Search

Spin Hall nano-oscillators (SHNOs) are emerging spintronic devices for microwave signal generation and oscillator-based neuromorphic computing combining nano-scale footprint, fast and ultra-wide microwave frequency tunability, CMOS compatibility, and strong non-linear properties providing robust large-scale mutual synchronization in chains and two-dimensional arrays. While SHNOs can be tuned via magnetic fields and the drive current, neither approach is conducive to individual SHNO control in large arrays. Here, we demonstrate electrically gated W/CoFeB/MgO nano-constrictions in which the voltage-dependent perpendicular magnetic anisotropy tunes the frequency and, thanks to nano-constriction geometry, drastically modifies the spin-wave localization in the constriction region resulting in a giant 42% variation of the effective damping over four volts. As a consequence, the SHNO threshold current can be strongly tuned. Our demonstration adds key functionality to nano-constriction SHNOs and paves the way for energy-efficient control of individual oscillators in SHNO chains and arrays for neuromorphic computing., Spin Hall nano-oscillators can be tuned via magnetic fields and the drive current, but individual oscillator control in large arrays remains a challenge. Here, the authors provide individual control of the threshold current and the auto-oscillation frequency by voltage-controlled magnetic anisotropy.

We demonstrate an optimized fabrication process for electric field (voltage gate) controlled nano-constriction spin Hall nano-oscillators (SHNOs), achieving feature sizes of x. The optimized tilted etching process reduces sidewalls by 75% compared to no tilting. Moreover, the HfOx encapsulation avoids any sidewall shunting and improves gate breakdown. Our experimental results on W/CoFeB/MgO/SiO2 SHNOs show significant frequency tunability (6 MHz/V) even for moderate perpendicular magnetic anisotropy. Circular patterns with diameter of 45 nm are achieved with an aspect ratio better than 0.85 for 80% of the population. The optimized fabrication process allows incorporating a large number of individual gates to interface to SHNO arrays for unconventional computing and densely packed spintronic neural networks., 8 Pages, 5 Figures

In spin Hall nano-oscillators (SHNOs), pure spin currents drive local regions of magnetic films and nanostructures into auto-oscillating precession. If such regions are placed in close proximity to each other they can interact and may mutually synchronize. Here, we demonstrate robust mutual synchronization of two-dimensional SHNO arrays ranging from 2 × 2 to 8 × 8 nano-constrictions, observed both electrically and using micro-Brillouin light scattering microscopy. On short time scales, where the auto-oscillation linewidth $$\Delta f$$ is governed by white noise, the signal quality factor, $$Q=f/\Delta f$$, increases linearly with the number of mutually synchronized nano-constrictions (N), reaching 170,000 in the largest arrays. We also show that SHNO arrays exposed to two independently tuned microwave frequencies exhibit the same synchronization maps as can be used for neuromorphic vowel recognition. Our demonstrations may hence enable the use of SHNO arrays in two-dimensional oscillator networks for high-quality microwave signal generation and ultra-fast neuromorphic computing. Synchronization of oscillators can be used to carry out cognitive tasks. Large two-dimensional arrays of synchronized spin Hall nano-oscillators have now been demonstrated, and may in future enable neuromorphic computing on the nanoscale.

Metal-assisted chemical etching process is exploited to realise deep-etched silicon structures. Gold as the noble metal, hydrogen peroxide and hydrofluoric acid solutions are used to achieve deep vertical structures. By controlling the solution concentrations, thickness and morphology of the deposited metal, several hundred micrometre-sized silicon structures can be achieved. This method, upon achieving more controllability and repeatability, can be a good substitute for dry etching, due to its high etch-rate, low-cost materials and non-requirement to complex equipment. In this work, the effect of different etching parameters on the etching process is studied to gain more control over the etching conditions.

Ferromagnetic materials dominate as the magnetically active element in spintronic devices, but come with drawbacks such as large stray fields, and low operational frequencies. Compensated ferrimagnets provide an alternative as they combine the ultrafast magnetization dynamics of antiferromagnets with a ferromagnet-like spin-orbit-torque (SOT) behavior. However to use ferrimagnets in spintronic devices their advantageous properties must be retained also in ultrathin films (t < 10 nm). In this study, ferrimagnetic Gdx(Fe87.5Co12.5)1-x thin films in the thickness range t = 2-20 nm were grown on high resistance Si(100) substrates and studied using broadband ferromagnetic resonance measurements at room temperature. By tuning their stoichiometry, a nearly compensated behavior is observed in 2 nm Gdx(Fe87.5Co12.5)1-x ultrathin films for the first time, with an effective magnetization of Meff = 0.02 T and a low effective Gilbert damping constant of {\alpha} = 0.0078, comparable to the lowest values reported so far in 30 nm films. These results show great promise for the development of ultrafast and energy efficient ferrimagnetic spintronic devices., Comment: 7 Pages, 4 Figures

Synchronization of large spin Hall nano-oscillator (SHNO) arrays is an appealing approach toward ultrafast non-conventional computing. However, interfacing to the array, tuning its individual oscillators and providing built-in memory units remain substantial challenges. Here, we address these challenges using memristive gating of W/CoFeB/MgO/AlOx-based SHNOs. In its high resistance state, the memristor modulates the perpendicular magnetic anisotropy at the CoFeB/MgO interface by the applied electric field. In its low resistance state the memristor adds or subtracts current to the SHNO drive. Both electric field and current control affect the SHNO auto-oscillation mode and frequency, allowing us to reversibly turn on/off mutual synchronization in chains of four SHNOs. We also demonstrate that two individually controlled memristors can be used to tune a four-SHNO chain into differently synchronized states. Memristor gating is therefore an efficient approach to input, tune and store the state of SHNO arrays for non-conventional computing models.

A single nanoconstriction spin-Hall nano-oscillator (NC-SHNO) in out-of-plane fields is presented as a nonlinear amplitude and frequency modulator operated by radio-frequency (RF) current modulation. The current modulation was carried out in different NC-SHNO nonlinearity regimes corresponding to negative, zero, and positive values of $df/dI$ in order to investigate the device response to an 80 MHz modulating current. Our study shows that current modulation of SHNOs can be quantitatively predicted by a nonlinear frequency and amplitude modulation (NFAM) model using the values of $df/dI$ and $d^2f/dI^2$ extracted from the free-running frequency $f$ versus current $I$ profile. The NFAM model reproduces the asymmetric sideband amplitude as well as the red and blue shift of the frequency in excellent agreement with the experimental results. The ability to predict the modulation process is a necessary benchmark in designing SHNO modulators for future integrated microwave circuits.

Propagating spin waves generated by a metal-based spin Hall nano-oscillator for highly energy-efficient spin wave technology., Spin-orbit torque (SOT) can drive sustained spin wave (SW) auto-oscillations in a class of emerging microwave devices known as spin Hall nano-oscillators (SHNOs), which have highly nonlinear properties governing robust mutual synchronization at frequencies directly amenable to high-speed neuromorphic computing. However, all demonstrations have relied on localized SW modes interacting through dipolar coupling and/or direct exchange. As nanomagnonics requires propagating SWs for data transfer and additional computational functionality can be achieved using SW interference, SOT-driven propagating SWs would be highly advantageous. Here, we demonstrate how perpendicular magnetic anisotropy can raise the frequency of SOT-driven auto-oscillations in magnetic nanoconstrictions well above the SW gap, resulting in the efficient generation of field and current tunable propagating SWs. Our demonstration greatly extends the functionality and design freedom of SHNOs, enabling long-range SOT-driven SW propagation for nanomagnonics, SW logic, and neuromorphic computing, directly compatible with CMOS technology.

Emerging spin-torque nano oscillators (STNOs) and spin-Hall nano oscillators (SHNOs) are potential candidates for microwave applications. Recent advances in three-terminal magnetic tunnel junction (MTJ)-based SHNOs opened the possibility to develop more reliable and well-controlled oscillators, thanks to individual spin Hall-driven precession excitation and read-out paths. To develop hybrid systems by integrating three-terminal SHNOs and CMOS circuits, an electrical model able to capture the analog characteristics of three-terminal SHNOs is needed. This model needs to be compatible with current electric design automation (EDA) tools. This work presents a comprehensive macrospin-based model of three-terminal SHNOs able to describe the dc operating point, frequency modulation, phase noise, and output power. Moreover, the effect of voltage-controlled magnetic anisotropy (VCMA) is included. The model shows good agreement with experimental measurements and could be used in developing hybrid three-terminal SHNO/CMOS systems. QC 20190930

We study the current tunable microwave signal properties of nano-constriction based spin Hall nano-oscillators (SHNOs) in oblique magnetic fields as a function of the nano-constriction width, $w=$~50--140 nm. The threshold current is found to scale linearly with $w$, defining a constant threshold current density of $J_{th}=$ 1.7 $\times$ 10$^{8}$ A/cm$^2$. While the current dependence of the microwave frequency shows the same generic non-monotonic behavior for all $w\geqslant$ 80 nm, the quality of the generated microwave signal improves strongly with $w$, showing a linear $w$ dependence for both the total power and the linewidth. As a consequence, the peak power for a 140 nm nano-constriction is about an order of magnitude higher than that of a 80 nm nano-constriction. The smallest nano-constriction, $w=$ 50 nm, exhibits a different behavior with a higher power and a worse linewidth indicating a crossover into a qualitatively different narrow-constriction regime.

We demonstrate an approach for improving the spectral linewidth of a spin torque nano-oscillator (STNO). Using He + ion irradiation, we tune the perpendicular magnetic anisotropy (PMA) of the STNO free layer such that its easy axis is gradually varied from strongly out-of-plane to moderate in-plane. As the PMA impacts the non-linearity N of the STNO, we can, in this way, control the threshold current, the current tunability of the frequency, and, in particular, the STNO linewidth, which dramatically improves by two orders of magnitude. Our results are in good agreement with the theory for nonlinear auto-oscillators, confirm theoretical predictions of the role of the nonlinearity, and demonstrate a straightforward path toward improving the microwave properties of STNOs.

We investigate the effect of hafnium (Hf) dusting on the magnetodynamical properties of NiFe/Pt bilayers using spin-torque-induced ferromagnetic resonance measurements on 6 μm wide microstrips on high-resistive Si substrates. Based on two series of NiFe(tNiFe)/Hf(tHf)/Pt(5) stacks, we first demonstrate that the zero-current magnetodynamic properties of the devices benefit from Hf dusting: (i) the effective magnetization of the NiFe layer increases by 4%–8% with Hf present and (ii) the damping α decreases linearly with tHf by up to 40%. The weaker anisotropic magnetoresistance (AMR≈0.3%–0.4%) of the 3 nm NiFe series is largely unaffected by the Hf, while the stronger AMR of the 5 nm NiFe series drops from 0.7% to 0.43% with increasing tHf. We find that the spin Hall efficiency ξSH is independent of the NiFe thickness, remaining unaffected (ξSH = 0.115) up to tHf = 0.4 nm and then decreasing linearly for higher tHf. The different trends of α and ξSH suggest that there is an optimum Hf thickness (≈0.4 nm) for which the threshold current for auto-oscillation should have a minimum, while the much lower damping should improve mutual synchronization. Our results also indicate that the spin-orbit torque is entirely damping-like with no field-like torque component. Finally, the internal spin Hall angle of Pt is estimated to be θSH = 0.22 by calculating the transparency of the interface. QC 20180911

We demonstrate low-operational-current W/Co$_{20}$Fe$_{60}$B$_{20}$/MgO spin Hall nano-oscillators (SHNOs) on highly resistive silicon (HiR-Si) substrates. Thanks to a record high spin Hall angle of the $\beta$-phase W ($\theta_{SH}$ = -0.53), a very low threshold current density of 3.3 $\times$ 10$^{7}$ A/cm$^2$ can be achieved. Together with their very wide frequency tunability (7-28 GHz), promoted by a moderate perpendicular magnetic anisotropy, this makes HiR-Si/W/CoFeB based SHNOs potential candidates for wide-band microwave signal generation. Their CMOS compatibility offers a promising route towards the integration of spintronic microwave devices with other on-chip semiconductor microwave components., Comment: 6 pages, 4 figures

We present the first experimental study for enhancement of PtSi Schottky detectors using photonic crystal (PC) structures. PCs can be used for simultaneous reduction of reflection and increase of absorption. The 2-D PCs are fabricated by interference lithography and reactive ion etching. In PC with 4.2- \(\mu \) m depth, the average responsivity is enhanced by a factor of \(\sim 7\) with respect to regular detectors. We show that the light absorption enhancement is not sufficient to explain efficiency enhancement. The extra enhancement may be due to nanoscale roughness on the PC walls that affect the carrier collection efficiency and cutoff wavelength.

A nanoporous tapered silicon (Si) photonic crystal (PC) is realized. The PCs with this structure, which may be called hybrid PC-porous can significantly reduce the surface reflection over the broad wavelength range of 400–2000 nm. Moreover, the absorption enhances in this structure significantly. The PCs are fabricated by interference lithography and then nanoporous structure is applied on it using metal assisted chemical etching. The measured reflectance and absorption across a spectral range of 400–2000 nm are, approximately 3% and 96%, respectively. The improvement on the reflectance and absorption are about 90% and 70% compared to bare Si respectively; which is promising in the utilization of this structure for various applications.

We present a two-dimensional nanodot-array single-electron transistor (2DA-SET) analytical current–voltage model on the basis of master equation and the orthodox theory. Electrical characteristics of these multi island single-electron transistors, which composed of several islands up with to 5 × 5 nanodot array, have been investigated using SIMON simulator to show the temperature dependence of the Coulomb oscillation in 2DA-SET by varying gate voltage in the temperature range from T = 5 to 50 K. In addition, the effects of self-island capacitance and inter-dot spacing variation on I–V characteristics of the device under study have been inspected. It illustrates that a proportional increase in current value resulting from higher temperature values and different distribution of nanodot capacitance will be suppressed by the capacitance of whole nanodot-array network in arrays with multiple elements. The 2DA-SET device behaves as a single-island SET device regarding high drain voltage. This is probably because the network of islands is electrically enlarged and merges into a single island owing to the high applied voltage. Finally, we compare the advantages of 2DA-SET face to single-island SET.

The paper describes how pore size and geometry of porous silicon can affect the electrical and optical properties of PtSi/Porous Si Schottky barrier detectors. In this study, two techniques used to prepare different porosities on silicon substrate which includes: electrochemical etching process and electroless metal-assisted chemical etching. Surface morphology and geometric properties of PtSi/Porous Si have been characterized using scanning electron microscopy (SEM) and also I–V characteristics of detectors at reverse bias performed at cryogenic temperature 77 K; between 1–5 μm for IR spectroscopy measurements. It was found that by reducing the overall size of pores from micrometric to nanometric size, the external quantum efficiency increases up to 80% for 1 μm radiation and the wavelength of maximum responsivity moves toward the lower IR spectra in 77 K. Furthermore, the produced micro metric pores using electrochemical porosification, show more constant responsivity than nanometric one's which obtained from metal-assisted chemical etching.

A method for selective formation of reproducible, high fidelity and controllable nano and micrometer size porous Si areas over n-type Si wafers is provided. A 400 nm thick Silicon Nitride layer was used as the mask layer while Platinum and Palladium nanoparticles were deposited over the unprotected areas to obtain porous areas through metal assisted chemical etching process. Nanoparticles were deposited by electroless plating solutions containing H 2 PtCl 6 and PdCl 2 . Good controls over pore size and depth were obtained with well defined and sharp edges of the patterned areas. The results were compared to porous structures obtained via electrochemical etching process, indicating the superiority of metal assisted etching in terms of its simplicity as well as the ability of Silicon Nitride layer acting as the mask layer.

One of the most fundamental features of digital image and the basic steps in image processing, analysis, pattern recognition and computer vision is the Edge of image where the preciseness and reliability of its results will affect directly the comprehension machine system made for objective world. Several edge detectors have been developed in the past decades, although no single edge detectors have been developed satisfactorily enough for all application. In this paper, a new edge detection technique is proposed basis on the BP neural network. Here, it is classified the edge patterns of binary images into 16 possible types of visual patterns. In the following, After training the pre-defined edge patterns, the BP neural network is applied to correspond any type of edges with its related visual pattern. The results demonstrated that the new proposed technique provides the better results compared with traditional edge detection techniques while improved the computations complexity.

In this paper, a newfound and simple silicon bulk micromachining process based on metal-assisted chemical etching (MaCE) is proposed which opens a whole new field of research in MEMS technology. This method is anisotropic and by controlling the etching parameters, deep vertical etching, relative to substrate surface, can be achieved in micrometer size for 〈1 0 0〉 oriented Si wafer. By utilizing gold as a catalyst and a photoresist layer as the single mask layer for etching, 60 µm deep gyroscope micromachined structures have been fabricated for 2 µm features. The results indicate that MaCE could be the only wet etching method comparable to conventional dry etching recipes in terms of achievable etch rate, aspect ratio, verticality and side wall roughness. It also does not need a vacuum chamber and the other costly instruments associated with dry etching techniques.

We combined photonic crystal (PC) and nanoporous structures in silicon (Si) to decrease the reflectance of the Si surface. Due to gradual increase of effective refractive index in this structure, reflection is reduced drastically over a broad spectral range. A two-dimensional Si PC was fabricated by interference lithography, then a nanoporous structure was made on the PC by using metal-assisted chemical etching. The obtained reflectance is about 3% across a spectral range of 400 to 2000 nm. This indicates an improvement of reflectance up to 90% compared to bare Si.

A simple and efficient method for selective formation of porous Si areas using regular photoresist as a masking layer is presented. Such a simple masking layer is sufficient to create a wide range of porosity levels through metal-assisted chemical etching (MaCE) using platinum, palladium and silver nanoparticles. Reproducible porous areas with a minimum feature size of 5 ?m were produced on Si wafers. The pore size and height are the functions of the etching time and type of nanoparticles. Using Ag nanopaticles we have been able to obtain Si nanowires of about 30 ?m height. Based on these results, a combination of positive photoresist and MaCE seems to be a reliable way for micron and submicron patterning of nanowire and porous areas on Si wafers, which is simple, inexpensive and compatible with integrated circuit fabrication techniques.

Spin-Hall-effect nano-oscillators are promising beyond the CMOS devices currently available, and can potentially be used to emulate the functioning of neurons in computational neuromorphic systems. As they oscillate in the 4–20 GHz range, they could potentially be used for building highly accelerated neural hardware platforms. However, due to their extremely low signal level and high impedance at their output, as well as their microwave-range operating frequency, discerning whether the SHNO is oscillating or not carries a great challenge when its state read-out circuit is implemented using CMOS technologies. This paper presents the first CMOS front-end read-out circuitry, implemented in 180 nm, working at a SHNO oscillation frequency up to 4.7 GHz, managing to discern SHNO amplitudes of 100 µV even for an impedance as large as 300 Ω and a noise figure of 5.3 dB300 Ω. A design flow of this front end is presented, as well as the architecture of each of its blocks. The study of the low-noise amplifier is deepened for its intrinsic difficulties in the design, satisfying the characteristics of SHNOs.

Combinatorial optimization problems are known for being particularly hard to solve on traditional von Neumann architectures. This has led to the development of Ising Machines (IMs) based on quantum annealers and optical and electronic oscillators, demonstrating speed-ups compared to central processing unit (CPU) and graphics processing unit (GPU) algorithms. Spin torque nano-oscillators (STNOs) have shown GHz operating frequency, nanoscale size, and nanosecond turn-on time, which would allow their use in ultrafast oscillator-based IMs. Here, we show using numerical simulations based on STNO auto-oscillator theory that STNOs exhibit fundamental characteristics needed to realize IMs, including in-phase/out-of-phase synchronization and second harmonic injection locking phase binarization. Furthermore, we demonstrate numerically that large STNO network IMs can solve Max-Cut problems on nanosecond timescales.