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Thermodynamics of far-from-equilibrium systems often require measurement of effective parameters such as temperature. Whether such approach is valid for the general case of resetting protocols, active systems, or of confined systems under time-varying fields is still under investigation. We report on the effect of switching ON-OFF of an asymmetric bistable potential to the mean first passage time (MFPT) of a probed particle to go from one potential minima to the other. Experimental results coupled with numerical simulations shows the potential becoming more symmetric at slow switching. Moreover, the MFPT deviates from equilibrium condition with an effective temperature, Teff < T, at slow switching but approaches room temperature, T, at fast switching. For each switching rate, we quantify how far the system is from equilibrium by measuring deviation from a detailed balance like relation and the net circulation of flux present in phase-space. Both analysis suggest equilibrium condition are met at high switching. [ABSTRACT FROM AUTHOR]

Perpendicular magnetic anisotropy (PMA) induced at the interface of the metallic magnetic layer/oxide layer plays a major role in scaling of state-of-the-art spin-transfer-torque magnetoresistive random access memory. The realization of PMA requires the development of mature interface manipulation techniques as well as materials constituting the interface. Herein, we report large PMA using stacks developed with an ultrathin (∼0.7 nm) Fe/rock-salt CrO(001) interface via an oxygen-driven diffusion process. The stacks were prepared by sputter-deposition and post-annealing of the Cr buffer/ultrathin Fe/MgO structure. Significant oxidation of the Fe layer and Cr diffusion into the MgO layer occurred during the deposition. After post-annealing, the oxidized Fe layer was reduced to form an Fe/rock-salt-type Cr-monoxide structure due to chemical layer exchange. The lattice-matched Fe/CrO interface with a large interfacial PMA energy of 1.55 mJ/m2 was confirmed after annealing at 500 °C. X-ray absorption spectroscopy measurements revealed that the post-annealing promoted the redox reaction from the Fe oxide to the metallic Fe and the formation of the CrO. The observed PMA indicates that the oxygen-driven diffusion process by annealing resulted in the well-controlled Fe/CrO interface. The demonstrated diffusion process provides a new chemical route to fabricate artificial, well-controlled PMA interfaces, even containing metastable materials, beyond the conventional sequential layer stacking for the development of spintronic devices. [ABSTRACT FROM AUTHOR]

Implementation of spin transfer torque magneto-resistive random access memory (STT-MRAM) in memory chips requires that the write margin of the MRAM cell, defined as the difference between breakdown voltage and write voltage for the specified endurance, write error rate, and write speed, is sufficiently large in order to accommodate resistance variations arising from external chip circuitry. We show that by increasing only the thickness of the MgO cap layer to make its resistance-area product close to that of the main MgO barrier, the write margin can be increased substantially without affecting the thermal stability of the cell. [ABSTRACT FROM AUTHOR]

The article explores the roadmap to neuromorphic computing with a focus on emerging technologies, challenges, and potential solutions in the field. It discusses the significance of materials engineering, various approaches to neuromorphic research, and the importance of balancing performance with sustainability. The text also delves into the potential of 2D-LMs in mimicking synapses and neurons in artificial neural networks, highlighting the stable resistive switching observed in certain materials. Efforts are ongoing to address challenges and further advance the use of 2D-LMs in neuromorphic computing, offering promising solutions to computational challenges faced by traditional digital computers. [Extracted from the article]

Understanding the magnetization reversal and temperature characteristics of synthetic antiferromagnets (SAFs) is helpful for optimizing the magnetic properties. In this study, a series of synthetic antiferromagnets with perpendicular magnetic anisotropy are deposited. The four stable magnetic states of the SAF are determined by the mutual alignment of magnetic moments in the layers and are controlled by both the magnetic interlayer exchange interaction and Zeeman energy. First order reversal curves were employed to investigate magnetization reversal behaviors and distinguish between reversible and irreversible components. An innovative approach to enhancing the antiferromagnetic coupling field and thermal stability involves introducing a Ru insertion and increasing the thickness of the adjacent magnetic layer. [ABSTRACT FROM AUTHOR]

This paper reports that the in situ growth magnetic field (Hg) during magnetic-phase CoFeB deposition impacts the electric-field control of magnetic anisotropy in Co40Fe40B20/(011)-Pb(Mg1/3Nb2/3)0.7Ti0.3O3 [CoFeB/(011)-PMN-PT] composite multiferroic heterostructures at room temperature. In the Hg1 mode (in situ Hg along the [ 01 1 ¯ ] direction of the ferroelectric PMN-PT substrate), the electric-field-controlled modulation ratios of the magnetic coercivity HC and saturation magnetic field HS are approximately −47% and +156%, respectively. However, in the Hg2 mode (in situ Hg along the [100] direction of the ferroelectric PMN-PT substrate) of the CoFeB/(011)-PMN-PT multiferroic heterostructure, the electric-field-controlled modulation ratios of the magnetic coercivity HC and saturation magnetic field HS can reach as high as +162% and +393%, respectively. Moreover, the electric-field-controlled magnetic coercive field HC exhibits a butterfly shape when plotted versus the applied electric fields in both modes, which matches the in-plane butterfly strain loop of the ferroelectric PMN-PT substrate. However, the electric-field-controlled saturation magnetic field HS presents a square loop, which is very consistent with the ferroelectric loop of the PMN-PT substrate. This result may be ascribed to the distinct pathway of the ferroelastic domain switching in the (011)-oriented PMN-PT substrate. This study provides a new idea for the design of spintronic devices based on multiferroic heterostructures. [ABSTRACT FROM AUTHOR]

We investigate the interaction between two magnetic layers separated with a normal metal insertion layer (Ti, Pt, and Ru) using x-ray ferromagnetic resonance (XFMR). We measure the amplitude and phase of the ferromagnetic resonance of both layers. Our results indicate that a ferromagnetic exchange coupling between two layers is a dominant coupling mechanism for a thick insertion metal layer. Based on the exchange coupling model, we extract the smallest value of the indirect exchange coefficient of 1.2 μJ/m2, which corresponds to an exchange field of about 0.36 mT. While this value is difficult to measure with other experimental tools, we were able to measure the small value because XFMR detects a resonance phenomenon of a thin layer generated by an oscillating indirect exchange and the Oersted fields with a phase and layer resolved observation. [ABSTRACT FROM AUTHOR]

This document provides information about the data availability for a study conducted by a group of authors. The authors' names are listed, and it is stated that the data supporting the findings of the study are available within the article. The document does not provide any further details about the study or its findings. [Extracted from the article]

Measuring thermal stability in magnetic random access memory devices is non-trivial. Recently, there has been much discussion on the appropriate model to use: single domain or domain wall nucleation. Of particular challenge is assessing the maximum size at which the single domain model can be assumed. Typically, this is estimated to be in the range of 20–30 nm based on a value of the exchange stiffness (A e x ) that is assumed, estimated using indirect measurements or derived from significantly thicker films. In this work, it is proposed that this maximum size can be measured directly via the "activation volume" (V a c t ) or the "activation diameter" (D a c t ), which originates from the concept of magnetic viscosity. This is conducted by measuring, using the time dependence of magnetization at different applied fields, D a c t in perpendicular magnetic tunnel junction pillars of varying effective anisotropy constant (K e f f ) and diameter. It is shown that the trend in D a c t follows 1 / K e f f dependence, in good agreement with the analytic model for the critical diameter of coherent switching. Critically, it is also found that the smallest size for which a single domain, with coherent reversal, occurs is 20 nm. Thus, in devices with technologically relevant values of K e f f , the macrospin model may only be used in 20 nm, or smaller, devices. [ABSTRACT FROM AUTHOR]

We report on tunnel magnetoresistance (TMR) sensors with two ferromagnetic free layers (FLs) on both sides of a MgO tunnel barrier. The magnetizations of these FLs are weakly (softly) pinned in an anti-parallel configuration by the exchange bias of IrMn antiferromagnetic layers through ferromagnetic and/or antiferromagnetic interlayer couplings of non-magnetic spacer layers. These dual soft-pinned FL (SPFL) sensors show a symmetric resistance (R) change with respect to the polarity of the external magnetic field (H) applied to the hard axis (HA) of the SPFLs and TMR ratios of over 200%, which is much higher than those of the single-SPFL sensors, where one side of the ferromagnetic layers is an SPFL and the other side is a strongly pinned reference layer. When the applied H exhibits an angular misalignment from the HA, dual-SPFL sensors show much-reduced asymmetry in the R–H curves than single-SPFL sensors. Macrospin simulations of the magnetization behavior and R–H curve for both dual- and single-SPFL sensors revealed that the small asymmetry in the R–H curves of dual-SPFL sensors was owing to their significantly smaller dependence of relative magnetization angle between the SPFLs on the magnetic field misalignment from the HA. These results suggest that dual-SPFL TMR sensors are promising for use in magnetic sensors such as encoder sensors for position and rotation detections. [ABSTRACT FROM AUTHOR]

We present detailed investigations of the magnetic properties of Pt/CoFeB/MgO layers as studied using the angle-resolved spin-torque ferromagnetic resonance method. Although the measurements provide reasonable magnetic parameters, we obtain an unusual value in the interfacial spin transparency if spin-pumping is assumed to be the dominant source for enhancing magnetic damping (αeff) in the nanometer thickness regime. However, the thickness dependence of the Landé g-factor (geff) for CoFeB indicates that the interfacial spin–orbit coupling plays a role in determining αeff. In addition, the azimuthal asymmetry in the magnetic system may not be related to the possibility of generating unconventional spin polarization. The results in this work are expected to aid in understanding various magnetic properties and current-induced spin-torques in a heavy-metal/ferromagnet bilayer. [ABSTRACT FROM AUTHOR]

Suppression of the Walker breakdown in confined wires is key to improving the operation and reliability of magnetic domain-wall-based devices, including logic, memory, and sensor applications. Here, via micromagnetic simulations, we demonstrate that periodical wire-width modulation with suitable geometric parameters can fully suppress the Walker breakdown of a field-driven domain wall, conserving its spin structure in the whole operating field range of a device. Key differences in the efficacy of the wire-width modulation are observed for wires with different widths and thicknesses such that different domain wall states are energetically stable. In particular, the approach is found to be effective in expanding the field-operating window of a device in the case of smaller wire widths and thicknesses (below 150 nm wide and 15 nm thick), whereas in larger wires, the advantages from the suppression in the Walker breakdown are counteracted by the increase in domain wall pinning and the reduction in the nucleation field for new domain walls. Simulations on intersecting magnetic wires prove the importance of suppression of the Walker breakdown. Since the domain wall behavior is chirality dependent, introducing periodical wire-width modulation conserves the spin structure, thus reducing stochasticity of the domain wall propagation. [ABSTRACT FROM AUTHOR]

A key element to tailor the properties of magnetic multilayers is the coupling between the individual magnetic layers. In the case of skyrmion hosting multilayers, coupling of skyrmions across the magnetic layers is highly desirable. Here, the magnetic interlayer coupling was studied in epitaxial all-oxide heterostructures of ferromagnetic perovskite SrRuO 3 layers separated by spacers of the strong spin–orbit coupling oxide SrIrO 3. This combination of oxide layers is being discussed as a potential candidate system to host Néel skyrmions. First order reversal curve (FORC) measurements were performed in order to distinguish between magnetic switching processes of the individual layers and to disentangle the signal of soft magnetic impurities from the sample signal. Additionally, FORC investigations enabled us to determine whether the coupling between the magnetic layers is ferromagnetic or antiferromagnetic. The observed interlayer coupling strength was very weak for the heterostructure with a two monolayer (ML) thick SrIrO 3 spacer, and no coupling was observed for spacers of 6 and 12 ML thickness. The decoupling of the magnetic SrRuO 3 layers due to the SrIrO 3 spacer is a disadvantage for the study of skyrmions in such multilayers and indicates that other oxides have to be identified for realizing strong magnetic coupling. [ABSTRACT FROM AUTHOR]

Write failure (WF) is a major reliability issue for applications of magnetoresistive random access memory (MRAM), and much effort has been devoted to reducing the write error rate (WER), which is the probability of write failures of a memory cell. Recently, it was shown that the WER of MRAM obeys a skewed probability distribution even though the variation in material parameters obeys a normal distribution. However, little is known about the effect of WER distribution on WF in a memory cell array. Here, we study WF in a memory cell array consisting of magnetic tunnel junction elements with distributed WERs based on numerical simulations. We simulated Bernoulli trials of writing, assuming that the WER obeys a beta distribution. The results show that for typical writing patterns, WF in a memory cell array obeys a binomial distribution, with the mean of the WER as the probability of success. The statistical properties of WF in a memory cell array are not affected by the variance and skewness of the WER. The results provide a basic understanding of the statistical properties of WF in a memory cell array and will be useful for the development of computing systems that exploit erroneous memories. [ABSTRACT FROM AUTHOR]

The use of voltage induced strain to switch magnetic tunnel junctions (MTJs) is a promising solution for reducing the switching energy in MRAM technologies. The MTJ is integrated with a piezoelectric layer to generate the strain. A very thin layer is needed to switch with small voltages and small energy dissipation. It is challenging to synthesize ultrathin piezoelectric layers that retain a high degree of piezoelectricity. An alternate approach is to use time-varying strain generated by a surface acoustic wave (SAW). This approach does not require a thin piezoelectric layer since the SAW is confined to the surface of the layer. In this study, we fabricated in-plane MTJs on piezoelectric LiNbO3 substrates and used IDTs to generate the SAW signal within the substrate. Our results showed that the SAW signal had a significant influence on the resistance and the tunneling magnetoresistance (TMR) ratio of the MTJs. The influence was much less significant in nanometer size MTJs than in micrometer sized ones. Most surprisingly, the SAW signal caused the tunneling magnetoresistance ratio (TMR) to drop below zero for the micrometer size MTJ, meaning that the antiparallel resistance RAP is temporarily less than the parallel resistance RP under SAW excitation. Our results provide insight into the dynamic behavior of MTJs under periodic strain and the dependence of this behavior on the device dimensions as they are scaled down to nanometer sizes. [ABSTRACT FROM AUTHOR]

Barrier interface condition is critical for spin-polarized tunneling and spin-transfer torque switching in CoFeB∣MgO∣CoFeB-based magnetic tunnel junctions. The differential tunnel conductance g V contains information on CoFeB's magnetic properties at tunnel interfaces. Experimentally, we find g V to follow a "cross-normalization" relationship between the parallel and antiparallel alignments. This we show originates from the leading order spin-flip scatter terms related to CoFeB interface magnetic properties such as its exchange-stiffness. By connecting the observable g V slopes to electrode-specific spin-flip scatter rates, we obtain an efficient measurement for mass-screening of junctions for interface magnetic differences. This provides valuable information for device and fabrication process optimization. [ABSTRACT FROM AUTHOR]

The discovery of perpendicular magnetic anisotropy (PMA) in Ta∣CoFeB∣MgO and the subsequent development of perpendicularly magnetized tunnel junctions at IBM is reviewed. The fast-turnaround method used for screening materials for interface PMA by measuring the moment/area and anisotropy field of in-plane materials as a function of CoFeB thickness is presented, including the data as a function of seed-layer material which led to the discovery of PMA in Ta∣CoFeB∣MgO. Magnetic and electrical data are reported for the first PMA magnetic tunnel junction we made using this material. By inserting a thin Fe layer at the Ta∣CoFeB interface, a substantial increase in the PMA energy density was obtained. Pure Fe layers (which required the use of a TaMg seed) greatly improved the thermal stability, allowing annealing up to 400 °C. [ABSTRACT FROM AUTHOR]

Utilizing the electronic analogs of optical phenomena such as anti-reflection coating and resonance for spintronic devices, we propose and theoretically analyze the design of a spin transfer torque-magnetoresistive random access memory (STT-MRAM) device. The proposed device consists of a superlattice heterostructure terminated with the anti-reflective regions sandwiched between the fixed and free ferromagnetic layers. Employing Green's function spin-transport formalism coupled self-consistently with the stochastic Landau–Lifshitz–Gilbert–Slonczewski equation, we design an STT-MRAM based on the anti-reflective superlattice magnetic tunnel junction (AR-SLMTJ) device having an ultrahigh tunnel magnetoresistance (≈ 3.5 × 10 4 %) and large spin current. We demonstrate that the STT-MRAM based on the AR-SLMTJ structure owing to the physics of bandpass spin filtering is nearly 1100% more energy efficient than trilayer magnetic tunnel junction (MTJ) based STT-MRAM. We also present detailed probabilistic switching and energy analysis to find out the optimal point of operation of a trilayer MTJ and AR-SLMTJ based STT-MRAM. Our predictions serve as a template to consider the heterostructures for next-generation spintronic device applications. [ABSTRACT FROM AUTHOR]

We report an experimental study that focuses on the magneto-optic interaction between light and perpendicularly magnetized ferromagnetic thin-film multilayers. In this study, samples containing multiple Co/Pt interfaces are measured via the polar magneto-optic Kerr effect. Thanks to a sample set with a continuous Co thickness variation and single and double Co-layered samples, interface and bulk contributions to the magneto-optic activity are separately determined. Kerr rotation and ellipticity signals are recorded as a function of the Co thickness. The results are analyzed via a transfer matrix method that accounts for the standard optical and magneto-optic effects in multi-layered thin-films. Bulk magneto-optic contribution of Co (Q Co , bulk ) that is in accordance with the literature is consistently determined despite the use of thin-films. Interfacial Co (Q Co , int. ) and magnetized Pt (Q Pt ) contributions are presented in terms of two models. Interface phenomena of the large spin–orbit coupling between Co and Pt and the proximity induced magnetization in Pt layer are represented in the models. The strength of interfacial magneto-optic activity is interpreted as an indicator of the relative interface quality among the samples. [ABSTRACT FROM AUTHOR]

The critical current density J c 0 required for switching the magnetization of the free layer (FL) in a spin-transfer torque magnetic random access memory (MRAM) cell is proportional to the product of the damping parameter, saturation magnetization, and thickness of the free layer, α M S t F. Conventional FLs have the structure CoFeB/nonmagnetic spacer/CoFeB. By reducing the spacer thickness, W in our case, and also splitting the single W layer into two layers of a sub-monolayer thickness, we have reduced t F while minimizing α and maximizing M S , ultimately leading to lower J c 0 while maintaining high thermal stability. Bottom-pinned MRAM cells with a device diameter in the range of 55–130 nm were fabricated, and J c 0 is the lowest for the thinnest (1.2 nm) FLs, down to 4 MA / cm 2 for 65 nm devices, ∼ 30 % lower than 1.7 nm FLs. The thermal stability factor Δ dw , as high as 150 for the smallest device size, was determined using a domain wall reversal model from field-switching probability measurements. With high Δ dw and the lowest J c 0 , the thinnest FLs have the highest spin-transfer torque efficiency. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In this paper, we investigated the dipolar magnetic coupling in ferromagnetic multi-layered structures. However, this kind of coupling has been extensively studied since the last few decades through the Néel model (orange peel coupling), but most of the analyses were based on mathematically modeling a simple sinusoidal rough interface that hides the details of such a coupling. Therefore, we add a generality to the Néel model via adapting the anisotropic morphological self-affine interfaces that can unravel the details of interesting effects that are technologically important to consider for future magnonic and spintronic devices. The tensorial coupling between the ferromagnetic (FM) layers has been obtained from the magnetostatic energy of a pseudo-spin valve structure (FM/NM/FM). Our findings show that the coupling strength is dependent not only on the roughness properties of the self-affine interfaces but also on the rotational angle between the patterned interfaces. The variation of this orientation angle along with the change of the interface correlation lengths can switch FM coupling to antiferromagnetic coupling and vice versa. These results are advantageous for the engineering and fabrication of magnonic waveguide circuits and spintronic devices specifically in spin valves, magnetoresistive elements, and magnetic tunneling junctions. [ABSTRACT FROM AUTHOR]

Perpendicular magnetic anisotropy (PMA) in the Ta/CoFeB/MgO system has been studied using x-ray magnetic circular dichroism and vibrating sample magnetometry. The ratios of the orbital to spin magnetic moments of Co atoms in the Ta/CoFeB/MgO structures with PMA have been found to be enhanced by 100%, compared with the Ta/CoFeB/Ta structure without PMA. The orbital moments of Co are as large as 0.30 μ B , more than half of their spin moments in the perpendicularly magnetized Ta/CoFeB/MgO structures. The results indicate that the PMA observed in the CoFeB/MgO structures is related to the increased spin–orbital coupling of the Co atoms. This work offers experimental evidence of the correlation between PMA and the element-specific spin and orbital moments in the Ta/CoFeB/MgO systems. [ABSTRACT FROM AUTHOR]

The study of interface spin effects in spintronic multilayer films requires distinguishing the effects generated by different interfaces. However, testing in atmospheric conditions requires a capping layer to protect the films, which introduces new interfaces and limits the study of interface spin-dependent effects. To address this challenge, we have developed an integrated ultra-high vacuum cluster system that includes magnetron sputtering equipment, ion irradiation equipment, and time-resolved magneto-optical Kerr effect (TR-MOKE) equipment. Our sputtering system integrates 12 cathodes in a single chamber, allowing the co-sputtering of four targets. The ultimate vacuum can reach 1 × 10−10 mbar, and the deposition resolution of 0.1 nm can be achieved. Ion irradiation equipment can ionize to produce He+, and by screening and accelerating the implantation of He+ into multilayer films, ion scanning is realized, and up to 30 keV energy can be applied to the films. The TR-MOKE equipment can detect ultra-fast magnetic dynamics processes in vacuum conditions, and its external magnetic field can be rotated 360°. Our vacuum cluster system connects the three subsystems, allowing in situ film deposition, regulation, and characterization. By accurately detecting the effects of different layers, the system can distinguish the interface effects of multilayers. Experimental results demonstrate that the three subsystems can work independently or coordinate to observe the interface effects of multilayers. [ABSTRACT FROM AUTHOR]

We report measurements of optically detected ferromagnetic resonance (ODFMR) in thin films using nitrogen-vacancy (NV) centers in diamond, whose fluorescence intensity changes in response to the ferromagnetic resonance (FMR) excitation of a proximal ferromagnet. Here, we extend the study of the off-resonant and broadband detection of FMR, first observed in the magnetic insulator YIG to a diverse set of ferromagnetic materials. We measure ODFMR signals from several technologically relevant metallic ferromagnetic materials including Py, Co, Co 2 (Mn 0.6 Fe 0.4 )Ge, and an insulating NiZnAl ferrite. These results show the generality of the spin-relaxation based coupling of the NV spins to the ferromagnetic dynamics that enables this detection. The observable field-frequency range of the ODFMR signal is dependent on material parameters such as saturation magnetization and damping. These results imply that NV-based ODFMR can give insights into nanoscale ferromagnetic dynamics and its damping processes, especially for samples with low magnetization and in the low field-frequency regime, which is important for several technologies. [ABSTRACT FROM AUTHOR]

We fabricated quasiantiferromagnetic (quasi-AFM) layers with alternating antiparallel magnetization in the neighboring domains via 90° magnetic coupling through an Fe-O layer. We investigated the magnetic properties and the relationship between the magnetic domain size and the 90° magnetic coupling via experiments and calculations. Two types of samples with a Ru buffer and a (Ni80Fe20)Cr40 buffer were prepared, and we found that with the NiFeCr buffer, the sample has a flatter Fe-O layer, leading to stronger 90° magnetic coupling and a smaller domain size compared with the Ru buffer sample. This trend is well explained by the bilinear and biquadratic coupling coefficients, A12 and B12, in Landau–Lifshitz–Gilbert simulations, suggesting the possibility of using both AFM and FM properties by controlling the quasi-AFM domain size. [ABSTRACT FROM AUTHOR]

We investigate the dependence of magnetic properties on the post-annealing temperature/time, the thickness of the soft ferromagnetic electrode, and the Ta dusting layer in the pinned electrode as well as their correlation with the tunnel magnetoresistance ratio, in a series of perpendicular magnetic tunnel junctions of materials sequence Ta/Pd/IrMn/CoFe/Ta(x)/CoFeB/MgO(y)/CoFeB(z)/Ta/Pd. We obtain a large perpendicular exchange bias of 79.6 kA/m for x = 0.3 nm. For stacks with z = 1.05 nm , the magnetic properties of the soft electrode resemble the characteristics of superparamagnetism. For stacks with x = 0.4 nm , y = 2 nm , and z = 1.20 nm , the exchange bias presents a significant decrease at post-annealing temperature T ann = 330 ° C for 60 min, while the interlayer exchange coupling and the saturation magnetization per unit area sharply decay at T ann = 340 ° C for 60 min. Simultaneously, the tunnel magnetoresistance ratio shows a peak of 65.5 % after being annealed at T ann = 300 ° C for 60 min, with a significant reduction down to 10 % for higher annealing temperatures (T ann ≥ 330 ° C) and down to 14 % for longer annealing times (T ann = 300 ° C for 90 min). We attribute the large decrease of tunnel magnetoresistance ratio to the loss of exchange bias in the pinned electrode. [ABSTRACT FROM AUTHOR]

D019-Mn3Sn, an antiferromagnet having a non-collinear spin structure in a kagome lattice, has attracted great attention owing to various intriguing properties such as large anomalous Hall effect. Stability of a magnetic state against thermal fluctuation, characterized in general by the thermal stability factor Δ, has been well studied in ferromagnetic systems but not for antiferromagnets. Here, we study Δ of the antiferromagnetic Mn3Sn nanodots as a function of their diameter D. To quantify Δ, we measure the switching probability as a function of the pulse-field amplitude and analyze the results based on a model taking account of two and sixfold magnetic anisotropies in the kagome plane. We observe no significant change in Δ down to D = 300 nm below which it decreases with D. The obtained D dependence is well explained by a single-domain and nucleation-mediated reversal models. These findings provide a basis to understand the thermal fluctuation and reversal mechanism of antiferromagnets for device applications. [ABSTRACT FROM AUTHOR]

We demonstrate tunnel magnetoresistance (TMR) ratios of up to 631% at room temperature (RT) using CoFe/MgO/CoFe(001) epitaxial magnetic tunnel junctions (MTJs). The TMR ratio increased up to 1143% at 10 K. The large TMR ratios resulted from fine-tuning of atomic-scale structures of the MTJs, such as crystallographic orientations and MgO interface oxidation by interface insertion of ultrathin CoFe and Mg layers, which are expected to enhance the well-known Δ1 coherent tunneling transport. Interestingly, the TMR oscillation effect, which is not covered by the standard coherent tunneling theory, also became significant. A 0.32-nm period TMR oscillation with increasing MgO thickness dominates the transport in a wide range of MgO thicknesses; the peak-to-valley difference of the TMR oscillation exceeds 140% at RT, which is attributed to the appearance of large oscillatory components in the resistance area product. [ABSTRACT FROM AUTHOR]

The exceptional capabilities of the human brain provide inspiration for artificially intelligent hardware that mimics both the function and the structure of neurobiology. In particular, the recent development of nanodevices with biomimetic characteristics promises to enable the development of neuromorphic architectures with exceptional computational efficiency. In this work, we propose biomimetic neurons comprised of domain wall-magnetic tunnel junctions that can be integrated into the first trainable CMOS-free recurrent neural network with biomimetic components. This paper demonstrates the computational effectiveness of this system for benchmark tasks and its superior computational efficiency relative to alternative approaches for recurrent neural networks. [ABSTRACT FROM AUTHOR]

The possibility of higher electrical efficiency in computing by operating at low temperatures raises the need for non-volatile memory cells optimized for cryogenic operation. We report a study on low temperature spin transfer torque switching of magnetic tunnel junctions with 20 to 100 nm in diameter with thermal stability adapted to low temperature operation. The evolution of magnetic and electrical properties are characterized for four different stacks from 300 to 10 K comprising insertions of Mg, Ru and permalloy (Py) in the storage layer to reduce its effective anisotropy. Two figures of merit are used to compare different devices and stacks, Δ/Ic and Δ/Esw, normalizing the thermal stability Δ by the critical current or switching energy. Devices with a Py insertion layer show a higher FOM (3.78 kBTop/μA) and switching energy Esw below 655 fJ for 100 ns pulses at Top = 10 K. A procedure to optimize the reference layer stray field was also implemented to achieve full compensation using a synthetic antiferromagnetic layer for 20 nm diameter devices. [ABSTRACT FROM AUTHOR]

Ferromagnetic Resonance mode intensity, I, field linewidth, ΔH, and frequency linewidth, Δf, have been investigated for a weakly coupled magnetic tunnel junction (MTJ) system. The analysis applies for a coupled trilayer, i.e., two ferromagnetic thin films separated by a non magnetic thin film and for a MTJ system consisting of the trilayer and an antiferromagnetic layer. For this particular weakly coupled system, analytical expressions for I, ΔH, and Δf have been determined. The linewidth expressions can be separated into the parts characterizing the individual thin films and magnetic coupling dependent parts; the mode linewidths can be viewed as those of uncoupled thin films with equivalent damping parameters. The intensity and the linewidths are discussed for different applied magnetic fields, for the saturated and unsaturated systems, and for the two types of coupling. It will be shown how the mode intensity behavior can be used as a probe to distinguish the ferromagnetic from the antiferromagnetic coupling. [ABSTRACT FROM AUTHOR]

Spin-Transfer-Torque Magnetic Random Access Memory (STT-MRAM) is a memory which has bit cells made of magnetic tunnel junctions (MTJs), which comprise a storage switchable magnetic layer ("free layer") and, typically, one thin insulating barrier and one stable magnetic layer providing reference spin polarization for read and write operations ("reference layer"). STT-MRAM may compete with conventional dynamic and static RAM on technological nodes below 22 nm, if its switching current is reduced. This goal may be achieved for MTJ, which has two insulating barriers and reference layers. Building such a double-barrier MTJ, however, faces tremendous material challenges. In this work, a new double-barrier MTJ design with a switchable reference layer is introduced. We show that its efficiency is similar to its counterpart with stable reference layers, but it is much easier to be built. [ABSTRACT FROM AUTHOR]

The conditions of field and voltage for inducing steady state excitations in fully perpendicular magnetic tunnel junctions (pMTJs), adapted for memory applications, were numerically investigated by the resolution of the Landau-Lifshitz-Gilbert equation in the macrospin approach. Both damping-like and the field-like spin transfer torque terms were taken into account in the simulations, as well as the contribution of the second order uniaxial anisotropy term (K2), which has been recently revealed in MgO-based pMTJs. An in-plane applied magnetic field balances the out of plane symmetry of the pMTJ and allows the signal detection. Using this model, we assessed the states of the free layer magnetization as a function of strength of K2 and polar θH angle of the applied field (varied from 90° to 60°). There are two stable states, with the magnetization in-plane or out of plane of the layer, and two dynamic states with self-sustained oscillations, called in-plane precession state (IPP) or out of plane precession state (OPP). The IPP mode, with oscillation frequencies up to 7 GHz, appears only for positive voltages if θH = 90°. However, it shows a more complex distribution when the field is slightly tilted out of plane. The OPP mode is excited only if K2 is considered and reaches a maximum oscillation frequency of 15 GHz. Large areas of dynamic states with high frequencies are obtained for strong values of the field-like torque and K2, when applying a slightly tilted external field toward the out of plane direction. The non-zero temperature does not modify the phase diagrams but reduces drastically the power spectral density peak amplitudes. [ABSTRACT FROM AUTHOR]

Perpendicular magnetic tunnel junctions (pMTJs) with a CoFeB/W/CoFeB/MgO composite free layer are experimentally investigated. Magnetic anisotropy and tunneling magnetoresistance (TMR) are determined as functions of ferromagnetic (CoFeB) and capping (MgO) layer thicknesses using the ferromagnetic resonance technique. In pMTJs fabricated into 130 and 150-nm diameters, the TMR ratio > 160% and the thermal stability factor Δ > 60 are measured for the MgO capping layer thickness of tMgO ≥ 1 nm. The voltage vs. magnetic field stability diagram indicates field-free current induced switching with voltages | V S | < 0.5 V. [ABSTRACT FROM AUTHOR]

Since the resurgence of multiferroics research, significant advancement has been made in the theoretical and experimental investigation of the electric field control of magnetization, magnetic anisotropy, magnetic phase, magnetic domains, and Curie temperature in multiferroic heterostructures. As a result of these advances, multiferroic heterostructures are on a trajectory to impact spintronics applications through the significantly reduced energy consumption per unit area for magnetization switching (1–500 μJ cm−2) when compared to that of current-driven magnetization switching (0.2–10 mJ cm−2). Considering this potential impact, it becomes necessary to understand magnetoelectric switching dynamics and characteristic switching times. The body of experimental work investigating magnetoelectric switching dynamics is rather limited, with the majority of room temperature converse magnetoelectric switching measurements reported having employed relatively long voltage pulses. Recently, however, the field has started to consider the kinetics of the switching path in multiferroic (and ferroelectric) switching. Excitingly, the results are challenging our understanding of switching processes while offering new opportunities to engineer the magnetoelectric effect. Considering the prospects of multiferroics for beyond-CMOS applications and the possible influence on operational speed, much remains to be understood regarding magnetoelectric switching kinetics and dynamics, particularly at reduced dimensions and under the influence of boundary effects resulting from strain, electrostatics, and orientation. In this article, we review magnetoelectric switching in multiferroic heterostructures for the electric field control of magnetism. We then offer perspectives moving toward the goal of low energy-delay spintronics for computational applications. [ABSTRACT FROM AUTHOR]

In this work, we present a detailed ferromagnetic resonance (FMR) study of two FeCoB layers coupled across a Ta spacer. The structures of studied samples are FM1/Ta(d)/FM2 where FM1 is a magnetic layer composed of Fe/FeCoB, FM2 is a magnetic layer composed of FeCoB/NiFe, d is the thickness of the Ta layer in nm, and d is varied from 0.375 nm to 4 nm. The FeCoB within FM1 is strongly coupled to high saturation magnetization Fe, and the FeCoB in FM2 is strongly coupled to lower saturation magnetization NiFe in order to separate the FMR resonance positions of these two layers. This is required to determine the strength of interlayer exchange coupling (J) between FM1 and FM2. We solved a system of coupled Landau Lifshitz Gilbert equations, representing the coupled magnetic layers FM1 and FM2, and used it to fit the FMR data and determine J, magnetic anisotropy, Gilbert damping, and g-factor of each magnetic layer as a function of Ta spacer layer thickness and annealing temperature. This study reveals that the dependence of the coupling strength on the Ta thickness is the same for non-annealed samples and for those annealed at 200 °C: the coupling drops to 0 above approximately 0.475 nm and increases rapidly below 0.45 nm. For samples annealed at 300 °C coupling begins to increase below approximately 0.7 nm. It is found that the coupling between FM1 and FM2 for non-annealed samples goes to zero for a Ta thickness two times less than the spin diffusion length. [ABSTRACT FROM AUTHOR]

Low temperature Co K-edge x-ray magnetic circular dichroism spectra at different field cooling conditions were recorded to study the imprinted magnetization in antiferromagnetic (AFM) Co doped ZnO (Co:ZnO) films which manifests itself in a vertical exchange bias effect. Co:ZnO films with 50% and 60% doping concentrations were investigated to provide a high degree of pinned magnetic moments. The measurements reveal a change at the main absorption energy of the spectra, while the signal obtained at the pre-edge stays unaffected by the cooling conditions. Therefore, the pinned uncompensated AFM moments, resulting in an imprinted magnetization, are predominantly of orbital character and are independent of ferromagnetic layers. [ABSTRACT FROM AUTHOR]

Fabrication of perpendicularly magnetized ferromagnetic films on various buffer layers, especially on numerous newly discovered spin–orbit torque (SOT) materials to construct energy-efficient spin-orbitronic devices, is a long-standing challenge. Even for the widely used CoFeB/MgO structures, perpendicular magnetic anisotropy (PMA) can only be established on limited buffer layers through post-annealing above 300 °C. Here, we report that the PMA of CoFeB/MgO films can be established reliably on various buffer layers in the absence of post-annealing. Further results show that precise control of MgO thickness, which determines oxygen diffusion in the underneath CoFeB layer, is the key to obtain the as-deposited PMA. Interestingly, contrary to the previous understanding, post-annealing does not significantly influence the well-established as-deposited PMA but indeed enhances unsaturated PMA with a thick MgO layer by modulating oxygen distributions, rather than crystallinity or Co– and Fe–O bonding. Moreover, our results indicate that oxygen diffusion also plays a critical role in PMA degradation at high temperatures. These results provide a practical approach to build spin-orbitronic devices based on various high-efficient SOT materials. [ABSTRACT FROM AUTHOR]

We present a comprehensive benchmarking for spin-transfer torque (STT) and spin–orbit torque (SOT) based random-access memories. Based on experimentally validated micromagnetic simulations along with the use of rare event enhancement techniques, we show various tradeoffs among the write error rate, write time, and write current. We consider both in-plane and perpendicular devices. For SOT driven perpendicular devices, we include various write mechanisms, such as field-assisted, STT-assisted, and switching due to out-of-plane spin torque, usually present in low symmetry materials. [ABSTRACT FROM AUTHOR]

A coplanar waveguide shorted in one end is proposed, designed, and implemented successfully to measure the properties of magnetic thin films as a part of the vector network analyzer ferromagnetic resonance (FMR) spectroscopy setup. Its simple structure, potential applications, and easy installation inside the cryostat chamber made it advantageous especially for low-temperature measurements. It provides a wide band of frequencies in the gigahertz range essential for FMR measurements. Our spectroscopy setup with a short-circuited coplanar waveguide has been used to extract the Gilbert damping coefficient and effective magnetization values for standard ferromagnetic thin films, such as Permalloy (Py) and Cobalt (Co). The thickness and temperature-dependent studies of those magnetic parameters have also been done here for the afore-mentioned magnetic samples. [ABSTRACT FROM AUTHOR]

In ferromagnets, alloying provides tunability of magnetic, electrical, and mechanical properties; however, its effect on magnetization has mostly been studied in terms of static properties. In this study, we investigate the stiffness and damping of spin waves in a cobalt-boron (Co100−xBx) alloy using an optical pump-probe technique. Coherent spin waves in Co100−xBx were excited by an optical spin–orbit torque from Pt. The dynamics of the spin wave were monitored on a picosecond timescale using the magneto-optical Kerr effect. From the oscillation period and relaxation of the spin wave, the stiffness and damping of the spin waves were determined. As the boron concentration increases from 0% to 35%, the spin wave stiffness decreases from 4.3 to 1.0 meV nm2, and the damping constant of the spin wave increases from 0.02 to 0.11. Such a large change suggests that B alloying has a significant impact on the spin-wave properties. [ABSTRACT FROM AUTHOR]

Magnetic tunnel junctions (MTJs) that consist of two ferromagnets separated by a thin insulator are among the core devices used in spintronic applications such as magnetic sensors. Since magnetic sensors require high sensitivity for nondestructive eddy current testing, we developed and demonstrated magnetic sensors based on various configurations of serial MTJs. We fabricated sensors with 4, 16, 28, 40, and 52 serial MTJs in various numbers of rows (1, 4, 7, 10, and 13) to detect surface cracks via eddy current testing. All of the sensors could detect and discriminate between surface cracks 0.1mm in width and 0.1 to 1.0mm in depth on an aluminum specimen. Systematic studies on the effect of the number of MTJs showed a signal to noise ratio as high as 115 dB when detecting 0.1mm deep cracks with 28 serial MTJs in 7 rows. This suggests that suitably configured serial MTJ sensors can offer an excellent performance in the detection of tiny surface defects via eddy current testing. [ABSTRACT FROM AUTHOR]

Recently, magnetic tunnel junctions with perpendicular magnetized electrodes combined with exchange bias films have attracted great interest. In this paper, we examine the tunnel magnetoresistance of Ta/Pd/IrMn/Co-Fe/Ta/Co-Fe-B/MgO/Co-Fe-B/capping/Pd magnetic tunnel junctions dependent on the capping layer, i.e., Hf or Ta. In these stacks, perpendicular exchange bias fields of 500 Oe along with perpendicular magnetic anisotropy are combined. A tunnel magnetoresistance of (47.2 ± 1.4)% for the Hf-capped sample was determined compared to the Ta one (42.6 ± 0.7)% at room temperature. Interestingly, this observation is correlated with the higher boron absorption of Hf compared to Ta, which prevents the suppression of the △1 channel and leads to higher tunnel magnetoresistance values. Furthermore, the temperature dependent coercivities of the soft electrodes of both samples are mainly described by the Stoner-Wohlfarth model including thermal fluctuations. Slight deviations at low temperatures can be attributed to a torque on the soft electrode which is generated by the pinned magnetic layer system. [ABSTRACT FROM AUTHOR]

With the rapid development of new device miniaturization technology, there is invigorated interest in magnetic nanostructures for potential application in novel multifunctional devices. In continuation to our search for a suitable magnetic material having Curie temperature (Tc) well above room temperature for multifunctional applications, we have studied the dielectric, optical, magnetic, and resistance switching characteristics of Ni0.65Zn0.35Fe2O4 (NZFO) thin films. The observation of only (004) reflection in the X-ray diffraction patterns confirms the c-axis orientation and high quality growth of NZFO thin films. The presence of mixed valences of Fe2+/Fe3+ cations is probed by X-ray photon spectroscopy, which supports the cationic ordering-mediated large dielectric response. Our investigations reveal NZFO to be an indirect band gap material (~1.8eV) with a direct gap at ~2.55eV. These nanostructures exhibit high saturation magnetization and a low coercive field with a ferrimagnetic-paramagnetic phase transition of ~713K. Magnetic force microscopy studies revealed the stripe-like domain structure of the investigated thin films. In addition, these thin films exhibit reliable and repeatable unipolar resistive switching characteristics. The observed high dielectric permittivity with low loss tangent, large magnetization with soft magnetic behavior, striped magnetic domain structure and reliable resistance switching in NZFO thin films above room temperature suggest potential application in memory, spintronics, and multifunctional devices. [ABSTRACT FROM AUTHOR]

We study the annealing stability of bottom-pinned perpendicularly magnetized magnetic tunnel junctions based on dual MgO free layers and thin fixed systems comprising a hard [Co/Ni] multilayer antiferromagnetically coupled to thin a Co reference layer and a FeCoB polarizing layer. Using conventional magnetometry and advanced broadband ferromagnetic resonance, we identify the properties of each sub-unit of the magnetic tunnel junction and demonstrate that this material option can ensure a satisfactory resilience to the 400 °C thermal annealing needed in solid-state magnetic memory applications. The dual MgO free layer possesses an anneal-robust 0.4 T effective anisotropy and suffers only a minor increase of its Gilbert damping from 0.007 to 0.010 for the toughest annealing conditions. Within the fixed system, the ferro-coupler and texture-breaking TaFeCoB layer keeps an interlayer exchange above 0.8 mJ/m², while the Ru antiferrocoupler layer within the synthetic antiferromagnet maintains a coupling above -0.5 mJ/m². These two strong couplings maintain the overall functionality of the tunnel junction upon the toughest annealing despite the gradual degradation of the thin Co layer anisotropy that may reduce the operation margin in spin torque memory applications. Based on these findings, we propose further optimization routes for the next generation magnetic tunnel junctions. [ABSTRACT FROM AUTHOR]

Spin currents are used to write information in magnetic random access memory (MRAM) devices by switching the magnetization direction of one of the ferromagnetic electrodes of a magnetic tunnel junction (MTJ) nanopillar. Different physical mechanisms of conversion of charge current to spin current can be used in two-terminal and three-terminal device geometries. In two-terminal devices, charge-to-spin conversion occurs by spin filtering in the MTJ's ferromagnetic electrodes and present day MRAM devices operate near the theoretically expected maximum charge-to-spin conversion efficiency. In three-terminal devices, spin–orbit interactions in a channel material can also be used to generate large spin currents. In this Perspective article, we discuss charge-to-spin conversion processes that can satisfy the requirements of MRAM technology. We emphasize the need to develop channel materials with larger charge-to-spin conversion efficiency—that can equal or exceed that produced by spin filtering—and spin currents with a spin polarization component perpendicular to the channel interface. This would enable high-performance devices based on sub-20 nm diameter perpendicularly magnetized MTJ nanopillars without need of a symmetry breaking field. We also discuss MRAM characteristics essential for CMOS integration. Finally, we identify critical research needs for charge-to-spin conversion measurements and metrics that can be used to optimize device channel materials and interface properties prior to full MTJ nanopillar device fabrication and characterization. [ABSTRACT FROM AUTHOR]

We offer a perspective on the prospects of ultrafast spintronics and opto-magnetism as a pathway to high-performance, energy-efficient, and non-volatile embedded memory in digital integrated circuit applications. Conventional spintronic devices, such as spin-transfer-torque magnetic-resistive random-access memory (STT-MRAM) and spin–orbit torque MRAM, are promising due to their non-volatility, energy-efficiency, and high endurance. STT-MRAMs are now entering into the commercial market; however, they are limited in write speed to the nanosecond timescale. Improvement in the write speed of spintronic devices can significantly increase their usefulness as viable alternatives to the existing CMOS-based devices. In this article, we discuss recent studies that advance the field of ultrafast spintronics and opto-magnetism. An optimized ferromagnet–ferrimagnet exchange-coupled magnetic stack, which can serve as the free layer of a magnetic tunnel junction (MTJ), can be optically switched in as fast as ∼3 ps. Integration of ultrafast magnetic switching of a similar stack into an MTJ device has enabled electrical readout of the switched state using a relatively larger tunneling magnetoresistance ratio. Purely electronic ultrafast spin–orbit torque induced switching of a ferromagnet has been demonstrated using ∼6 ps long charge current pulses. We conclude our Perspective by discussing some of the challenges that remain to be addressed to accelerate ultrafast spintronics technologies toward practical implementation in high-performance digital information processing systems. [ABSTRACT FROM AUTHOR]

A method is developed to enable a fast bit error rate (BER) characterization of spin-transfer-torque magnetic random access memory magnetic tunnel junction (MTJ) cells without integrating with complementary metal-oxide semiconductor circuit. By utilizing the reflected signal from the devices under test, the measurement setup allows a fast measurement of bit error rates at >106, writing events per second. It is further shown that this method provides a time domain capability to examine the MTJ resistance states during a switching event, which can assist write error analysis in great detail. BER of a set of spin-transfer-torque MTJ cells has been evaluated by using this method, and bit error free operation (down to 10-8) for optimized in-plane MTJ cells has been demonstrated. [ABSTRACT FROM AUTHOR]

Deterministic magnetization switching driven by current-induced spin–orbit torque (SOT) without an external magnetic field has potential applications in magnetic random access memory. Here, we realized the field-free magnetization switching in a T-type structure (CoFeB/W/CoFeB), where the two CoFeB layers have perpendicular magnetic anisotropy and in-plane magnetic anisotropy (IMA), respectively. We discovered that the direction of symmetry-breaking field is parallel to the magnetization of the bottom CoFeB (IMA), which cannot be explained by a stray field of this layer. In addition, by placing a 2.5-nm thick insulating layer of MgO between the bottom CoFeB and W layer (CoFeB/MgO/W/CoFeB) to block the interlayer exchange coupling and the spin current from the bottom CoFeB, the field-free SOT switching was still achieved, primarily due to the Néel orange-peel effect in our devices. By using micromagnetic simulations, the roughness of angstrom magnitude was introduced into the model to calculate the symmetry-breaking field, finding a qualitative agreement with experiments. Moreover, we obtained the spin Hall angle of CoFeB ( θ SH = −0.024) by the current-induced hysteresis loop shift method, and the contribution of the effective efficiency χ from the bottom CoFeB was accounted for about 26% of the total in the current-induced SOT switching process. These results indicated that an in-plane ferromagnet layer in the T-type structure provides not only the symmetry-breaking field but also spin current for the field-free SOT magnetization switching. [ABSTRACT FROM AUTHOR]