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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]

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]

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]

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]

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]

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]

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]

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]

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]