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The synchronisation of magnetic tunnel junctions in the high frequency domain has attracted significant interest in the context of novel computation paradigms, specifically neuromorphic spintronics and probabilistic computing. In this work, a design for the coupling and synchronization of spin torque vortex oscillators (STVOs) is implemented. The geometry comprises the fabrication of adjacent pairs of STVO nanopillars (MgO-based magnetic tunnel junctions), with an edge-to-edge distance down to 100 nm, together with individual top contacts that allow an independent electrical access to each device. In this geometry, the magneto-dipolar coupling promotes the synchronization of the two oscillators, at the same time as the access to the frequency and phase of each individual oscillator is possible. Both frequency and time domain measurements confirm a successful synchronization, with the coupling being controlled by the relative DC bias in each oscillator. As a proof-of-concept towards an oscillator-based Ising machine, it is also shown that the second harmonic injection locking of an STVO can be controlled by tuning the magneto-dipolar coupling to its correspondent STVO pair. These results represent a step forward for the implementation of magneto-dipolar coupled magnetic tunnel junctions, specifically in the field of unconventional computing hardware. Spin torque nano-oscillators (STNOs) have been proposed as the building blocks of Ising machines for solving combinatorial optimization problems. In this study, the authors experimentally demonstrate how the short-range magnetic coupling between a pair of STNOs controls their ability to lock with an external second harmonic signal. [ABSTRACT FROM AUTHOR]

NiFe-based vortex spin-torque nano-oscillators (STNO) have been shown to be rich dynamic systems which can operate as efficient frequency generators and detectors, but with a limitation in frequency determined by the gyrotropic frequency, typically sub-GHz. In this report, we present a detailed analysis of the nature of the higher order spin wave modes which exist in the Super High Frequency range (3–30 GHz). This is achieved via micromagnetic simulations and electrical characterisation in magnetic tunnel junctions, both directly via the spin-diode effect and indirectly via the measurement of the coupling with the gyrotropic critical current. The excitation mechanism and spatial profile of the modes are shown to have a complex dependence on the vortex core position. Additionally, the inter-mode coupling between the fundamental gyrotropic mode and the higher order modes is shown to reduce or enhance the effective damping depending upon the sense of propagation of the confined spin wave. The control and manipulation of spin waves holds promise for miniaturized radio-frequency spintronic devices. The authors demonstrate electrical access to the dynamics of magnetic vortices in confined geometries: a new avenue of research for future applications exploiting super high frequency behaviour for microwave communications and computing applications. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

One of the critical issues in spintronics‐based technologies is to increase the data storage density. Current strategy is based on shrinking the devices size down to tens of nanometers, or several nanometers, which is reaching its limit. A new proposal is to use multi‐level cells (MLCs) to store more than two bits in each cell. In this work, the multi‐level switching is realized in CoFeB/MgO/CoFeB based nano‐scale single perpendicular magnetic tunnel junctions (p‐MTJs) with three or four stable resistance states. A large range of writing currents for each state is obtained, accompanying with a good repeatability of set‐reset operations between different states. The developed multi‐domain model perfectly matches the experimental results, reflecting the magnetic behaviors during multi‐level switching. Furthermore, current‐driven domain wall (DW) motion is revealed in the circular p‐MTJs, where the DW position can be reversibly manipulated by applied current. To design high‐performance multi‐level p‐MTJs, the parameter diagrams are calculated, suggesting various feasible strategies to improve the multi‐level switching through materials optimization and devices geometry. In summary, the demonstration of multi‐level switching in single p‐MTJ shows the high potential of realizing the new generation of p‐MTJ‐based multi‐level spintronic devices, such as multi‐level memories and spin‐neuron devices. [ABSTRACT FROM AUTHOR]

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

Recently, the perpendicular magnetic tunnel junctions (p-MTJs) arouse great interest because of its unique features in the application of spin-transfer-torque magnetoresistive random access memory (STT-MRAM), such as low switching current density, good thermal stability and high access speed. In this paper, we investigated current induced switching (CIS) in ultrathin MgO barrier p-MTJs with dimension down to 50 nm. We obtained a CIS perpendicular tunnel magnetoresistance (p-TMR) of 123.9% and 7.0 Ω·μm2 resistance area product (RA) with a critical switching density of 1.4×1010 A/m2 in a 300 nm diameter junction. We observe that the extrinsic breakdown mechanism dominates, since the resistance of our p-MTJs decreases gradually with the increasing current. From the statistical analysis of differently sized p-MTJs, we observe that the breakdown voltage (Vb) of 1.4 V is 2 times the switching voltage (Vs) of 0.7 V and the breakdown process exhibits two different breakdown states, unsteady and steady state. Using Simmons’ model, we find that the steady state is related with the barrier height of the MgO layer. Furthermore, our study suggests a more efficient method to evaluate the MTJ stability under high bias rather than measuring Vb. In conclusion, we developed well performant p-MTJs for the use in STT-MRAM and demonstrate the mechanism and control of breakdown in nano-scale ultrathin MgO barrier p-MTJs. [ABSTRACT FROM AUTHOR]

The ability to fabricate MgO magnetic tunnel junction nanopillars with low RA and high TMR opens the door to new devices such as magnetic memories or nanooscillators. Here, we address the dynamic behavior of a CoFeB/MgO/CoFeB MTJ with a synthetic-antiferromagnetic pinned layer as a potential candidate for spin-transfer torque (STT) driven applications. The unpatterned stack presents an RA of \sim 1.4~\Omega \mu \textm^2 and both free layer and barrier are patterned down to a $90 \times 180$ nm2 elliptical shape. The observed high-frequency spectra displayed deviations from the uniform precession modes due to additional coupling fields. A dependence of the STT magnitude on the analyzed mode was obtained, differing from previous reports, most likely due to the very low RA employed. [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]

Highly sensitive nanosensors with high spatial resolution provide the necessary features for high-accuracy imaging of isolated magnetic nanoparticles or mapping of magnetic fields. Here, we fabricated nanosensor devices based on MgO-magnetic tunnel junctions with soft pinned sensing layer. The exchange interaction at the free-layer is tuned to yield distinct linear operation ranges for the nanosensors. Circular (diameter D = 120–500 nm) and elliptical pillars with low aspect ratio (120 nm \times 130 nm–120 nm \times 200 nm) displaying a linear non-hysteretic transfer curves with tunnel magnetoresistance values up to 143% were obtained. A noticeable improvement in the sensitivity for circular structures from an average value of \sim 1 %/mT up to \sim 2 Hz with Hooge parameters within 1– 3 \times 10^{-9} \mu m ^{2}$ in the linear range. Nevertheless, such high sensitivity values are a major improvement in comparison with those reported previously for nanometric sensors, and extremely competitive with values reported for micrometric spin-valve sensors, with the advantage of providing a reduced device footprint suitable for highly resolved measurements. [ABSTRACT FROM AUTHOR]

Spin valves (SVs) with synthetic-antiferromagnet (SAF) pinned layers and synthetic-ferrimagnet (SF) free-layers deposited by ion beam deposition are optimized for incorporation in nanometric sensors. The results on combined SAF-SF structures indicate a reduced saturation and offset fields when compared with the simple top-pinned or SAF structure. Therefore, SAF-SF SV display sensitivities of $\sim 0.025$ %/Oe (200 nm sensor), \sim 0.1$ %/Oe (500 nm sensor), and \sim 0.2$ %/Oe (1 sensor), which correspond to an improvement of $2\times $ , $4.5\times $ , and $7\times $ , respectively, when compared with all other SV stacks tested. The results are relevant for geometries where nanometric SVs are incorporated within very narrow gaps of magnetic flux concentrators leading to superior and competitive gains in sensitivity. These geometries have the unique feature of submicrometric spatial resolution, and have high impact on surface scanning applications. [ABSTRACT FROM AUTHOR]

The switching field dependence on the size of nanometric magnetic tunnel junctions was studied. CoFe/Ru/CoFeB/MgO/CoFeB nano-pillars were fabricated down to 150\,\times\,300 nm^2 and characterized, revealing a squared transfer curve with a sharp transition between magnetic states. A micromagnetic finite element tool was then used to simulate the magnetic behavior of the studied nanopillar. The simulations indicated a single-domain like state at remanence, also displaying a sharp transition between parallel/antiparallel free-layer configurations. Overall, the experimentally measured switching fields (Hsw) were smaller than those obtained from simulations. Such trend was consistent with the presence of a particular free layer profile, signature of the two angle etching step used for pillar definition. Further decrease of experimental Hsw was attributed to local defects and thermal activated processes. This study was able to validate this particular simulation tool for the control of the nanofabrication process. [ABSTRACT FROM AUTHOR]

Full Wheatstone bridge electrical current sensor incorporating 114 MgO-based magnetic tunnel junction elements (3\times 30\ \mum^2) connected in series was produced for improved electrical robustness. To that end, magnetic tunnel junctions with R\timesA\sim 7\ k\Omega\ \mum^2, tunneling magnetoresistance of 200%, were produced. The sensor was designed with an integrated Ru thin film resistive thermal detector (RTD) for temperature drift monitoring and compensation. In order to achieve a full bridge signal, a U-shaped copper trace was placed under a printed circuit board (PCB) specifically designed for this type of device. The resulting device exhibit sensitivities of 63.9 V/Oe/A in a 75 Oe linear range biased with 1 mA current, providing a significantly advantageous alternative to AMR and GMR based bridges. [ABSTRACT FROM AUTHOR]

We present an investigation into the in-plane field induced free layer state transitions found in magnetic tunnel junctions. By applying an ac current into an integrated field antenna, the magnetisation of the free layer can be switched between the magnetic vortex state and the quasi-uniform anti-parallel state. When in the magnetic vortex state, the vortex core gyrates a discrete number of times, and at certain frequencies there is a 50% chance of the core gyrating n or n − 1 times, leading to the proposal of a novel nanoscale continuous digital true random bit generator. [ABSTRACT FROM AUTHOR]

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

In the field of spin caloritronics recent theoretical models suggested a significant influence of the interfaces of the magnetic tunnel junction (MTJ) on the thermal transport. In this work magnetothermopower measurements are carried out on CoFeB/MgO/CoFeB nanopillars and an unexpected increase of the thermovoltage with the diameter of the nanopillars is observed. To understand this behavior the thermal profiles are computed by finite element simulations. The observed behavior with the pillar diameter could only be reproduced in simulations by considering a far lower effective thermal conductivity of the MgO than the intrinsic thin-film value. In agreement with theoretical predictions, a finite thermal conductivity of the MgO/CoFeB interface can explain this observation. This is experimental evidence of the influence of the thermal resistance of the MgO/CoFeB interfaces on magnetothermovoltage measurements and is in agreement with recent theoretical predictions. The measured magnetothermovoltage is around 4.5 μV and the simulated temperature difference is about 2 K across the tunnel barrier, which resulted in a magnetic contribution of the thermopower of ΔSMTJ ≈ -2.25μVK-1. This value was about 20 times smaller than the result obtained by the typically used thermal conductivity of MgO thin films. [ABSTRACT FROM AUTHOR]