without increasing the intrinsic switching current.

Spin-torque nano-oscillator based on a synthetic antiferromagnet free layer and perpendicular to plane polarizer

Coupled free layers systems used as active elements in spin-transfer nano-oscillators (STNOs) can have improved microwave performances in terms of linewidth or tunability. Here, we report a numerical study on the magnetization dynamics of a synthetic antiferromagnet (SAF) driven by a perpendicularly spin-polarized current and an in-plane applied magnetic field. Compared to the single free layer STNO, the current-field state diagrams, derived for strong and weak RKKY exchange coupling strength inside the SAF, show a more complex structure, with new static or dynamic states (chaotic dynamics), and also a larger zone of out-of-plane precession state (OPP) oscillations. The OPP frequency behaviour is generally similar to that of the single free layer STNO except for the zone near the chaotic dynamics and for fields larger than the spin-flop field of the SAF, where several frequency jumps have been observed.

Spin torque switching and scaling in synthetic antiferromagnet free layers with in-plane magnetization

We study the field-induced and the current-induced switching of synthetic antiferromagnet free layers with in-plane magnetization. In practical cases, the energy barrier relevant for the thermal stability depends mainly on the magneto-crystalline anisotropies of the layers. We derive the spin-flop and the direct overwrite fields and their dependence on the asymmetry of the thickness of the two layers of the SyF. This is used to explain why the SyF magnetizations are much more robust to external fields than the anisotropy would tell. We then calculate the spin-torque instability threshold current densities for the acoustical and the optical excitations of the SyF, taking into account that there are spin torques acting on the two layers of the SyF. The acoustical excitation has the lowest instability current. Based on these findings, we finally discuss the scaling of SyF free layers by combining thermal stability requirements and dielectric breakdown avoidance constraints. Scaling can be conducted with either the aim of minimizing the write current or with the aim of minimizing the free layer surface. An absolute minimum of write current can be reached for relatively large cells (166×166 nm2) provided a zero effective magnetization is induced by a proper choice of the free layer thicknesses. Depending on how close the technology can reliably approach to the thickness yielding the easy axis reorientation transition while keeping bulk damping, this current minimum could in principle be as low as 14 μA in the macrospin approximation. If in contrast one aims at small free layer surfaces, then the present CoFeB alloy-based technology cannot reach dimensions below 90×90 nm2.

Design Space Exploration of Typical STT MTJ Stacks in Memory Arrays in the Presence of Variability and Disturbances

High leakage power in sub-100-nm memory technology nodes drives the need for nonvolatile memory devices to reduce power consumption and enhance battery life. Spin-transfer torque magnetic tunneling junction (STT-MTJ) is a promising nonvolatile memory device with comparable read and write performances as SRAM and eDRAM with almost zero standby power. In this paper, we present a simulation framework that can solve transport [using nonequilibrium Green's function (NEGF) formalism] and magnet dynamics [using Landau-Lifshitz-Gilbert (LLG) equation] self-consistently to study the read and write performances of STT-MTJ. Due to process variations, thermal disturbances, and stray fields, the performance of STT-MTJ degrades and results in one transistor-one STT-MTJ (IT-ISTTMTJ) memory failures. A thorough memory design space investigation can help us to reduce such failures. Hence, we present a design space exploration framework for IT-ISTTMTJ memory, which consists of magnetic materials with different RAPA products, different genres of MTJ stacks, and a transistor. A comprehensive study based on critical memory performance metrics such as tunneling magnetoresistance, JC, and write cycle shows the relative merits and demerits of each MTJ stack for embedded memory applications. Finally, the benefits of synthetic antiferromagnet free layer in providing immunity against stray fields are shown illustrating the need for coupled free-layer stacks in scaled technology nodes.

Current-induced magnetization switching of synthetic antiferromagnetic free layer in magnetic tunnel junctions

Spin transport in a magnetic tunnel junction with a synthetic antiferromagnetic (SAF) free layer is investigated using the drift-diffusion model. Although the diffusive transport is inappropriate for the MgO tunnel barrier, the drift-diffusion model is found to capture the core features of in-plane spin-transfer torque (STT) through the tunnel barrier, and more importantly, it can describe non-negligible STT exerting on two ferromagnets in a SAF-free layer. STT in a SAF-free layer substantially changes the magnetization dynamics and induces a shift of the critical switching current. STT in a SAF-free layer suppresses current-induced parallel-to-antiparallel switching, whereas it encourages antiparallel-to-parallel switching.