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Modern computing schemes require large circuit areas and large energy consumption for neuromorphic computing applications, such as recognition, classification, and prediction. This is because these tasks require parallel processing on large datasets. Stochastic computing (SC) is a promising alternative to conventional binary computing schemes due to its low area cost, low processing power, and robustness to noise. However, the large area and energy costs for random number generation with CMOS-based circuits make SC impractical for most hardware implementations. For this reason, beyond-CMOS approaches to random number generation have been investigated in recent years. Spintronics is one of the most promising approaches due to the intrinsic stochasticity of the magnetic tunnel junction (MTJ). In this review article, we provide an overview of the literature published in recent years investigating the tunable, intrinsic stochasticity of MTJs and proposing practical methods for random number generation using spintronic hardware.

Perpendicular magnetic tunnel junctions with double-interface free layer (p-DMTJs), which exhibit enhanced tunnel magnetoresistance (TMR) and thermal stability (Δ) at the nanoscale, have received considerable interest as building blocks for spintronic data storage devices. Heat-assisted magnetic recording (HAMR) techniques have been widely employed in mainstream magnetic storage to enable ultrahigh storage density. However, the data access is achieved by sensing the stray field of the selected magnetic element using a mechanical “read head”, resulting in an unfavorable speed limitation and design complexity. To address this issue, integrating laser-assisted switching with a high-performance magnetic tunnel junction has received interest in spintronic R&D; however, it has not yet been achieved. In this study, we experimentally explored femtosecond (fs) laser-assisted switching in a p-DMTJ device using a direct electrical TMR readout. We demonstrate two reconfigurable switching operations, i.e., binary “write” and unidirectional “reset”, by the interplay of the fs laser and synchronized magnetic field sequence. We further explored the joint effect, and a switching phase diagram was obtained. The effect of the stray field of p-DMTJ, as well as laser helicity, on switching is also discussed. Results show the feasibility of fs laser-assisted writing p-DMTJs, which can pave the way in high-density optospintronic storage applications. [ABSTRACT FROM AUTHOR]

• Manuscript include advancement in two and three terminal spintronic devices. • Review included artificial neurons based spintronic devices for artificial intelligence. • Yearly development in spintronic technology has been illustrate by flow chart. • The parametric comparisons of MRAM and logic circuit has been included. • Include features of cutting-edge quantum & magnetic tunnel junction. The scaling of metal oxide semiconductor field effect transistors (MOSFETs) for data storing and logic circuit operation has reached a critical point, beyond which further scaling poses various secondary issues. These problems include short channel effects, hot carrier effects (HCEs), and reliability concerns. However, a promising alternative called spintronics has recently emerged as a highly exciting technology. Spintronics considers both the charge and spin of electrons in device operations and offers superior properties compared to MOSFETs. Researchers have reported numerous spintronics devices that exhibit significant potential in memory and logic circuits when integrated with complementary metal oxide semiconductor (CMOS) technology. These devices not only possess excellent scalability but also consume less power than MOSFETs at the nano-scale level. This article aims to provide a comprehensive review of the current state, future prospects, and challenges associated with spintronics devices. [ABSTRACT FROM AUTHOR]

Spin-transfer torque magnetic random access memory (STT-MRAM) is a promising solution for onboard, radiation-tolerant memory for space environments. This is because magnetic tunnel junctions (MTJs) have demonstrated outstanding resilience to ionizing radiation. In this study, we examine the effect of total ionizing dose (TID) on the MTJ’s thermal stability factor, intrinsic critical switching voltage, and write energy. These parameters were calculated from switching probability distribution curves taken at three different pulsewidths. Additionally, dc current hysteresis plots were obtained to measure the MTJ’s tunnel magnetoresistance ratio, its dependence on voltage, and the dc current required to switch states. Tests were carried out at two TID exposure levels up to 1 Mrad(Si) on a total of 24 MTJs of varying cross section dimensions in order to identify the influence of scaling as well as to obtain statistical variation for any changes observed. Results indicate that TID exposure had small effects on the MTJ’s thermal stability and critical switching voltage, which affected the write energy, especially for switching from the antiparallel (AP) to parallel (P) direction. For switching from P to AP, these changes fluctuated, suggesting that TID has multiple effects, which have competing influences on the MTJ’s switching properties. The work presented here shows that, while MTJs are highly resistant to ionizing radiation, a subtle influence on some properties for the STT-MRAM write operation should be considered in radiation environment assessments for both terrestrial and space applications. [ABSTRACT FROM AUTHOR]

Flip-flops (FFs) account for a very significant portion of the entire power consumption of VLSI circuits. Magnetic tunnel junctions (MTJs) can be combined with conventional FFs to achieve nonvolatility, almost unlimited write times, and higher energy efficiency. The mainstream MTJ switching mechanisms include spin-transfer torque (STT), spin-orbit torque (SOT), and voltage-controlled magnetic anisotropy (VCMA) effect, among them the first two mechanisms all facing the disadvantages of both large writing energy and writing latency. Instead, utilizing the VCMA effect to switch MTJ only needs a sub-nanosecond and has low energy consumption, which is more energy-efficient. In this article, first, a double-stage pre-charge sense amplifier (DPCSA) is proposed, reducing 53.9% of sensing delay compared with the conventional one. Second, based on DPCSA, a VCMA-MTJ-based multi-bit shared magnetic nonvolatile FF (MNV-FF) with a self-adaptive writing circuit is designed, achieving high speed and energy efficiency for backup and restore operations. Finally, by extending the MNV-FF, a magnetic nonvolatile register group (MNV-RG) with high area efficiency can be obtained. An SMIC 55 nm CMOS design kit and a VCMA-MTJ compact model described by Verilog-A language are used for hybrid simulation on the Cadence/Specter Platform. Simulation results in 0.7 × and 1.3 × of the improvements in data backup time and backup energy, respectively, compared with the efficient VCMA-MTJ-based MNV-FF. Two more improvements of 158.7 × and 19.6 × in data restore time and restore energy are also achieved in the same comparison. [ABSTRACT FROM AUTHOR]

Spin-transfer torque magnetic random-access memory (STT-MRAM) based on a single-barrier magnetic tunnel junction (SBMTJ) and a double-barrier magnetic tunnel junction (DBMTJ) has evolved along with a low switching current and low energy consumption to obtain a high areal density and a fast switching speed. The increment of initial temperature, ${T}_{{\text {in}}}$ , in STT-MRAM can achieve low switching energy, ${E}_{{\text {SW}}}$. However, this leads to an unavoidable decrease in $\Delta $ and switching efficiency. In this paper, SBMTJ and DBMTJ were analyzed in terms of the switching efficiency factor with ${E}_{{\text {SW}}}$ reduction by increasing the ${T}_{{\text {in}}}$. The switching temperature, ${T}_{{\text {SW}}}$ , was investigated using a finite-element method simulation. The results show that the DBMTJ(A) and SBMTJ with the same MgO layer thickness of 0.9 nm provide a higher switching current, ${I}_{{\text {C}}}$ , than the DBMTJ(B) with a MgO layer thickness of 1.3 nm. The ${T}_{{\text {SW}}}$ in a DBMTJ(A) is higher than that in an SBMTJ, while ${T}_{{\text {SW}}}$ for a DBMTJ(B) is the smallest because of its low ${I}_{{\text {C}}}$. The DBMTJ(A) and DBMTJ(B) can be applied in a higher temperature range than the SBMTJ at a $\Delta $ of 40. In addition, the STT efficiency factor, $\Delta /{I}_{{\text {C0}}}$ , for a DBMTJ is better than the factor for an SBMTJ. Although the temperature increment would cause an undesirable reduction in $\Delta $ , it can reach a low ${E}_{{\text {SW}}}$ for the requirement of a fast write access time. Therefore, the control device for increasing the ${T}_{{\text {in}}}$ in the MTJs is attractive and should be promoted in the advancement of memory technology. [ABSTRACT FROM AUTHOR]

We investigated the effect that Ta spacer had on the interfacial microstructures and magnetic properties of MgO/CoFeB/Ta/CoFeB/MgO films. We found that when the Ta spacer thickness was optimized, the double MgO interface led to an enhancement in the perpendicular magnetic anisotropy (PMA) and thermostability. By systematically studying the changes for different structures in the chemical state, we interpreted how elements diffuse and alter the chemical states and properties of the magnetic tunnel junctions (MTJs). We found that increasing the Fe oxide is crucial to the PMA in the films and we attribute the increase in the of Fe oxide content to the oxygen liberated by the reduction of boron oxide. • The magnetic properties and thermal stability of MgO/CoFeB/Ta/CoFeB/MgO films were studied. • Fe oxide was increased through the oxygen liberated by the reduction of boron oxide. • The formation of minor Fe(Co)oxide at the CoFeB/MgO interface enhanced the PMA. [ABSTRACT FROM AUTHOR]

Magnetic tunnel junction nanopillar devices were patterned into three-terminal spin Hall nano-oscillators starting from a 200 mm wafer with an MgO barrier wedge with a nominal thickness variation between 0.72 and 1.36 nm. The resulting devices, with $R \times A$ values in the range between 1 and $110~\Omega \cdot \mu \text {m}^{2}$ , were characterized in the frequency domain in an attempt to optimize the MgO barrier thickness with respect to the nano-oscillators output power. While all the devices exhibit oscillations, the integrated matched output power reaches a maximum of 12.5 nW for an $R \times A$ of $34~\Omega \cdot \mu \text{m}^{2}$ with a nominal MgO thickness of 1.2 nm. [ABSTRACT FROM AUTHOR]

In the Internet of Things era, security concerns may require a cryptography system in every connected device. True random number generators (TRNGs) are preferred instead of pseudorandom number generators in the cryptography systems to achieve a higher level of security. For on-chip applications, we seek scalable and CMOS-compatible devices and designs for TRNGs. In this paper, the stochastic behavior of the spin transfer torque magnetic tunnel junction (STT-MTJ) is utilized for the source of randomness. However, variations and correlations exist in MTJs due to fabrication limitations, so TRNG designs based on a single MTJ have to be postprocessed or tracked in real time to ensure an acceptable level of randomness. Two novel designs are proposed in this paper, which can produce random sequences with high variation resilience. The first design uses a parallel structure to minimize variation effects, and the second design leverages the symmetry of an MTJ pair to take advantage of any correlations. Moreover, a universal circuit for quality improvement is proposed and it can be used with any random number generator. All of the designs are validated in a 28-nm CMOS process by Monte Carlo simulation with a compact model of the MTJ. The National Institute of Standards and Technology (NIST) statistical test suite is used to test the randomness quality of the generated sequences under the scenario of encryption keys in the transport layer security or secure sockets layer (TLS/SSL) cryptographic protocol. [ABSTRACT FROM AUTHOR]

This paper describes a robust, modular, and physics-based circuit framework to model the conventional and emerging magnetic tunnel junction (MTJ) devices. Magnetization dynamics are described by the stochastic Landau–Lifshitz–Gilbert (sLLG) equation whose results are rigorously benchmarked with a Fokker–Planck equation description of the magnet dynamics. We then show how sLLG is coupled to the transport equations of MTJ-based devices in a unified circuit platform. Step by step, we illustrate how the physics-based MTJ model can be extended to include different spintronics phenomena, including spin-transfer torque, voltage-controlled magnetic anisotropy (VCMA), and spin–orbit torque phenomena by the experimentally benchmarked examples. To demonstrate how our approach can be used in the exploration of novel MTJ-based devices, we present a recently proposed MEMS resonator-driven spin-torque nano-oscillator (STNO) that can reduce the phase noise of STNOs. We briefly elaborate on the use of our framework beyond conventional devices. [ABSTRACT FROM AUTHOR]

In this investigation, the low-frequency alternate-current (AC) magnetic susceptibility (χac) and hysteresis loop of various MgO thickness in CoFeB/MgO/CoFeB magnetic tunneling junction (MTJ) determined coercivity (Hc) and magnetization (Ms) and correlated that with χac maxima. The multilayer films were sputtered onto glass substrates and the thickness of intermediate barrier MgO layer was varied from 6 to 15 Å. An experiment was also performed to examine the variation of the highest χac and maximum phase angle (θmax) at the optimal resonance frequency (fres), at which the spin sensitivity is maximal. The results reveal that χac falls as the frequency increases due to the relationship between magnetization and thickness of the barrier layer. The maximum χac is at 10 Hz that is related to the maximal spin sensitivity and that this corresponds to a MgO layer of 11 Å. This result also suggests that the spin sensitivity is related to both highest χac and maximum phase angle. The corresponding maximum of χac is related to high exchange coupling. High coercivity and saturation magnetization contribute to high exchange-coupling χac strength.

This paper reports on the integration of magnetic tunnel junction (MTJ) sensing devices with magnetoresistance responses above 150% on flexible substrates. The sensors are integrated in a fabrication process based on polyimide materials, and demonstrate sensitivities up to 250~\mu \textV /Oe for bias currents in the 100- \mu \textA range. Assessment of the sensors performance is done under controlled mechanical load conditions, and the magnetoresistance is only slightly affected (1% variation) while sensitivity changes by 7.5% when the bending radii reduce down to 5 mm. The results demonstrate the high potential of the MTJ sensors to be used in applications requiring bending and conforming to non-planar geometries, bringing magnetoelectronic technologies to hard-to-reach regions of space. [ABSTRACT FROM PUBLISHER]

This investigation studies CoFeB/AlOx/Co magnetic tunneling junction (MTJ) in the magnetic field of a low-frequency alternating current, for various thicknesses of the barrier layer AlOx. The low-frequency alternate-current magnetic susceptibility (χac) and phase angle (θ) of the CoFeB/AlOx/Co MTJ are determined using an cac analyzer. The driving frequency ranges from 10 to 25,000 Hz. These multilayered MTJs are deposited on a silicon substrate using a DC and RF magnetron sputtering system. Barrier layer thicknesses are 22, 26, and 30 Å. The X-ray diffraction patterns (XRD) include a main peak at 2θ = 44.7° from hexagonal close-packed (HCP) Co with a highly (0002) textured structure, with AlOx and CoFeB as amorphous phases. The full width at half maximum (FWHM) of the Co(0002) peak, decreases as the AlOx thickness increases; revealing that the Co layer becomes more crystalline with increasing thickness. χac result demonstrates that the optimal resonance frequency (fres) that maximizes the χac value is 500 Hz. As the frequency increases to 1000 Hz, the susceptibility decreases rapidly. However, when the frequency increases over 1000 Hz, the susceptibility sharply declines, and almost closes to zero. The experimental results reveal that the mean optimal susceptibility is 1.87 at an AlOx barrier layer thickness of 30 Å because the Co(0002) texture induces magneto-anisotropy, which improves the indirect CoFeB and Co spin exchange-coupling strength and the χac value. The results concerning magnetism indicate that the magnetic characteristics are related to the crystallinity of Co.

Bioinspired hardware holds the promise of low-energy, intelligent, and highly adaptable computing systems. Applications span from automatic classification for big data management, through unmanned vehicle control, to control for biomedical prosthesis. However, one of the major challenges of fabricating bioinspired hardware is building ultrahigh-density networks out of complex processing units interlinked by tunable connections. Nanometer-scale devices exploiting spin electronics (or spintronics) can be a key technology in this context. In particular, magnetic tunnel junctions (MTJs) are well suited for this purpose because of their multiple tunable functionalities. One such functionality, nonvolatile memory, can provide massive embedded memory in unconventional circuits, thus escaping the von-Neumann bottleneck arising when memory and processors are located separately. Other features of spintronic devices that could be beneficial for bioinspired computing include tunable fast nonlinear dynamics, controlled stochasticity, and the ability of single devices to change functions in different operating conditions. Large networks of interacting spintronic nanodevices can have their interactions tuned to induce complex dynamics such as synchronization, chaos, soliton diffusion, phase transitions, criticality, and convergence to multiple metastable states. A number of groups have recently proposed bioinspired architectures that include one or several types of spintronic nanodevices. In this paper, we show how spintronics can be used for bioinspired computing. We review the different approaches that have been proposed, the recent advances in this direction, and the challenges toward fully integrated spintronics complementary metal–oxide–semiconductor (CMOS) bioinspired hardware. [ABSTRACT FROM PUBLISHER]

In this paper, a review of the developments in MRAM technology over the past 20 years is presented. The various MRAM generations are described with a particular focus on spin-transfer torque MRAM (STT-MRAM) which is currently receiving the greatest attention. The working principles of these various MRAM generations, the status of their developments, and demonstrations of working circuits, including already commercialized MRAM products, are discussed. [ABSTRACT FROM PUBLISHER]

We investigated the tunneling magnetoresistance (TMR) properties for the magnetic sensor application in MgO-based magnetic tunnel junctions (MTJs) using a perpendicularly magnetized [Co/Pd]-based reference layer and an in-plane magnetized CoFeB sensing layer with various thicknesses ( $t_{{\mathrm{CoFeB}}}$ ). Linear TMR curves to an out-of-plane magnetic field were successfully obtained with a dynamic range of more than 600 Oe, corresponding to the coercivity of the [Co/Pd]-based reference layer. The MTJs showed the highest sensitivity of 0.026%/Oe for $t_{{\mathrm{CoFeB}}} = 1.8 nm and the smallest nonlinearity of 0.11% full scale for t\mathrm{CoFeB} = 3 nm. We clarified that the sensitivity and the nonlinearity in the MTJs are significantly associated with t{\mathrm{CoFeB}} , which is attributed to the change in the anisotropy field of the CoFeB sensing layer. [ABSTRACT FROM PUBLISHER]

We report the electronic transport properties of the MnAs(001)/InP(001) magnetic tunnel junctions by first principles calculations. The half-metallic MnAs was predicted as a result of 100% spin polarization and then minority spin current is absolutely inhibited when the bias voltage is applied to terminals of both MnAs and InP. Majority spin current is inhibited when the bias voltage is less than 0.8 V and “relaxed” when the bias voltage increases. Spin-up and spin-down currents flowing in opposite directions are proved to be induced by temperature difference between left and right electrodes called Seebeck Effect rather than external electrical bias. Thermal current was demonstrated perfect spin filtering effects to indicating possible use for fabrication of highly-efficient spin filter devices in this study. [ABSTRACT FROM AUTHOR]

This investigation studies CoFeB/AlOx/Co magnetic tunneling junction (MTJ) in the magnetic field of a low-frequency alternating current, for various thicknesses of the barrier layer AlOx. The low-frequency alternate-current magnetic susceptibility (Xac) and phase angle (ƒÆ) of the CoFeB/AlOx/Co MTJ are determined using an Xac analyzer. The driving frequency ranges from 10 to 25,000 Hz. These multilayered MTJs are deposited on a silicon substrate using a DC and RF magnetron sputtering system. Barrier layer thicknesses are 22, 26, and 30 A. The X-ray diffraction patterns (XRD) include a main peak at 2ƒÆ = 44.7° from hexagonal close-packed (HCP) Co with a highly (0002) textured structure, with AlOx and CoFeB as amorphous phases. The full width at half maximum (FWHM) of the Co(0002) peak, decreases as the AlOx thickness increases; revealing that the Co layer becomes more crystalline with increasing thickness. Xac result demonstrates that the optimal resonance frequency (fres) that maximizes the Xac value is 500 Hz. As the frequency increases to 1000 Hz, the susceptibility decreases rapidly. However, when the frequency increases over 1000 Hz, the susceptibility sharply declines, and almost closes to zero. The experimental results reveal that the mean optimal susceptibility is 1.87 at an AlOx barrier layer thickness of 30 A because the Co(0002) texture induces magneto-anisotropy, which improves the indirect CoFeB and Co spin exchange-coupling strength and the Xac value. The results concerning magnetism indicate that the magnetic characteristics are related to the crystallinity of Co. [ABSTRACT FROM AUTHOR]

In this study, we report the effects of deposition rate and composition of CoFeB on the magnetic properties of CoFeB/MgO/CoFeB structures. We found that the asymmetry exists not only in different deposition stack sequences (bottom structure Ta/CoFeB/MgO and top structure MgO/CoFeB/Ta), but also in different compositions of CoFeB. Co{40}Fe{40}B{20} is more suitable for the bottom structure, while Co{20}Fe{60}B{20} has better perpendicular magnetic anisotropy for the top structure. Based on the results The full pMTJ stacks of structure Ta 5/Co{40}Fe{40}B{20} 1.3/MgO 1/Co{20}Fe{60}B{20}2.2/Top layer (nm unit) were fabricated with different deposition rates and annealing temperatures. We found that low deposition rate (0.03 nm/s) can prevent overlapping magnetization reversal of both free and fixed layer. The antiferromagnetic coupling between the free and fixed layer was found to increase with annealing temperature which can be identified as the consequence of orange-peel coupling. [ABSTRACT FROM AUTHOR]

An investigation of barrier breakdown in MgO-based magnetic tunnel junctions (MTJs) submitted to pulsed electrical stress is presented. By studying the effect of delay between successive pulses, we observed that a very pronounced optimum in endurance of MTJs is obtained for an intermediate value of the delay between pulses corresponding to the characteristic time for a trapped electron in the barrier to escape from its trap. A charge trapping-detrapping model was proposed which consistently explains our experimental data. The delay between successive pulses affects the density of electrons trapped in the barrier. The average value in time and the time-modulation of the density of trapped charge give rise to distinct breakdown mechanisms. Our model allows evaluating the MTJ probability of breakdown for different applied pulse conditions. An expected endurance of the MTJs is then derived depending on the characteristics of the electrical stress in terms of delay, amplitude, unipolarity versus bipolarity. [ABSTRACT FROM PUBLISHER]

We investigated gain and fan-out in a current-field driven spin transistor that is driven by a current-induced magnetic field. The basic structure of the transistor consisted of a coplanar waveguide for magnetic field application and an MgO-based magnetic tunnel junction with high magnetoresistance and low resistance area product. Under an assisting ac magnetic field and a dc bias voltage of 0.4 V, we demonstrated substantial power gain of 130, current gain of 4.9, and a fan-out value of 5.7. The scaling of the fan-out value in this device is discussed in detail. The current-field driven spin transistor is proven to be a promising candidate as a basic component of a nonvolatile logic device. [ABSTRACT FROM PUBLISHER]

We have fabricated perpendicular magnetic tunnel junctions consisting of hcp Ru underlayer/hcp CoPt alloy/CoFeB/MgO/CoFeB/CoPt alloy (or Co)/Pt capping layer. By inserting the CoPt alloy (or Co) between the CoFeB and Pt capping layers, it is possible to increase a perpendicular magnetic anisotropy of the top electrode. Using a top electrode of \Co40\Fe40\B20(0.4\ \nm)/\Co72\Pt28(1.6\ \nm)/\Pt, we have obtained an effective magnetic anisotropy of 3.3\times 10^6\ \erg/cm^3 and a tunnel magnetoresistance (TMR) of 9.4%. The analysis of the crystal structure reveals that the low TMR is a consequence of the fcc (111) texture of CoFeB layers. [ABSTRACT FROM PUBLISHER]

Nanostructured magnetic tunnel junction rings (MTJ) based on CoFeB free layer were fabricated to investigate the corresponding magnetization reversal processes. The dimensions of the MTJ rings were with outer diameter/linewidth of 1 μm/100 nm and 0.7μm/100 nm, respectively, in which the fabrication was carried out by a top-down technique in combination with electron beam lithography, ion-milling and etch-back processes. Double transition processes in magnetoresistance (MR) responses are observed in these nanostructured MTJ rings. With considering the surface roughness, an OOMMF simulation was employed to explore the magnetization evolution. The simulated hysteresis loops are consistent with the MR behaviors, unfolding that the magnetic switching process initiates from the forward to reversed onion states through an incomplete vortex state. This two-step switching behavior is reasonably assumed to be due to a pinning site resulting from the surface roughness in a region of the ring. In addition, a series of in situ magnetic force microscopy images extracted from nanostructured MTJ rings reveals the similar magnetization evolution with the simulated results. [ABSTRACT FROM AUTHOR]

A relationship between boron (B) diffusion into the MgO barrier and pinhole creation in CoFeB/MgO/CoFeB-magnetic tunnel junctions (MTJs) was investigated. The diffused B in the MgO layer was identified by secondary ion mass spectrometry for the MTJs annealed at 350° C, which provide the giant magnetoresistance (TMR) ratio. The pinhole density, estimated from the statistic distribution of break-down voltage of the TMR properties, increased as either the thickness or the B content of the CoFeB layer became thicker or higher. These experimental findings imply that the diffused B into the MgO barrier creates pinholes to short-circuit the tunnel conduction, since the amount of diffused B into the MgO barrier might be related to the total amount of the B content in the CoFeB layer. Three different techniques were found to be useful for the reduction of diffused B into the MgO barrier layer; usage of materials having boron affinity for capping layer, decrease of the total amount of B-content in CoFeB layer, and reduction of grain boundaries in the MgO barrier layer. [ABSTRACT FROM AUTHOR]

Epitaxial Fe/MgO/Fe magnetic tunnel junctions (MTJs) were grown on MgO(001) substrates by molecular beam epitaxy. The epitaxial structure has been characterized using in-situ reflection high energy diffraction, ex-situ X-ray diffraction, and high-resolution transmission electron microscopy. The magnetic and transport properties have been investigated using vibrating sample magnetometry and physical properties measurement system. The temperature dependence of the TMR ratio, whose value rises from 170% at room temperature to 318% at 10 K, is found to be nonlinear and is fitted using a model based on the effect of magnetic disorder of the electrodes on the spin polarized tunneling. [ABSTRACT FROM AUTHOR]

We report on a systematic investigation of rapid thermal anneal (RTA) effects on the properties of FeMn exchange-biased magnetic tunnel junctions (MTJs). The tunneling magnetoresistance (TMR) in an as-grown MTJ is found to be ∼27%, whereas the TMR in MTJs annealed by RTA increases with annealing temperature up to 300 °C, reaching ∼46%. A significant change in the effective barrier thickness (teff) and height (Φeff) occurs within 10 s during RTA. Transmission electron microscopy and X-ray reflectivity studies demonstrate that the interface of the alumina tunnel barrier for the MTJ annealed by RTA became sharper and clearer, giving rise to the enhanced TMR. [Copyright &y& Elsevier]

This study investigated Co40Fe40W20 single-layer thin films according to their corresponding structure, grain size, contact angle, and surface energy characteristics. Co40Fe40W20 alloy thin films of different thicknesses, ranging from 10 to 50 nm, were sputtered on Si(100) substrates by DC magnetron sputtering. The thin films were annealed under three conditions: as-deposited, 250 °C, and 350 °C temperatures, respectively. The Scherrer equation was applied to calculate the grain size of Co40Fe40W20 thin films. The results show that the grain size of CoFe(110) increased simultaneously with the increase of post-annealing temperature, suggesting that the crystallinity of Co40Fe40W20 thin films increased with the post-annealing temperature. Moreover, the contact angles of all Co40Fe40W20 thin films were all less than 90°, suggesting that Co40Fe40W20 thin films show changes in the direction of higher hydrophilicity. However, we found that their contact angles decreased as the grain size of CoFe increased. Finally, the Young equation was applied to calculate the surface energy of Co40Fe40W20 thin films. After post-annealing, the surface energy of Co40Fe40W20 thin films increased with the rising post-annealing temperature. This is the highest value of surface energy observed for 350 °C. In addition, the surface energy increased as the contact angle of Co40Fe40W20 thin films decreased. The high surface energy means stronger adhesion, allowing the formation of multilayer thin films with magnetic tunneling junctions (MTJs). The sheet resistance of the as-deposited and thinner CoFeW films is larger than annealed and thicker CoFeW films. When the thickness is from 10 nm to 50 nm, the hardness and Young's modulus of the CoFeW film also show a saturation trend. [ABSTRACT FROM AUTHOR]

In this study, we report the effects of electrode and annealing on the magnetic properties of CoFeB–MgO–CoFeB–TbFeCo structures. The samples of structure bottom electrode/Co20Fe60B20(x\ nm)/MgO(1)/Co20Fe60B20(0 or 1 nm)/Tb23(Fe80Co20)77(5\ nm)/top electrode were deposited on SiO2 substrates at room temperature. The structures were then annealed at the temperature ranging from 250 to 400^\circC under 5 kOe perpendicular applied field for two hours. We found that the magnetic properties of the CoFeB free layer depend on the electrode stack structures. The perpendicular magnetic easy axis of the CoFeB free layer can be maintained after annealing up to 400^\circC. [ABSTRACT FROM AUTHOR]

This paper reports the results obtained using TbFe and FeCo alloys as the electrodes of MgO-based perpendicular magnetic tunnel junctions (pMTJs). Without annealing treatment, the as-deposited structure showed MgO (001) crystalline texture. The tunnel magnetoresistance (TMR) ratio measured from the patterned cell of size 3\,\times\,6 \mum^2 was 9.5% at room temperature. However, the coercivities of both free and fixed layers were reduced considerably due to the damage caused from the patterning process. The hysteresis loop measurements showed that the magnetic properties of the structures can still be maintained after annealing up to 350 ^\circC. [ABSTRACT FROM AUTHOR]

In this paper, we present experimental results of the ferromagnetic interlayer coupling effect in magnetic tunnel junction devices. This investigation is focused on the sensor size dependency of the ferromagnetic interlayer coupling. The geometry-dependent magnetostatic edge-coupling contribution is estimated by micromagnetic simulation and then subtracted from the net coupling field in order to obtain the correlation between ferromagnetic interlayer coupling and the sensor size. [ABSTRACT FROM AUTHOR]

Abstract: Magnetic tunneling junctions (MTJs) have a sandwiched structure, which comprises a top ferromagnetic (FM1) layer, an insulating tunneling layer (spacer), and a bottom ferromagnetic (FM2) layer. Exchange coupling in MTJs has been extensively widely examined because the effect of spacer thickness on the ferromagnetic spin-coupling can be exploited in read-head sensors, spin-valve structures, and magntoresistance random access memories (MRAMs). In this investigation, MTJs were deposited in the sequence, glass/CoFeB(50Å)/AlO x (d)/Co(100Å), where the thickness of the AlO x layer d =12, 17, 22, 26 or 30Å. Saturation magnetization (M s) results demonstrate that the exchange coupling strength and coercivity (H c) can be varied considerably by varying the tunneling barrier AlO x spacer. The X-ray diffraction patterns (XRD) include a main peak from hexagonal close-packed (HCP) Co with a highly (002) textured structure at 2θ =44.7°, and AlO x and CoFeB are amorphous phases. The full width at half maximum (FWHM) of the Co (002) peak declines as the AlO x thickness increases, revealing that the Co layer becomes more crystalline. The magnetic results reveal that the magnetic characteristics are related to the Co crystallinity. The exchange coupling strength increases with AlO x thickness. The coercivity (H c) also increases, because the Co crystallinity is eliminated. [Copyright &y& Elsevier]

This letter presents energy-efficient MgO based magnetic tunnel junction (MTJ) bits for high-speed spin transfer torque magnetoresistive random access memory (STT-MRAM). We present experimental data illustrating the effect of device shape, area, and tunnel-barrier thickness of the MTJ on its switching voltage, thermal stability, and energy per write operation in the nanosecond switching regime. Finite-temperature micromagnetic simulations show that the write energy changes with operating temperature. The temperature sensitivity increases with increasing write pulsewidth and decreasing write voltage. We demonstrate STT-MRAM cells with switching energies of < 1 pJ for write times of 1–5 ns. [ABSTRACT FROM AUTHOR]

High tunneling magnetoresistance (TMR) ratio and low RA in magnetic tunnel junctions are necessary condition for application in magnetic random access memory. To get high TMR ratio and low RA, good quality of MgO (002) insulating layer is important. To increase crystalline quality of MgO (002) layer, we applied negative bias on the substrate during MgO deposition. We report the results of the tunneling magnetoresistance (TMR) ratio and the resistance-area product (RA) for CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) with substrate bias voltage, and showed TMR increase and RA decrease with substrate bias. The effects of substrate bias voltage on the TMR ratio, RA and MgO (002) peak intensity are discussed. [ABSTRACT FROM AUTHOR]

Cross-strip magnetic tunnel junctions (MTJs) were fabricated following this sequence: Si(100)/Ta(30 Å)/Co(75 Å)/AlO[subx] (δt[subo])/Co (75 Å)/ IrMn(90 Å)/Ta(100 Å) was deposited under an in-plane deposition field (h) = 500 Oe, where δt[subo] = 12, 17, 22, and 26, and 30 Å was the thickness of the AlO[subx] layer. Tunneling magnetoresistance (TMR) rose initially from 21% to 36% and then fell to 24% as δt[subo] increased monotonically. This means the spin tunneling weakened as δt[subo] ≥ 26 Å. The magnetostriction (λ[subs]) of the Co/AIO[subx]/Co MTJs was generally in the range of -23 ppm to -12 ppm. Moreover, the λ[subs] vs. δt[subo] plot shows the concave-up feature. High-resolution sectional transmission electron microscope (HR X-TEM) with nanoprobe energy dispersive x-ray (EDX) spectroscopy was used to analyze the Co, Al, and O concentrations across each AIO[subx] layer. The different concentration dependences along δt[subo] resulted in the λ[subs] feature. In addition, HR X-TEM, showed that in the microstructure of each Co/AlO[subx]/Co MTJ, the interfaces of Co/AlO[subx] and AlO[subx]/Co are smooth. We find that the optimal MTJ in this series has the following properties: TMR = 36% and λ[subs] = -15 ppm. [ABSTRACT FROM AUTHOR]

Scanning tunneling microscopy (STM) is used to study transport in magnetic double tunnel junctions (DTJs) formed using a fixed transparency barrier of a patterned tunnel junction (TJ), and a variable tunnel barrier between the top electrode of the patterned junction and the STM tip. A sufficiently thin top electrode has been predicted to result in a rectification of charge current through a DTJ when the two barriers have different transparency. Our measurements indeed show a high current rectification ratio for 3-nm-thick, continuous film top electrodes, which is observed for junctions with asymmetric tunnel barriers. [ABSTRACT FROM AUTHOR]

Magnetic tunnel junctions (MTJs) with partially oxidized 9-Å AlOx-barriers were recently shown to have the necessary characteristics to be used as magnetoresistive sensors in high-density storage devices. Here we study dielectric breakdown in such underoxidized MTJs, focusing on its dependence on tunnel junction area and oxidation time. A clear relation between breakdown mechanism and junction area is observed for the MTJs with the highest studied oxidation time: samples with large areas fail usually due to extrinsic causes (characterized by a smooth resistance decrease at dielectric breakdown). Small area junctions fail mainly through an intrinsic mechanism (sharp resistance decrease at breakdown). However, this dependence changes for lower oxidation times, with extrinsic breakdown becoming dominant. In fact, in the extremely underoxidized magnetic tunnel junctions, failure is exclusively related with extrinsic causes, independently of MTJ area. These results are related with the presence of defects in the barrier (weak spots that lead to intrinsic breakdown) and of metallic unoxidized Al nanoconstrictions (leading to extrinsic breakdown). [ABSTRACT FROM AUTHOR]

30-nm scale fabrication of magnetic tunnel junctions (MTJs) was demonstrated. A scanning electron microscope (SEM) was used for chemical-vapor deposition (CVD) of carbon hard masks. Using electron beam (EB)-CVD, less than several 10-nm scale carbon pillar could be formed on MTJ films. Argon ion milling, of which incident angle from the normal of the film plane was determined 45° and 75°, was utilized to pattern the MTJs. TMR properties of 80-nm scale MTJs were successfully measured using DC-four-probe at room temperature. [ABSTRACT FROM AUTHOR]

Thermal stability factor \Delta of the recording layer was studied in perpendicular anisotropy CoFeB/MgO magnetic tunnel junctions (p-MTJs) with various CoFeB recording layer thicknesses and junction sizes. In all series of p-MTJs with different thicknesses, \Delta is virtually independent of the junction sizes of 48–81 nm in diameter. The values of \Delta increase linearly as the recording layer thickness increases. The slope of the linear fit is explained well by a model based on nucleation-type magnetization reversal. [ABSTRACT FROM PUBLISHER]

In this investigation, the low-frequency alternate-current (AC) magnetic susceptibility (χ ac ) and hysteresis loop of various MgO thickness in CoFeB/MgO/CoFeB magnetic tunneling junction (MTJ) determined coercivity ( H c ) and magnetization ( M s ) and correlated that with χ ac maxima. The multilayer films were sputtered onto glass substrates and the thickness of intermediate barrier MgO layer was varied from 6 to 15 Å. An experiment was also performed to examine the variation of the highest χ ac and maximum phase angle (θ max ) at the optimal resonance frequency ( f res ), at which the spin sensitivity is maximal. The results reveal that χ ac falls as the frequency increases due to the relationship between magnetization and thickness of the barrier layer. The maximum χ ac is at 10 Hz that is related to the maximal spin sensitivity and that this corresponds to a MgO layer of 11 Å. This result also suggests that the spin sensitivity is related to both highest χ ac and maximum phase angle. The corresponding maximum of χ ac is related to high exchange coupling. High coercivity and saturation magnetization contribute to high exchange-coupling χ ac strength.

This investigation studies CoFeB/AlO x /Co magnetic tunneling junction (MTJ) in the magnetic field of a low-frequency alternating current, for various thicknesses of the barrier layer AlO x . The low-frequency alternate-current magnetic susceptibility (χ ac ) and phase angle (θ) of the CoFeB/AlO x /Co MTJ are determined using an c ac analyzer. The driving frequency ranges from 10 to 25,000 Hz. These multilayered MTJs are deposited on a silicon substrate using a DC and RF magnetron sputtering system. Barrier layer thicknesses are 22, 26, and 30 Å. The X-ray diffraction patterns (XRD) include a main peak at 2θ = 44.7° from hexagonal close-packed (HCP) Co with a highly (0002) textured structure, with AlO x and CoFeB as amorphous phases. The full width at half maximum (FWHM) of the Co(0002) peak, decreases as the AlO x thickness increases; revealing that the Co layer becomes more crystalline with increasing thickness. χ ac result demonstrates that the optimal resonance frequency ( f res ) that maximizes the χ ac value is 500 Hz. As the frequency increases to 1000 Hz, the susceptibility decreases rapidly. However, when the frequency increases over 1000 Hz, the susceptibility sharply declines, and almost closes to zero. The experimental results reveal that the mean optimal susceptibility is 1.87 at an AlO x barrier layer thickness of 30 Å because the Co(0002) texture induces magneto-anisotropy, which improves the indirect CoFeB and Co spin exchange-coupling strength and the χ ac value. The results concerning magnetism indicate that the magnetic characteristics are related to the crystallinity of Co.