arXiv subjects

Roman Khymyn

Publications and source records attributed to Roman Khymyn.

At least 19 recordsLinked to original sources

arXiv2026

Reconfigurable field-free spin Hall nano-oscillators enabled by crystallographic anisotropy in epitaxial Co/Pt

Spin Hall nano-oscillators (SHNOs) are nanoscale microwave sources for wireless communication, neuromorphic computing and oscillator-based Ising machines, but conventional devices require a global magnetic bias. Here we replace this bias through crystallographic anisotropy in epitaxial Co/Pt. Growth of hcp Co with its c-axis in the film plane produces an anisotropy field of about 0.36 T and enables field-free auto-oscillations above 10 GHz in nanoconstriction SHNOs. The active current polarity is selected by the remanent magnetization, providing nonvolatile reconfiguration of the oscillation state. Micro-focused Brillouin light scattering confirms that the nonlinear response is confined to the nanoconstriction region. Lithographic control of the angle between the current and anisotropy axes tunes the excitation threshold and drives two spectral branches from separated modes to a dominant single branch, consistent with mutual synchronization. These results establish epitaxial crystallographic anisotropy as a route to reconfigurable field-free spintronic oscillators and oscillator networks.

cond-mat.mes-hall↗

arXiv2026

Robust stabilization of high-frequency magnetic droplets in W-CoFeB-MgO nanoconstriction spin Hall nano-oscillators

Magnetic droplets are highly nonlinear spin-wave solitons that can be excited in nanoscale spintronic devices with strong perpendicular magnetic anisotropy. Although extensively studied in nanocontact-based spin-torque oscillators, their stabilization in pure spin current-driven devices such as spin Hall nano-oscillators (SHNOs) has remained elusive. Here, we micromagnetically demonstrate the robust stabilization of non-propagating high-frequency droplets in CMOS-compatible W/CoFeB/MgO nanoconstriction SHNOs under oblique magnetic fields. While the constriction geometry gives rise to noncircular droplet shapes in an inhomogeneous effective field landscape, stable droplet modes exhibiting complete core magnetization reversal and pronounced hysteresis are observed. At lower current densities, droplets display breathing oscillations with periodic expansion and contraction, whereas higher drive currents lead to drift, deformation, and the emergence of sidebands around the fundamental frequency. Tuning the strength and orientation of the applied magnetic field alters the effective field landscape, allowing droplets to escape confinement and propagate over distances exceeding 2$μ$m.

cond-mat.mes-hall↗

arXiv2026

Spin-Orbital Hall Nano-Oscillators using PtCr/NiFe

The orbital Hall effect provides a promising route for generating angular-momentum currents beyond conventional spin Hall physics. PtCr alloys exhibit unusually large current-induced torques, but the contribution of orbital transport and the ability of these torques to sustain coherent nonlinear magnetization dynamics remain unresolved. Here we demonstrate spin-orbital Hall nano-oscillators by exploiting a homogeneous heavy-metal/light-metal alloy in which orbital Hall currents generated by Cr are converted by Pt into spin currents, producing giant spin-orbit torques. Using PtCr/NiFe heterostructures, the effective torque efficiency increases from ~0.14 in Pt/NiFe to ~0.40 in Pt0.38Cr0.62/NiFe despite substantial Pt dilution, enabling coherent auto-oscillations with the threshold current density reduced from ~ 1.07 x 10^12 to ~ 4.4 x 10^11 A m^-2. First-principles calculations show that Cr alloying suppresses the intrinsic spin Hall conductivity while enhancing the orbital Hall conductivity, and reproduce the observed torque enhancement only when orbital transport is included. Our combined experimental and first-principles results show that alloy engineering enables giant spin-orbit torques through an intrinsic orbital-mediated contribution, enabling coherent auto-oscillations without engineered multilayers and establishing a scalable materials platform for low-power nonlinear spintronic and orbitronic devices.

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arXiv2026

A 2048-spin bulk acoustic wave Ising machine for number partitioning and Sudoku

Optical coherent Ising machines based on time-multiplexing have demonstrated significant progress in terms of connectivity and spin scalability. However, they are constrained by large physical footprints, high power consumption, poor thermal stability, and high cost. Here, we present a time-multiplexed Ising machine leveraging propagating wave packets in solid-state delay lines at microwave frequencies, enabling thermally stable, robust, low-power, tabletop, and affordable design. We use two serially connected 20.5 MHz, 707 microseconds bulk acoustic wave delay lines supporting 2,048 spins. Our design provides all-to-all connectivity with 15-bit coupling resolution and finds approximate MAX-CUT solutions in 341 ms, potentially scalable to sub-ms by using higher frequency delay lines. Additionally, we demonstrate solutions to number partitioning and Sudoku problems. Compared with state-of-the-art Coherent Ising machines, our machine exhibits four orders of magnitude higher thermal stability. Against the simulated bifurcation algorithm, our design achieves comparable results on the MAX-CUT problem, while outperforming it on the more complex number-partitioning and Sudoku problems.

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arXiv2025

Ultra-large mutually synchronized networks of 10 nm spin Hall nano-oscillators

While mutually interacting spin Hall nano-oscillators (SHNOs) hold great promise for wireless communication, neural networks, neuromorphic computing, and Ising machines, the highest number of synchronized SHNOs remains limited to $N$ = 64. Using ultra-narrow 10 and 20-nm nano-constrictions in W-Ta/CoFeB/MgO trilayers, we demonstrate mutually synchronized SHNO networks of up to $N$ = 105,000. The microwave power and quality factor scale as $N$ with new record values of 9 nW and $1.04 \times 10^6$, respectively. An unexpectedly strong array size dependence of the frequency-current tunability is explained by magnon exchange between nano-constrictions and magnon losses at the array edges, further corroborated by micromagnetic simulations and Brillouin light scattering microscopy. Our results represent a significant step towards viable SHNO network applications in wireless communication and unconventional computing.

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arXiv2024

Mutual synchronization in spin torque and spin Hall nano-oscillators

This chapter reviews the state of the art in mutually synchronized spin-torque and spin Hall nano-oscillator (STNO and SHNO) arrays. After briefly introducing the underlying physics, we discuss different nano-oscillator implementations and their functional properties with respect to frequency range, output power, phase noise, and modulation rates. We then introduce the concepts and the theory of mutual synchronization and discuss the possible coupling mechanisms in spintronic nano-oscillators, such as dipolar, electrical, and spin-wave coupling. We review the experimental literature on mutually synchronized STNOs and SHNOs in one- and two-dimensional arrays and discuss ways to increase the number of mutually synchronized nano-oscillators. Finally, the potential for applications ranging from microwave signal sources/detectors and ultrafast spectrum analyzers to neuromorphic computing elements and Ising machines is discussed together with the specific electronic circuitry that has been designed so far to harness this potential.

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arXiv2024

A frequency tunable low-noise YIG-GGG based oscillator with strong magneto-elastic coupling

We present a frequency tunable magneto-acoustic oscillator (MAO) operating in low-phase-noise and complex dynamical regimes based on a single composite YIG-GGG resonator. The magneto-acoustic resonator (MAR) is based on a YIG (yttrium iron garnet) layer epitaxially grown on a GGG (gadolinium gallium garnet) substrate. By optimizing the YIG thickness, we obtain a high magneto-elastic coupling of around 1 MHz between the ferromagnetic resonance (FMR) in YIG and high overtone acoustic resonances (HBARs) in the YIG-GGG structure in the 1-2 GHz frequency range. It allows to eliminate the need for pre-selectors and bulky circulators, thus simplifying the MAO design while maintaining the possibility to lock to HBAR YIG-GGG modes. With an adjustment in the loop over-amplification parameter, the MAO can be locked either only to high-Q magneto-acoustic HBARs or to both types of resonance including HBARs and the FMR mode of the YIG film. In a low-phase-noise regime, MAO generates only at certain values of the applied field and exhibits discrete frequency tunability with a 3.281 MHz step corresponding to the frequency separation between the adjacent HBAR modes in a YIG-GGG structure. In a complex regime where oscillation conditions expand to include both HBAR and FMR modes, MAO demonstrates continuous generation as the function of the applied field with variable phase noise parameters. Moreover, in low-phase-noise regime, MAO phase noise plot improves by 30 dB compared to the operational regime locked to the pure FMR in YIG which is in agreement with the measured FMR and HBAR Q-factors.

physics.app-ph↗

arXiv2024

Magnetic droplet solitons

Magnetic droplets are nanoscale, non-topological, dynamical solitons that can be nucleated in different spintronic devices, such as spin torque nano-oscillators (STNOs) and spin Hall nano-oscillators (SHNOs). This chapter first briefly discusses the theory of spin current driven dissipative magnetic droplets in ferromagnetic thin films with uniaxial anisotropy. We then thoroughly review the research literature on magnetic droplets and their salient features, as measured using electrical, microwave, and synchrotron techniques, and as envisaged by micromagnetic simulations. We also touch upon a closely related soliton, the dynamical skyrmion. Finally, we present an outlook of new routes in droplet science.

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arXiv2024

Ultra-low-current-density single-layer magnetic Weyl semimetal spin Hall nano-oscillators

Topological quantum materials can exhibit unconventional surface states and anomalous transport properties. Still, their applications in spintronic devices are restricted as they require the growth of high-quality thin films with bulk-like properties. Here, we study 10--30 nm thick epitaxial ferromagnetic Co$_{\rm 2}$MnGa films with high structural order and very high values of the anomalous Hall conductivity, $σ_{\rm xy}=1.35\times10^{5}$ $Ω^{-1} m^{-1}$ and the anomalous Hall angle, $θ_{\rm H}=15.8\%$, both comparable to bulk values. We observe a dramatic crystalline orientation dependence of the Gilbert damping constant of a factor of two and a giant intrinsic spin Hall conductivity, $\mathit{σ_{\rm SHC}}=(6.08\pm 0.02)\times 10^{5}$ ($\hbar/2e$) $Ω^{-1} m^{-1}$, an order of magnitude higher than literature values of multilayer Co$_{\rm 2}$MnGa stacks [1-3] and single-layer Ni, Co, Fe [4], and Ni$_{\rm 80}$Fe$_{\rm 20}$~[4,5]. As a consequence, spin-orbit-torque driven auto-oscillations of a 30 nm thick magnetic film are observed for the first time, at an ultralow threshold current density of $J_{th}=6.2\times10^{11}$ $Am^{-2}$. Theoretical calculations of the intrinsic spin Hall conductivity, originating from a strong Berry curvature, corroborate the results and yield values comparable to the experiment. Our results open up for the design of spintronic devices based on single layers of magnetic topological quantum materials.

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arXiv2024

Spintronic devices as next-generation computation accelerators

The ever increasing demand for computational power combined with the predicted plateau for the miniaturization of existing silicon-based technologies has made the search for low power alternatives an industrial and scientifically engaging problem. In this work, we explore spintronics-based Ising machines as hardware computation accelerators. We start by presenting the physical platforms on which this emerging field is being developed, the different control schemes and the type of algorithms and problems on which these machines outperform conventional computers. We then benchmark these technologies and provide an outlook for future developments and use-cases that can help them get a running start for integration into the next generation of computing devices.

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arXiv2024

Spin wave-driven variable-phase mutual synchronization in spin Hall nano-oscillators

Spin-orbit torque can drive auto-oscillations of propagating spin wave (PSW) modes in nano-constriction spin Hall nano-oscillators (SHNOs). These modes allow both long-range coupling and the potential of controlling its phase -- critical aspect for nano-magnonics, spin wave logic, and Ising machines. Here, we demonstrate PSW-driven variable-phase coupling between two nano-constriction SHNOs and study how their separation and the PSW wave vector impact their mutual synchronization. In addition to ordinary in-phase mutual synchronization, we observe, using both electrical measurements and phase-resolved $μ-$Brillouin Light Scattering microscopy, mutual synchronization with a phase that can be tuned from 0 to $π$ using the drive current or the applied field. Micromagnetic simulations corroborate the experiments and visualize how the PSW patterns in the bridge connecting the two nano-constrictions govern the coupling. These results advance the capabilities of mutually synchronized SHNOs and open up new possibilities for applications in spin wave logic, unconventional computing, and Ising Machines.

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arXiv2023

A 50-spin surface acoustic wave Ising machine

Time-multiplexed Spinwave Ising Machines (SWIMs) have unveiled a route towards miniaturized, low-cost, and low-power solvers of combinatorial optimization problems. While the number of supported spins is limited by the nonlinearity of the spinwave dispersion, other collective excitations, such as surface acoustic waves (SAWs), offer a linear dispersion. Here, we demonstrate an all-to-all, fully FPGA reprogrammable, 50-spin surface acoustic wave-based Ising machine (SAWIM), using a 50-mm-long Lithium Niobate SAW delay line, off-the-shelf microwave components, and a low-cost FPGA. The SAWIM can solve any 50-spin MAX-CUT problem, with arbitrary coupling matrices, in less than 340 $μ$s consuming only 0.62 mJ, corresponding to close to 3000 solutions per second and a figure of merit of 1610 solutions/W/s. We compare the SAWIM computational results with those of a 100-spin optical Coherent Ising machine and find a higher probability of solution. Moreover, we demonstrate that there is an optimum overall coupling strength between spins at which the probability of the exact solution reaches 100%. The SAWIM illustrates the general merits of solid state wave-based time-multiplexed Ising machines in the microwave domain as versatile platforms for commercially feasible high-performance solvers of combinatorial optimization problems.

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arXiv2023

Global biasing using a Hardware-based artificial Zeeman term in Spinwave Ising Machines

A spinwave Ising machine (SWIM) is a newly proposed type of time-multiplexed hardware solver for combinatorial optimization that employs feedback coupling and phase sensitive amplification to map an Ising Hamiltonian into phase-binarized propagating spin-wave RF pulses in an Yttrium-Iron-Garnet (YIG) film. In this work, we increase the mathematical complexity of the SWIM by adding a global Zeeman term to a 4-spin MAX-CUT Hamiltonian using a continuous external electrical signal with the same frequency as the spin pulses and phase locked with with one of the two possible states. We are able to induce ferromagnetic ordering in both directions of the spin states despite antiferromagnetic pairwise coupling. Embedding a planar antiferromagnetic spin system in a magnetic field has been proven to increase the complexity of the graph associated to its Hamiltonian and thus this straightforward implementation helps explore higher degrees of complexity in this evolving solver.

cond-mat.mes-hall↗

arXiv2023

Phase noise analysis of mutually synchronized spin Hall nano-oscillators

The reduction of phase noise in electronic systems is of utmost importance in modern communication and signal processing applications and requires an understanding of the underlying physical processes. Here, we systematically study the phase noise in mutually synchronized chains of nano-constriction spin Hall nano-oscillators (SHNOs). We find that longer chains have improved phase noise figures at low offset frequencies (1/f noise), where chains of two and ten mutually synchronized SHNOs have 2.8 and 6.2 dB lower phase noise than single SHNOs. This is close to the theoretical values of 3 and 10 dB, and the deviation is ascribed to process variations between nano-constrictions. However, at higher offset frequencies (thermal noise), the phase noise unexpectedly increases with chain length, which we ascribe to process variations, a higher operating temperature in the long chains at the same drive current and phase delays in the coupling between nano-constrictions.

cond-mat.mes-hall↗

arXiv2023

Robust mutual synchronization in long spin Hall nano-oscillator chains

Mutual synchronization of N serially connected spintronic nano-oscillators increases their coherence by a factor $N$ and their output power by $N^2$. Increasing the number of mutually synchronized nano-oscillators in chains is hence of great importance for better signal quality and also for emerging applications such as oscillator-based neuromorphic computing and Ising machines where larger N can tackle larger problems. Here we fabricate spin Hall nano-oscillator chains of up to 50 serially connected nano-constrictions in W/NiFe, W/CoFeB/MgO, and NiFe/Pt stacks and demonstrate robust and complete mutual synchronization of up to 21 nano-constrictions, reaching linewidths of below 200 kHz and quality factors beyond 79,000, while operating at 10 GHz. We also find a square increase in the peak power with the increasing number of mutually synchronized oscillators, resulting in a factor of 400 higher peak power in long chains compared to individual nano-constrictions. Although chains longer than 21 nano-constrictions also show complete mutual synchronization, it is not as robust and their signal quality does not improve as much as they prefer to break up into partially synchronized states. The low current and low field operation of these oscillators along with their wide frequency tunability (2-28 GHz) with both current and magnetic fields, make them ideal candidates for on-chip GHz-range applications and neuromorphic computing.

cond-mat.mes-hall↗

arXiv2022

Voltage control of frequency, effective damping and threshold current in nano-constriction-based spin Hall nano-oscillators

Using micromagnetic simulations, we study the interplay between strongly voltage-controlled magnetic anisotropy (VCMA), $ΔK = \pm$200 kJ/m$^3$, and gate width, $w=$ 10--400 nm, in voltage-gated W/CoFeB/MgO based nano-constriction spin Hall nano-oscillators. The VCMA modifies the local magnetic properties such that the magnetodynamics transitions between regimes of \emph{i}) confinement, \emph{ii}) tuning, and \emph{iii}) separation, with qualitatively different behavior. We find that the strongest tuning is achieved for gate widths of the same size as the the constriction width, for which the effective damping can be increased an order of magnitude compared to its intrinsic value. As a consequence, voltage control remains efficient over a very large frequency range, and subsequent manufacturing advances could allow SHNOs to be easily integrated into next-generation electronics for further fundamental studies and industrial applications.

cond-mat.mes-hall↗

arXiv2022

A spinwave Ising machine

We demonstrate a spin-wave-based time-multiplexed Ising Machine (SWIM), implemented using a 5 $μ$m thick Yttrium Iron Garnet (YIG) film and off-the-shelf microwave components. The artificial Ising spins consist of 34--68 ns long 3.125 GHz spinwave RF pulses with their phase binarized using a phase-sensitive microwave amplifier. Thanks to the very low spinwave group velocity, the 7 mm long YIG waveguide can host an 8-spin MAX-CUT problem and solve it in less than 4 $μ$s while consuming only 7 $μ$J. Using a real-time oscilloscope, we follow the temporal evolution of each spin as the SWIM minimizes its energy and find both uniform and domain-propagation-like switching of the spin state. The SWIM has the potential for substantial further miniaturization, scalability, speed, and reduced power consumption, and may become a versatile platform for commercially feasible optimization problem solvers with high performance.

cond-mat.mes-hall↗

arXiv2022

Optothermal control of spin Hall nano-oscillators

We investigate the impact of localized laser heating on the auto-oscillation properties of a 170 nm wide nano-constriction spin Hall nano-oscillators (SHNO) fabricated from a NiFe/Pt bilayer on a sapphire substrate. A 532 nm continuous wave laser is focused down to a spot size of about 500 nm at a power ranging from 0 to 12 mW. Through a comparison with resistive heating, we estimate a local temperature rise of about 8 K/mW. We demonstrate reversible laser tuning of the threshold current, the frequency, and the peak power, and find that the SHNO frequency can be tuned by up to 350 MHz, which is over three times more than the current tuning alone. Increasing the temperature also results in increased signal jitter, an increased threshold current, and a reduced maximum current for auto-oscillations. Our results open up for optical control of single SHNOs in larger SHNO networks without the need for additional voltage gates.

physics.app-ph↗