Magnetic Tunnel Junction Stack Research Articles

Non‐Volatile Analog Control and Reconfiguration of a Vortex Nano‐Oscillator Frequency

AbstractMagnetic tunnel junctions are nanoscale devices that have recently attracted interest in the context of frequency multiplexed spintronic neural networks, due to their interesting dynamical properties, which are defined during the fabrication process, and depend on the material parameters and geometry. This work proposes an approach to extending the functionality of a standard magnetic tunnel junction (MTJ) by introducing an additional ferromagnet/antiferromagnet (FM/AFM) storage layer (SL) vertically integrated with the standard vortex MTJ stack into the nanopillar. The magnetostatic field created by this storage layer acts on the free layer and can be used to change its static and dynamic properties. To tune the magnitude and direction of this magnetostatic field, magnetic reconfiguration is carried out through a thermally assisted switching mechanism using a voltage pulse that heats the AFM layer in the SL above the Néel temperature in the presence of an external field. It is experimentally shown that using an MTJ based on a 600 nm diameter nanopillar with a vortex in the free layer, reconfiguration of the SL allows to continuously change the core precession frequency in the 15 MHz range, or ≈10% of the device frequency.

Towards fully electrically controlled domain-wall logic

Utilizing magnetic tunnel junctions (MTJs) for write/read and fast spin-orbit-torque (SOT)-driven domain-wall (DW) motion for propagation, enables non-volatile logic and majority operations, representing a breakthrough in the implementation of nanoscale DW logic devices. Recently, current-driven DW logic gates have been demonstrated via magnetic imaging, where the Dzyaloshinskii-Moriya interaction (DMI) induces chiral coupling between perpendicular magnetic anisotropy (PMA) regions via an in-plane (IP) oriented region. However, full electrical operation of nanoscale DW logic requires electrical write/read operations and a method to pattern PMA and IP regions compatible with the fabrication of PMA MTJs. Here, we study the use of a Hybrid Free Layer (HFL) concept to combine an MTJ stack with DW motion materials, and He+ ion irradiation to convert the stack from PMA to IP. First, we investigate the free layer thickness dependence of 100-nm diameter HFL-MTJ devices and find an optimal CoFeB thickness, from 7 to 10 Å, providing high tunneling magnetoresistance (TMR) readout and efficient spin-transfer torque (STT) writing. We then show that high DMI materials, like Pt/Co, can be integrated into an MTJ stack via interlayer exchange coupling with the CoFeB free layer. In this design, DMI values suitable for SOT-driven DW motion are measured by asymmetric bubble expansion. Finally, we demonstrate that He+ irradiation reliably converts the coupled free layers from PMA to IP. These findings offer a path toward the integration of fully electrically controlled DW logic circuits.

Influence of Free Layer Surface Roughness on Magnetic and Electrical Properties of 300 mm CMOS-Compatible MTJ Stacks

The magnetic tunnel junction (MTJ) is a highly versatile device widely used in today’s spintronic applications such as magnetoresistive random-access memory (MRAM), magnetic sensors and prospectively as a read device in racetrack memory. Tuning the perpendicular (p-)MTJ stack to match the desired properties, such as tunnel magnetoresistance (TMR), magnetic anisotropies or coercive field of the free layer, requires careful optimization of the deposition parameters as well as precise layer thickness control. Here, the deposition of individual layers in a wedged manner across 300 mm wafers is proposed to engineer the thicknesses within the stack more efficiently. Furthermore, this technique provides detailed insights into effects related to surface roughness, magnetic anisotropy and TMR.

Weighted spin torque nano-oscillator system for neuromorphic computing

Neuromorphic computing is a promising strategy to overcome fundamental limitations, such as enormous power consumption, by massive parallel data processing, similar to the brain. Here we demonstrate a proof-of-principle implementation of the weighted spin torque nano-oscillator (WSTNO) as a programmable building block for the next-generation neuromorphic computing systems (NCS). The WSTNO is a spintronic circuit composed of two spintronic devices made of magnetic tunnel junctions (MTJs): non-volatile magnetic memories acting as synapses and non-linear spin torque nano-oscillator (STNO) acting as a neuron. The non-linear output based on the weighted sum of the inputs is demonstrated using three MTJs. The STNO shows an output power above 3 µW and frequencies of 240 MHz. Both MTJ types are fabricated from a multifunctional MTJ stack in a single fabrication process, which reduces the footprint, is compatible with monolithic integration on top of CMOS technology and paves ways to fabricate more complex neuromorphic computing systems.

Engineering buffer layers to improve temperature resilience of magnetic tunnel junction sensors

Improving the thermal resilience of magnetic tunnel junctions (MTJs) broadens their applicability as sensing devices and is necessary to ensure their operation under harsh environments. In this work, we are address the impact of temperature on the degradation of the magnetic reference in field sensor stacks based on MgO-MTJs. Our study starts by simple MnIr/CoFe bilayers to gather enough insights into the role of critical morphological and magnetic parameters and their impact in the temperature dependent behavior. The exchange bias coupling field (H ex), coercive field (H c), and blocking temperature (T b) distribution are tuned, combining tailored growth conditions of the antiferromagnet and different buffer layer materials and stackings. This is achieved by a unique combination of ion beam deposition and magnetron sputtering, without vaccum break. Then, the work then extends beyond bilayers into more complex state-of-the-art MgO MTJ stacks as those employed in commercial sensing applications. We systematically address their characteristic fields, such as the width of the antiferromagnetic coupling plateau ΔH, and study their dependence on temperature. Although, [Ta/CuN] buffers showed higher key performance indications (e.g. H ex) at room temperature in both bilayers and MTJs, [Ta/Ru] buffers showed an overall wider ΔH up to 200 °C, more suitable to push high temperature operations. This result highlights the importance of properly design a suitable buffer layer system and addressing the complete MTJ behavior as function of temperature, to deliver the best stacking design with highest resilience to high temperature environments.

Modeling and Analysis of Magnetic Field-Induced Coupling on Hexagonal MTJ Array

The hexagonal array geometry offered considerable merit for storage density enhancement and magnetic tunnel junction (MTJ) etch convenience. However, with the increase in storage density, the magnetic coupling will become an important factor affecting the performance of devices. We modeled the magnetic coupling of the MTJ stack with square and hexagonal arrays, and calculated the intra-cell and inter-cell stray fields with different structural parameters such as the electrical Critical Diameter (eCD) of the MTJ stack and the spacing between MTJ stacks (pitch), and further investigated the inter-cell magnetic coupling coefficient ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Psi$ </tex-math></inline-formula> ) and critical switching current ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$I_{c}$ </tex-math></inline-formula> ) to characterize the performance of the device. Simulation results show that the intra-cell stray field increases with the decrease of eCD, while the inter-cell stray field decreases with the decrease of eCD and increases with the decrease of pitch. In order to reduce the influence of the inter-cell stray field, the pitch needs to be more than twice of eCD. Therefore, building a hexagonal MTJ storage array with a pitch greater than twice the eCD can maximize the storage density.

Design of VGSOT-MTJ-Based Logic Locking for High-Speed Digital Circuits

Emerging spintronics devices in recent research have received much interest in various fields. Their unique physical aspects are being explored to keep Moore’s law alive. Therefore, the hardware security aspects of system-on-a-chip (SoC) designs using spintronics devices becomes important. Magnetic tunnel junctions (MTJ) are a potential candidate in spintronics-based devices for beyond-CMOS applications. This work uses voltage-gated spin-orbit torque-assisted magnetic tunnel junction (VGSOT-MTJ) based on the Verilog-A behavioral model to design a possible logic-locking system for hardware security. Compared with the SOT MTJ, which uses a heavy metal strip below the MTJ stack, VGSOT-MTJ has an antiferromagnetic (AFM) strip that utilizes the voltage-controlled magnetic anisotropy (VCMA) effect to significantly reduce the JSOT,critical. To design the logic-locking block, we performed a Monte Carlo analysis to account for the effect of process variation (PV) on critical MTJ parameters. Eye diagram tests and mask designing were performed, which included the effect of thermal noise and PV for high-speed digital circuit operations. Finally, transient performance was analyzed to demonstrate the VGSOT-MTJ’s ability to design logic-locking blocks from the circuit operation perspective.

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.

Unconventional Seedless Multilayers with Large Perpendicular Anisotropy for Back-End-of-Line Compatible Spintronic Devices

An unconventional class of hard and out-of-plane magnetized seedless multilayers (SL-MLs) of the form [Co/insertion layer/Pt]n, grown as a top reference layer in magnetic tunnel junctions (MTJ), was investigated. Among different non-ferrous insertion layers (Ta, Cu, Mg, W, Ru, and Al), Ta produces the highest perpendicular magnetic anisotropy. A back-end-of-line (BEOL) compatible hard top reference layer using Ta-inserted SL-MLs was achieved, exhibiting more than twice effective perpendicular anisotropy compared to a conventional top reference layer. Using this top reference layer, we developed three types of spintronic memory stacks: (a) top-pinned MTJ stacks that can withstand annealing up to 425 °C, (b) BEOL compatible double-MTJ memory cells with read/write mode control layer enabling a faster readout and low-voltage writing, and (c) 2-bit MTJ stacks combining spin-transfer torque and spin-orbit torque writing. The magnetic properties of these three types of memory stacks are discussed.

Optimization of asymmetric reference structures through non-evenly layered synthetic antiferromagnet for full bridge magnetic sensors based on CoFeB/MgO/CoFeB

Industrial sensor applications rely on the implementation of full Wheatstone bridge architectures, demanding the development of low-cost and mass production methods of magnetic tunnel junctions (MTJ) based on CoFeB/MgO/CoFeB. In particular, monolithic bridge microfabrication has been demonstrated through the double deposition of MTJ stacks engineered by asymmetric reference layers with non-evenly layered synthetic antiferromagnet (SAF) structures. However, extending the standard double magnetic layered SAF into a triple magnetic multilayer system brings critical changes in the overall performance of the reference structure, which directly influences the magnetic stability of the device. Consequently, a theoretical model of a triple magnetic layered AF/SAF structure was developed to support the understanding of the magnetic response of the reference layers, aiming to improve the magnetic stability around zero field. A full MTJ Wheatstone bridge incorporating the optimized double and triple reference structures was microfabricated with a linear and hysteresis-free response. Furthermore, a high thermal endurance of both structures was verified through the measurement of the magnetotransport behavior of each type of MTJ structure within a reversible magnetic field range of ±2 kOe and a temperature sweep from room temperature up to 200 °C.

A novel BIST for monitoring aging/temperature by self-triggered scheme to improve the reliability of STT-MRAM

This paper proposes a novel methodology to design high reliable STT-MRAM, with self-activated built-in-self-test (BIST) against aging/temperature-induced degradation. During sensing operation, tunneling magnetoresistance (TMR) is monitored, and real-time BIST is activated prior to permanent damage in Magnetic tunnel junction (MTJ) stack. To evaluate the feasibility of the test scheme, the proposed technique was involved in MRAM array implementation using 28-nm CMOS and 40-nm MTJ. HSPICE MOS Reliability Analysis (MOSRA) is used to evaluate the amount of electrical stress to the actual device aging degradation. Compared with previous periodical BIST method, the proposed self-triggered BIST saves ~31.1% cumulative power consumption over 12 years. And the proposed technique can improve reliability in the wear-out failure period.

Asymmetric magnetization reversal of the Heusler alloy Co2FeSi as free layer in an CoFeB/MgO/Co2FeSi magnetic tunnel junction

We report the in-plane magnetization reversal behavior of the Co2FeSi Heusler alloy free layer in a bottom-pinned magnetic tunnel junction film with 149% TMR. Using magneto-optic indicator film imaging, we visualize the magnetic domain dynamics of this buried layer integrated within a full magnetic tunnel junction stack. The magnetic domain dynamics reveal anisotropic magnetization reversal within Co2FeSi under applied in-plane magnetic fields. While the reversal behavior under in-plane fields applied perpendicular to the in-plane exchange bias direction indicates smooth, coherent rotation away from the easy axis, asymmetric magnetic reversal behavior is observed along the easy axis. Reversed domains propagate from the interior of the film laterally outward toward the edges as the free layer magnetization switches into parallel alignment with the pinned synthetic antiferromagnetic reference layer (IrMn/CoFe/Ru/CoFeB). On the other hand, domains propagate from the film edges inward for the free layer transition into antiparallel alignment with the reference layer. These results have important implications for Heusler magnetic tunnel junction device performance.

Impact of Fe80B20 insertion on the properties of dual-MgO perpendicular magnetic tunnel junctions

We explore the impact of Fe80B20 inserted at both Co20Fe60B20/MgO interfaces of dual-MgO free layers (FLs) in bottom-pinned magnetic tunnel junctions (MTJs). MTJ stacks are annealed for 30 min at 350 °C and 400 °C in a vacuum after film deposition. Current-in-plane tunneling measurements are carried out to characterize magnetotransport properties of the MTJs. Conventional magnetometry measurements and ferromagnetic resonance (FMR) are conducted to estimate the saturation magnetization, the effective perpendicular anisotropy field and the Gilbert damping of dual-MgO FLs as a function of the Fe80B20 thickness and annealing temperatures. With ultrathin Fe80B20 (0.2–0.4 nm) inserted, perpendicular magnetic anisotropy (PMA) of FLs increases with similar tunnel magneto-resistance (TMR) and low damping values. As Fe80B20 layer thickness further increases (0.6–1.2 nm), both TMR and PMA degrade, and damping increases dramatically. This study demonstrates a novel approach to tune properties of MTJ stacks with dual-MgO FLs up to 400 °C annealing, which enables MTJ stacks for various applications.

Atomistic investigation of the temperature and size dependence of the energy barrier of CoFeB/MgO nanodots

The balance between low power consumption and high efficiency in memory devices is a major limiting factor in the development of new technologies. Magnetic random access memories (MRAMs) based on CoFeB/MgO magnetic tunnel junctions (MTJs) have been proposed as candidates to replace the current technology due to their non-volatility, high thermal stability, and efficient operational performance. Understanding the size and temperature dependence of the energy barrier and the nature of the transition mechanism across the barrier between stable configurations is a key issue in the development of MRAM. Here, we use an atomistic spin model to study the energy barrier to reversal in CoFeB/MgO nanodots to determine the effects of size, temperature, and external field. We find that for practical device sizes in the 10–50 nm range, the energy barrier has a complex behavior characteristic of a transition from a coherent to domain wall driven reversal process. Such a transition region is not accessible to simple analytical estimates of the energy barrier preventing a unique theoretical calculation of the thermal stability. The atomistic simulations of the energy barrier give good agreement with experimental measurements for similar systems, which are at the state of the art and can provide guidance to experiments identifying suitable materials and MTJ stacks with the desired thermal stability.

Ar+ ion irradiation of magnetic tunnel junction multilayers: impact on the magnetic and electrical properties

The impact of 400 keV Ar+ irradiation on the magnetic and electrical properties of in-plane magnetized magnetic tunnel junction (MTJ) stacks was investigated by ferromagnetic resonance, vibrating sample magnetometry and current-in-plane tunneling techniques. The ion fluences ranged from to . Below , the anisotropy of the Ta-capped FeCoB free layer was weakly modulated, following a decrease in the saturation magnetization. The tunnel magnetoresistance (TMR), along with the exchange-bias and the interlayer exchange coupling providing a stable magnetic configuration to the reference layer, decreased continuously. Above , a strong decrease in the saturation magnetization was accompanied by a loss of the magnetic coupling and of the TMR. We show there is an ion-fluence window where the modulation of magnetic anisotropy can occur while preserving a large TMR and stable magnetic configuration of the MTJ, and demonstrate that the layers surrounding the free layer play a decisive role in determining the trend of the magnetic anisotropy modulation resulting from the irradiation. Our results provide guidance for the tailoring of MTJ parameters via ion irradiation, which we propose as a potentially suitable technique for setting the magnetic easy-cone state in MTJ for attaining field-free, fast, and non-stochastic magnetization switching.

Etch Characteristics of Nanoscale Patterned Magnetic Tunnel Junction Stacks Using Pulse-Modulated Radio Frequency Source Plasma.

Magnetic tunnel junctions (MTJs) patterned with 70 × 70 nm² square arrays were etched in a CH₄/O₂/Ar gas mixture by pulse-modulated inductively coupled plasma reactive ion etching (ICPRIE). A good etch profile of MTJs with etch slope of approximately 82° was achieved by adjusting the on-off duty ratio of the plasma and pulse frequency. Langmuir probe analysis and optical emission spectroscopy confirmed that the balance between the formation of the passivation layer as an etch byproduct and sputtering effect is responsible for the etch selectivity and etch profile with a high degree of anisotropy. It is concluded that the application of pulse-modulated plasma on ICPRIE can be an effective method to obtain the anisotropic etch profile of nanometer-scale MTJs.

High-Temperature Magnetic Tunnel Junction Magnetometers Based on L1$_0$-PtMn Pinned Layer

Magnetometer sensors with high sensitivity based on magnetic tunnel junctions (MTJs) were developed to operate at high temperatures of more than 250 °C. We replaced the commonly used IrMn antiferromagnetic pinning layer with an optimized PtMn antiferromagnet, which has a high magnetic thermal stability due to its L1 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> crystal structure. We investigate the effect of measurement temperature on magnetoresistance (MR), exchange bias, and sensitivity in sensor MTJ stacks using either IrMn or PtMn pinning layers. Upon increasing the temperature up to 250 °C, the MR ratio decreases from 146% to 50% for IrMn, and 133% to 80% for PtMn pinning layers. The PtMn-based MTJ sensors sustain a constant sensitivity (4-5 %/Oe) up to 250 °C, compared with IrMn-based sensors, which decrease drastically. This high-temperature performance links to the thermal stability of exchange bias in the pinned layer by using PtMn.

Short- and long-term memory functions are mimicked through artificial synapse

Researchers find biological short- and long-term memory mechanisms can be imitated in neural networks using precise engineering of magnetic tunnel junction stacks.

Investigation on the structural property of the sputtered hcp-phase boron nitride tunnel barrier for spintronic applications

Recently, two-dimensional (2D) materials have attracted considerable interest for use in spintronic applications, especially hexagonal close-packed (hcp)-phase boron nitride (BN) as a tunnel barrier. In this paper, we experimentally investigated the structural properties of a sputtered hcp-BN thin film. By optimizing the experimental conditions, we obtained the stoichiometric BN thin film with a ratio of 1:1 of the Ar/N2 sputtering gas. Then the Co/BN/Co magnetic tunnel junction (MTJ) stacks were prepared to study the crystalline structure of the BN tunnel barrier and their epitaxial relationship. We found that the as-deposited BN tunnel barrier layer follows the texture of the bottom Co layer and forms a polycrystalline structure. After the high-temperature treatment of the MTJ stack, texturing of the BN tunnel barrier layer is observed, however, this annealing process makes the BN tunnel barrier noncontinuous and induces serious interdiffusion between layers. These results will open the door for development of spintronic devices based on MTJs with hcp-phase BN tunnel barrier and hcp-phase perpendicular magnetic anisotropy ferromagnetic layer.

A multifunctional standardized magnetic tunnel junction stack embedding sensor, memory and oscillator functionality

The objective of this study is to co-integrate multiple digital and analog functions together within CMOS by developing a universal magnetic tunneling junction stack (MTJ) capable of realizing logic, memory, and analog functions, within a single baseline technology. This will allow monolithic heterogeneous integration, fast and low-power processing, and high integration density, particularly useful for Internet of Things (IoT) platforms. This unique spintransfer-torque (STT) MTJ is called Multifunctional Standardized Stack (MSS). This paper presents the progress regarding memory, oscillator and sensor functionalities targeted for the technology. We show that a single magnetic stack deposition can be used to obtain these three functionalities on the same wafer.