Showing 1–49 of 49 results for author: Khymyn, R

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

Authors: Hind Prakash, Arunima TM, Roman Khymyn, Himanshu Fulara

Abstract: 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 robu… ▽ More 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. △ Less

Submitted 18 August, 2026; originally announced August 2026.

Spin-Orbital Hall Nano-Oscillators using PtCr/NiFe

Authors: Utkarsh Shashank, Akash Kumar, Daegeun Jo, Thi Ngoc Anh Nguyen, Jong-Guk Choi, Sambit Ghosh, Michal Strach, Lunjie Zeng, Andrew B. Yankovich, Roman Khymyn, Ahmad A. Awad, Eva Olsson, Peter M. Oppeneer, Johan Åkerman

Abstract: 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 exp… ▽ More 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. △ Less

Submitted 9 August, 2026; originally announced August 2026.

Comments: 20 pages, 4 figures

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

Authors: Venkatesh Vadde, Roman Ovcharov, Victor H. González, Roman Khymyn, Artem Litvinenko, Johan Åkerman

Abstract: 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 freque… ▽ More 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. △ Less

Submitted 2 July, 2026; originally announced July 2026.

Mutual synchronization of two asymmetric-nano-constriction-based spin-Hall nano-oscillators

Authors: Roman V. Ovcharov, Roman S. Khymyn, Akash Kumar, Johan Åkerman

Abstract: We propose an asymmetric-nanoconstriction (ANC) design of spin-Hall nano-oscillators (SHNOs) and investigate mutual synchronization of a pair of such devices using micromagnetic simulations. The ANC geometry enables strong dipolar coupling at sub-50 nm separations while preserving independent current bias for each oscillator. We first characterize the auto-oscillation of a single ANC-SHNO, reveali… ▽ More We propose an asymmetric-nanoconstriction (ANC) design of spin-Hall nano-oscillators (SHNOs) and investigate mutual synchronization of a pair of such devices using micromagnetic simulations. The ANC geometry enables strong dipolar coupling at sub-50 nm separations while preserving independent current bias for each oscillator. We first characterize the auto-oscillation of a single ANC-SHNO, revealing a broad frequency tuning range and a field-controlled crossover between negative and positive nonlinearities. We then demonstrate that two such oscillators can mutually synchronize solely via dipolar stray fields, without electrical or spin-wave coupling. Depending on the bias conditions, the coupled pair exhibits robust in-phase (0°) or out-of-phase (180°) locking. Notably, we find a bias-dependent amplitude correlation: when the oscillators sustain comparable amplitudes, both in-phase and out-of-phase synchronization are accessible, whereas amplitude imbalance drives the system into an out-of-phase state accompanied by suppression of the weaker oscillator. By combining strong conservative coupling with independent frequency and gain control, the ANC-SHNO platform provides a scalable route toward phased oscillator arrays, neuromorphic computing architectures, and experimental exploration of non-Hermitian spintronic dynamics. △ Less

Submitted 15 September, 2025; originally announced September 2025.

Comments: 7 pages, 5 figures

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

Authors: Nilamani Behera, Avinash Kumar Chaurasiya, Akash Kumar, Roman Khymyn, Artem Litvinenko, Lakhan Bainsla, Ahmad A. Awad, Johan Åkerman

Abstract: 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 m… ▽ More 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. △ Less

Submitted 30 January, 2025; originally announced January 2025.

Comments: 15 pages

Analytical modelling for the frequency behavior of two distinct modes in nano-constriction spin Hall nano-oscillator

Authors: H. Ghanatian, M. Rajabali, R. Khymyn, A. Kumar, V. H. González, H. Farkhani, J. Åkerman, F. Moradi

Abstract: Nano-constriction spin-Hall nano-oscillators (NC-SHNOs) have garnered considerable interest due to their potential use as efficient and adjustable nano-sized sources of microwave signals, with high-frequency tunability, adaptable design layout, and CMOS compatibility. In order to facilitate system- and circuit-level designs based on the NC-SHNOs, it is essential to have an analytical model capable… ▽ More Nano-constriction spin-Hall nano-oscillators (NC-SHNOs) have garnered considerable interest due to their potential use as efficient and adjustable nano-sized sources of microwave signals, with high-frequency tunability, adaptable design layout, and CMOS compatibility. In order to facilitate system- and circuit-level designs based on the NC-SHNOs, it is essential to have an analytical model capable of predicting the behavior of the NC-SHNO. In this paper, we introduce an analytical model to describe the frequency behavior of a single NC-SHNO in an in-plane magnetic field while considering the Oersted field. The model is divided into two regions based on the direct current value: the "linear-like" and "bullet" modes. Each region is characterized by distinct concepts and equations. The first region, the "linear-like mode," emerges from the nano-constriction edges and progresses toward the center of the active area of the NC-SHNO. In contrast, the second regime, the localized "bullet mode," exhibits negative nonlinearity, where increasing the current will lead to a decrease in frequency. The model's validity is confirmed through experimental data obtained from electrical RF measurements on a single 180nm wide NC-SHNO, and the model demonstrates excellent agreement with experimental data. △ Less

Submitted 3 February, 2025; v1 submitted 12 December, 2024; originally announced December 2024.

Comments: 5 pages, 4 figures

Instability in domain wall dynamics in almost compensated ferrimagnets

Authors: R. V. Ovcharov, B. A. Ivanov, E. G. Galkina, J. Åkerman, R. S. Khymyn

Abstract: Nanoscale self-localized topological spin textures, such as domain walls and skyrmions, are of interest for the fundamental physics of magnets and spintronics applications. Ferrimagnets (FiMs), in the region close to the angular momentum compensation point, are promising materials because of their ultrafast spin dynamics at nonzero magnetization. In this work, we study specific features of the FiM… ▽ More Nanoscale self-localized topological spin textures, such as domain walls and skyrmions, are of interest for the fundamental physics of magnets and spintronics applications. Ferrimagnets (FiMs), in the region close to the angular momentum compensation point, are promising materials because of their ultrafast spin dynamics at nonzero magnetization. In this work, we study specific features of the FiM domain wall (DW) dynamics, which are absent in both ferromagnets (FM) and antiferromagnets (AFM). In low-damping FMs and AFMs ($α\ll 1$), the non-stationary forced motion of DWs is characterized by slow ($t_{diss}\propto 1/α$) changes of the DW's velocity and internal structure for all accepted values of the DW energy $E$ and its linear momentum $P$ -- a consequence of the stability of DWs for any value of $P$. In contrast, the dispersion law of FiM DWs has specific points, $P=P_{cr}$ and $E_{cr}=E(P_{cr})$, such that stable DWs are only present for $P<P_{cr}$, i.e., $P_{cr}$ and $E_{cr}$ act as endpoints in the $E(P)$ dependence. We show that when a field-like torque driven DW reaches this endpoint, it falls into a highly-non-equilibrium state with the excitation of fast ($t \ll t_{diss}$) and highly-nonlinear intra-wall magnetization dynamics, covering a wide frequency range up until the frequencies of propagating spin waves. The domain wall finally throws off an "excessive" energy by a short "burst" of the propagating spin waves and returns to the stationary state; the full picture of the forced motion is a periodic repetition of such "explosive" events. △ Less

Submitted 28 October, 2024; v1 submitted 10 October, 2024; originally announced October 2024.

Comments: 6 pages, 3 figures; supplementary material 3 pages

A numerical model for time-multiplexed Ising machines based on delay-line oscillators

Authors: Roman V. Ovcharov, Victor H. González, Artem Litvinenko, Johan Åkerman, Roman S. Khymyn

Abstract: Ising machines (IM) have recently been proposed as unconventional hardware-based computation accelerators for solving NP-hard problems. In this work, we present a model for a time-multiplexed IM based on the nonlinear oscillations in a delay line-based resonator and numerically study the effects that the circuit parameters, specifically the compression gain $β_r$ and frequency nonlinearity $β_i$,… ▽ More Ising machines (IM) have recently been proposed as unconventional hardware-based computation accelerators for solving NP-hard problems. In this work, we present a model for a time-multiplexed IM based on the nonlinear oscillations in a delay line-based resonator and numerically study the effects that the circuit parameters, specifically the compression gain $β_r$ and frequency nonlinearity $β_i$, have on the IM solutions. We find that the likelihood of reaching the global minimum -- the global minimum probability (GMP) -- is the highest for a certain range of $β_r$ and $β_i$ located near the edge of the synchronization region of the oscillators. The optimal range remains unchanged for all tested coupling topologies and network connections. We also observe a sharp transition line in the ($β_i, β_r$) space above which the GMP falls to zero. In all cases, small variations in the natural frequency of the oscillators do not modify the results, allowing us to extend this model to realistic systems. △ Less

Submitted 11 June, 2024; originally announced June 2024.

Comments: 7 pages, 5 figures; supplementary material 2 pages

Optical heterodyne microscopy of operating spin Hall nano-oscillator arrays

Authors: A. Alemán, A. A. Awad, S. Muralidhar, R. Khymyn, A. Kumar, A. Houshang, D. Hanstorp, J. Åkerman

Abstract: Optical heterodyne detection is a powerful technique for characterizing a wide range of physical excitations. Here, we use two types of optical heterodyne detection techniques (fundamental and parametric pumping) to microscopically characterize the high-frequency auto-oscillations of single and multiple nano-constriction spin Hall nano-oscillators (SHNOs). To validate the technique and demonstrate… ▽ More Optical heterodyne detection is a powerful technique for characterizing a wide range of physical excitations. Here, we use two types of optical heterodyne detection techniques (fundamental and parametric pumping) to microscopically characterize the high-frequency auto-oscillations of single and multiple nano-constriction spin Hall nano-oscillators (SHNOs). To validate the technique and demonstrate its robustness, we study SHNOs made from two different material stacks, NiFe/Pt and W/CoFeB/MgO, and investigate the influence of both the RF injection power and the laser power on the measurements, comparing the optical results to conventional electrical measurements. To demonstrate the key features of direct, non-invasive, submicron, spatial, and phase-resolved characterization of the SHNO magnetodynamics, we map out the auto-oscillation magnitude and phase of two phase-binarized SHNOs used in Ising Machines. This proof-of-concept platform establishes a strong foundation for further extensions, contributing to the ongoing development of crucial characterization techniques for emerging computing technologies based on spintronics devices △ Less

Submitted 3 June, 2024; originally announced June 2024.

Comments: 7 pages

Emission of fast-propagating spin waves by an antiferromagnetic domain wall driven by spin current

Authors: Roman V. Ovcharov, B. A. Ivanov, Johan Åkerman, Roman S. Khymyn

Abstract: Antiferromagnets (AFMs) have great benefits for spintronic applications such as high frequencies (up to THz), high speeds (up to tens of km/s) of magnetic excitations, and field-free operation. Advanced devices will require high-speed propagating spin waves (SWs) as signal carriers, i.e., SWs with high k-vectors, the excitation of which remains challenging. We show that a domain wall (DW) in aniso… ▽ More Antiferromagnets (AFMs) have great benefits for spintronic applications such as high frequencies (up to THz), high speeds (up to tens of km/s) of magnetic excitations, and field-free operation. Advanced devices will require high-speed propagating spin waves (SWs) as signal carriers, i.e., SWs with high k-vectors, the excitation of which remains challenging. We show that a domain wall (DW) in anisotropic AFM driven by the spin current can be a source of such propagating SWs with high frequencies and group velocities. In the proposed generator, the spin current, with polarization directed along the easy anisotropy axis, excites the precession of the Néel vector within the DW. The threshold current is defined by the value of the anisotropy in the hard plane, and the frequency of the DW precession is tuneable by the strength of the spin current. We show that the above precession of spins inside the DW leads to robust emission of high-frequency propagating SWs into the AFM strip with very short wavelengths comparable to the exchange length, which is hard to achieve by any other method. △ Less

Submitted 20 March, 2024; originally announced March 2024.

Comments: 6 pages, 4 figures; supplementary material 4 pages

Spintronic devices as next-generation computation accelerators

Authors: Victor H. González, Artem Litvinenko, Akash Kumar, Roman Khymyn, Johan Åkerman

Abstract: 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 t… ▽ More 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. △ Less

Submitted 20 March, 2024; originally announced March 2024.

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

Authors: Akash Kumar, Avinash kumar Chaurasiya, Victor H. González, Nilamani Behera, Roman Khymyn, Ahmad A. Awad, Johan Åkerman

Abstract: 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 st… ▽ More 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. △ Less

Submitted 1 February, 2024; originally announced February 2024.

Comments: 19 pages

Journal ref: Nature Physics, 2025

Magnetic droplet solitons

Authors: Martina Ahlberg, Sheng Jiang, Roman Khymyn, Sunjae Chung, Johan Åkerman

Abstract: 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 resear… ▽ More 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. △ Less

Submitted 11 November, 2024; v1 submitted 22 December, 2023; originally announced December 2023.

Comments: 38 pages, 14 figures, book chapter

Journal ref: In: Bandyopadhyay, S., Barman, A. (eds) Nanomagnets as Dynamical Systems. Nanostructure Science and Technology. Springer, Cham (2024)

Mutual synchronization in spin torque and spin Hall nano-oscillators

Authors: Akash Kumar, Artem Litvinenko, Nilamani Behera, Ahmad A. Awad, Roman Khymyn, Johan Åkerman

Abstract: 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 m… ▽ More 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. △ Less

Submitted 15 December, 2024; v1 submitted 15 December, 2023; originally announced December 2023.

Comments: Chapter 5 of Book Nanomagnets as Dynamical Systems

Journal ref: Springer Nature Book Series 2024

Antiferromagnetic droplet soliton driven by spin current

Authors: Roman V. Ovcharov, Mohammad Hamdi, Boris A. Ivanov, Johan Åkerman, Roman S. Khymyn

Abstract: We demonstrate that a spin current flowing through a nano-contact into a uniaxial antiferromagnet with first- and second-order anisotropy can excite a self-localized dynamic magnetic soliton, known as a spin-wave droplet in ferromagnets. The droplet nucleates at a certain threshold current with the frequency of the Néel vector precession laying below the antiferromagnetic resonance. The frequency… ▽ More We demonstrate that a spin current flowing through a nano-contact into a uniaxial antiferromagnet with first- and second-order anisotropy can excite a self-localized dynamic magnetic soliton, known as a spin-wave droplet in ferromagnets. The droplet nucleates at a certain threshold current with the frequency of the Néel vector precession laying below the antiferromagnetic resonance. The frequency exhibits nonlinear behavior with the increasing of applied current. At the high value of applied torque, the soliton mode transforms, and the oscillator emits spin waves propagating in the antiferromagnetic layer. △ Less

Submitted 30 November, 2023; originally announced November 2023.

Comments: 6 pages, 5 figures

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

Authors: Lakhan Bainsla, Yuya Sakuraba, Avinash Kumar Chaurasiya, Akash Kumar, Keisuke Masuda, Ahmad A. Awad, Nilamani Behera, Roman Khymyn, Saroj Prasad Dash, Johan Åkerman

Abstract: 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… ▽ More 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. △ Less

Submitted 19 April, 2024; v1 submitted 14 November, 2023; originally announced November 2023.

Comments: 19 pages

Journal ref: ACS Nano 2025

A 50-spin surface acoustic wave Ising machine

Authors: Artem Litvinenko, Roman Khymyn, Roman Ovcharov, Johan Åkerman

Abstract: 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 FP… ▽ More 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. △ Less

Submitted 12 November, 2023; originally announced November 2023.

MSC Class: 90C27

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

Authors: Victor H. González, Artem Litvinenko, Roman Khymyn, Johan Åkerman

Abstract: 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… ▽ More 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. △ Less

Submitted 15 August, 2023; originally announced August 2023.

Comments: Index Terms: combinatorial optimization problems, Ising machines, spinwaves, unconventional computing, physical computing, spinwaves

Antiferromagnetic Bloch line driven by spin current as room-temperature analog of a fluxon in a long Josephson junction

Authors: R. V. Ovcharov, B. A. Ivanov, J. Åkerman, R. S. Khymyn

Abstract: Antiferromagnets (AFMs) are promising materials for future high-frequency field-free spintronic applications. Self-localized spin structures can enhance their capabilities and introduce new functionalities to AFM-based devices. Here we consider a domain wall (DW), a topological soliton that bridges a connection between two ground states, similar to a Josephson junction (JJ) link between two superc… ▽ More Antiferromagnets (AFMs) are promising materials for future high-frequency field-free spintronic applications. Self-localized spin structures can enhance their capabilities and introduce new functionalities to AFM-based devices. Here we consider a domain wall (DW), a topological soliton that bridges a connection between two ground states, similar to a Josephson junction (JJ) link between two superconductors. We demonstrate the similarities between DWs in bi-axial AFM with easy-axis primary anisotropy, driven by a spin current, and long Josephson junctions (LJJs). We found that the Bloch line (BL) in DWs resembles the fluxon state of JJs, creating a close analogy between the two systems. We propose a scheme that allows us to create, move, read, and delete such BLs. This transmission line operates at room temperature and can be dynamically reconfigured in contrast to superconductors. Results of a developed model were confirmed by micromagnetic simulations for Cr$_2$O$_3$ and DyFeO$_3$, i.e., correspondingly with weak and strong in-plane anisotropy. Overall, the proposed scheme has significant potential for use in magnetic memory and logic devices. △ Less

Submitted 16 October, 2023; v1 submitted 3 May, 2023; originally announced May 2023.

Comments: 8 pages, 5 figures; Published in Physical Review Applied

Phase noise analysis of mutually synchronized spin Hall nano-oscillators

Authors: Artem Litvinenko, Akash Kumar, Mona Rajabali, Ahmad A. Awad, Roman Khymyn, Johan Akerman

Abstract: 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… ▽ More 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. △ Less

Submitted 31 March, 2023; originally announced March 2023.

Journal ref: Applied Physics Letters 2023

Robust mutual synchronization in long spin Hall nano-oscillator chains

Authors: Akash Kumar, Himanshu Fulara, Roman Khymyn, Mohammad Zahedinejad, Mona Rajabali, Xiaotian Zhao, Nilamani Behera, Afshin Houshang, Ahmad A. Awad, Johan Åkerman

Abstract: 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… ▽ More 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. △ Less

Submitted 10 January, 2023; originally announced January 2023.

Comments: 9 Pages, 7 figures

Journal ref: Nano Letters 2023

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

Authors: Victor H. González, Roman Khymyn, Himanshu Fulara, Ahmad A. Awad, Johan Åkerman

Abstract: 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})… ▽ More 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. △ Less

Submitted 3 October, 2022; originally announced October 2022.

Journal ref: 121 (25): 252404, Appl. Phys. Lett, 2022

A spinwave Ising machine

Authors: Artem Litvinenko, Roman Khymyn, Victor H. González, Ahmad A. Awad, Vasyl Tyberkevych, Andrei Slavin, Johan Åkerman

Abstract: 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 wave… ▽ More 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. △ Less

Submitted 9 September, 2022; originally announced September 2022.

Optothermal control of spin Hall nano-oscillators

Authors: Shreyas Muralidhar, Afshin Houshang, Ademir Alemán, Roman Khymyn, Ahmad A. Awad, Johan Åkerman

Abstract: 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 tempe… ▽ More 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. △ Less

Submitted 28 January, 2022; originally announced January 2022.

Ultrathin ferrimagnetic GdFeCo films with very low damping

Authors: Lakhan Bainsla, Akash Kumar, Ahmad A. Awad, Chunlei Wang, Mohammad Zahedinejad, Nilamani Behera, Himanshu Fulara, Roman Khymyn, Afshin Houshang, Jonas Weissenrieder, J. Åkerman

Abstract: 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 dev… ▽ More 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 α = 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. △ Less

Submitted 16 November, 2021; originally announced November 2021.

Comments: 7 Pages, 4 Figures

Roadmap on Spin-Wave Computing

Authors: A. V. Chumak, P. Kabos, M. Wu, C. Abert, C. Adelmann, A. Adeyeye, J. Åkerman, F. G. Aliev, A. Anane, A. Awad, C. H. Back, A. Barman, G. E. W. Bauer, M. Becherer, E. N. Beginin, V. A. S. V. Bittencourt, Y. M. Blanter, P. Bortolotti, I. Boventer, D. A. Bozhko, S. A. Bunyaev, J. J. Carmiggelt, R. R. Cheenikundil, F. Ciubotaru, S. Cotofana , et al. (91 additional authors not shown)

Abstract: Magnonics is a field of science that addresses the physical properties of spin waves and utilizes them for data processing. Scalability down to atomic dimensions, operations in the GHz-to-THz frequency range, utilization of nonlinear and nonreciprocal phenomena, and compatibility with CMOS are just a few of many advantages offered by magnons. Although magnonics is still primarily positioned in the… ▽ More Magnonics is a field of science that addresses the physical properties of spin waves and utilizes them for data processing. Scalability down to atomic dimensions, operations in the GHz-to-THz frequency range, utilization of nonlinear and nonreciprocal phenomena, and compatibility with CMOS are just a few of many advantages offered by magnons. Although magnonics is still primarily positioned in the academic domain, the scientific and technological challenges of the field are being extensively investigated, and many proof-of-concept prototypes have already been realized in laboratories. This roadmap is a product of the collective work of many authors that covers versatile spin-wave computing approaches, conceptual building blocks, and underlying physical phenomena. In particular, the roadmap discusses the computation operations with Boolean digital data, unconventional approaches like neuromorphic computing, and the progress towards magnon-based quantum computing. The article is organized as a collection of sub-sections grouped into seven large thematic sections. Each sub-section is prepared by one or a group of authors and concludes with a brief description of the current challenges and the outlook of the further development of the research directions. △ Less

Submitted 30 October, 2021; originally announced November 2021.

Comments: 74 pages, 57 figures, 500 references

Journal ref: IEEE Transactions on Magnetics 58, 0800172 (2022)

Femtosecond laser comb driven perpendicular standing spin waves

Authors: Ahmad A. Awad, Shreyas Muralidhar, Ademir Alemán, Roman Khymyn, Dag Hanstorp, Johan Åkerman

Abstract: We study femtosecond laser comb driven sustained and coherent spin wave (SW) generation in permalloy (Py) films over a thickness range of $d =$ 40--100 nm. A simple rapid demagnetization model describes the dependence of the observed SW intensity on laser power for all film thicknesses. In the thicker films we observe laser comb excited perpendicular standing spin waves up to third order and to 18… ▽ More We study femtosecond laser comb driven sustained and coherent spin wave (SW) generation in permalloy (Py) films over a thickness range of $d =$ 40--100 nm. A simple rapid demagnetization model describes the dependence of the observed SW intensity on laser power for all film thicknesses. In the thicker films we observe laser comb excited perpendicular standing spin waves up to third order and to 18 multiples of the 1 GHz laser repetition rate. Our results demonstrate the versatility of femtosecond combs as contact-less SW point sources over a wide range of film thickness and type of SW modes. △ Less

Submitted 25 October, 2021; originally announced October 2021.

Freezing and thawing magnetic droplet solitons

Authors: Martina Ahlberg, Sunjae Chung, Sheng Jiang, Q. Tuan Le, Roman Khymyn, Hamid Mazraati, Markus Weigand, Iuliia Bykova, Felix Groß, Eberhard Goering, Gisela Schütz, Joachim Gräfe, Johan Åkerman

Abstract: Magnetic droplets are non-topological magnetodynamical solitons displaying a wide range of complex dynamic phenomena with potential for microwave signal generation. Bubbles, on the other hand, are internally static cylindrical magnetic domains, stabilized by external fields and magnetostatic interactions. In its original theory, the droplet was described as an imminently collapsing bubble stabiliz… ▽ More Magnetic droplets are non-topological magnetodynamical solitons displaying a wide range of complex dynamic phenomena with potential for microwave signal generation. Bubbles, on the other hand, are internally static cylindrical magnetic domains, stabilized by external fields and magnetostatic interactions. In its original theory, the droplet was described as an imminently collapsing bubble stabilized by spin transfer torque and, in its zero-frequency limit, as equivalent to a bubble. Without nanoscale lateral confinement, pinning, or an external applied field, such a nanobubble is unstable, and should collapse. Here, we show that we can freeze dynamic droplets into static nanobubbles by decreasing the magnetic field. While the bubble has virtually the same resistance as the droplet, all signs of low-frequency microwave noise disappear. The transition is fully reversible and the bubble can be thawed back into a droplet if the magnetic field is increased under current. Whereas the droplet collapses without a sustaining current, the bubble is highly stable and remains intact for days without external drive. Electrical measurements are complemented by direct observation using scanning transmission x-ray microscopy, which corroborates the analysis and confirms that the bubble is stabilized by pinning. △ Less

Submitted 30 April, 2021; originally announced April 2021.

Comments: 23 pages, 5 figures

Spin-Hall nanooscillator based on an antiferromagnetic domain wall

Authors: Roman V. Ovcharov, Elena G. Galkina, Boris A. Ivanov, Roman S. Khymyn

Abstract: We propose here a high-frequency spin-Hall nano-oscillator based on a simple magnetic texture, such as a domain wall, located in an antiferromagnet with easy-axis anisotropy type. We show that the spin current, polarized along the anisotropy axis, excites a conical precession of the Néel vector in such a domain wall, which allows obtaining a robust ac output signal, -- contrary to the planar prece… ▽ More We propose here a high-frequency spin-Hall nano-oscillator based on a simple magnetic texture, such as a domain wall, located in an antiferromagnet with easy-axis anisotropy type. We show that the spin current, polarized along the anisotropy axis, excites a conical precession of the Néel vector in such a domain wall, which allows obtaining a robust ac output signal, -- contrary to the planar precession in an uniform uniaxial antiferromagnet, where ac output is hard to achieve. The frequency of the auto-oscillations is easily tunable by the applied current up to the THz range, and the threshold current vanishes for pure uniaxial antiferromagnet. By micro-magnetic simulations, we demonstrate that the pinning of the domain wall is crucial for the oscillator design, which can be achieved in nano-constriction layout of the free layer. △ Less

Submitted 7 December, 2021; v1 submitted 10 April, 2021; originally announced April 2021.

Comments: 7 pages, 5 figures. Brief explanation of the dynamics near the threshold is added at P.3

Femtosecond laser pulse driven caustic spin wave beams

Authors: S. Muralidhar, R. Khymyn, A. A. Awad, A. Alemán, D. Hanstorp, J. Åkerman, .

Abstract: Controlling the directionality of spin waves is a key ingredient in wave-based computing methods such as magnonics. In this paper, we demonstrate this particular aspect by using an all-optical point-like source of continuous spin waves based on frequency comb rapid demagnetization. The emitted spin waves contain a range of k-vectors and by detuning the applied magnetic field slightly off the ferro… ▽ More Controlling the directionality of spin waves is a key ingredient in wave-based computing methods such as magnonics. In this paper, we demonstrate this particular aspect by using an all-optical point-like source of continuous spin waves based on frequency comb rapid demagnetization. The emitted spin waves contain a range of k-vectors and by detuning the applied magnetic field slightly off the ferromagnetic resonance (FMR), we observe X-shaped caustic spin-wave patterns at $70^{\circ}$ propagation angles as predicted by theory. When the harmonic of the light source approaches theFMR, the caustic pattern gives way to uniaxial spin-wave propagation perpendicular to the in-plane component of the applied field. This field-controlled propagation pattern and directionality of optically emitted short-wavelength spin waves provide additional degrees of freedom when designing magnonic devices. △ Less

Submitted 15 June, 2020; originally announced June 2020.

Comments: 5 pages, 4 figures

Journal ref: Phys. Rev. Lett. 126, 037204 (2021)

Width dependent auto-oscillating properties of constriction based spin Hall nano-oscillators

Authors: Ahmad A. Awad, Afshin Houshang, Mohammad Zahedinejad, Roman Khymyn, Johan Åkerman

Abstract: 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 micro… ▽ More 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. △ Less

Submitted 16 March, 2020; originally announced March 2020.

Terahertz frequency spectrum analysis with a nanoscale antiferromagnetic tunnel junction

Authors: P. Yu. Artemchuk, O. R. Sulymenko, S. Louis, J. Li, R. Khymyn, E. Bankowski, T. Meitzler, V. S. Tyberkevych, A. N. Slavin, O. V. Prokopenko

Abstract: A method to perform spectrum analysis on low power signals between 0.1 and 10 THz is proposed. It utilizes a nanoscale antiferromagnetic tunnel junction (ATJ) that produces an oscillating tunneling anisotropic magnetoresistance, whose frequency is dependent on the magnitude of an evanescent spin current. It is first shown that the ATJ oscillation frequency can be tuned linearly with time. Then, it… ▽ More A method to perform spectrum analysis on low power signals between 0.1 and 10 THz is proposed. It utilizes a nanoscale antiferromagnetic tunnel junction (ATJ) that produces an oscillating tunneling anisotropic magnetoresistance, whose frequency is dependent on the magnitude of an evanescent spin current. It is first shown that the ATJ oscillation frequency can be tuned linearly with time. Then, it is shown that the ATJ output is highly dependent on matching conditions that are highly dependent on the dimensions of the dielectric tunneling barrier. Spectrum analysis can be performed by using an appropriately designed ATJ, whose frequency is driven to increase linearly with time, a low pass filter, and a matched filter. This method of THz spectrum analysis, if realized in experiment, will allow miniaturized electronics to rapidly analyze low power signals with a simple algorithm. It is also found by simulation and analytical theory that for an ATJ with a 0.09 $μ$m$^2$ footprint, spectrum analysis can be performed over a 0.25 THz bandwidth in just 25 ns on signals that are at the Johnson-Nyquist thermal noise floor. △ Less

Submitted 30 November, 2019; originally announced December 2019.

Comments: 10 pages, 5 figures, submitted to the Journal of Applied Physics

Sustained coherent spin wave emission using frequency combs

Authors: A. A. Awad, S. Muralidhar, A. Alemán, R. Khymyn, M. Dvornik, D. Hanstorp, J. Åkerman

Abstract: We demonstrate sustained coherent emission of spin waves in NiFe films using rapid demagnetization from high repetition rate femtosecond laser pulse trains. As the pulse separation is shorter than the magnon decay time, magnons having a frequency equal to a multiple of the 1 GHz repetition-rate are coherently amplified. Using scanning micro-Brillouin Light Scattering (BLS) we observe this coherent… ▽ More We demonstrate sustained coherent emission of spin waves in NiFe films using rapid demagnetization from high repetition rate femtosecond laser pulse trains. As the pulse separation is shorter than the magnon decay time, magnons having a frequency equal to a multiple of the 1 GHz repetition-rate are coherently amplified. Using scanning micro-Brillouin Light Scattering (BLS) we observe this coherent amplification as strong peaks spaced 1 GHz apart. The BLS counts vs. laser power exhibit a stronger than parabolic dependence consistent with counts being proportional to the square of the magnetodynamic amplitude, and the demagnetization pulse strength being described by a Bloch law. Spatial spin wave mapping demonstrates how both localized and propagating spin waves can be excited, and how the propagation direction can be directly controlled. Our results demonstrate the versatility of BLS spectroscopy for rapid demagnetization studies and enable a new platform for photo-magnonics where sustained coherent spin waves can be utilized. △ Less

Submitted 16 March, 2020; v1 submitted 9 August, 2019; originally announced August 2019.

Comments: 7 pages, 7 figures

Journal ref: Phys. Rev. B 101, 224423 (2020)

Sub-terahertz ferrimagnetic spin-transfer torque oscillator

Authors: Ivan Lisenkov, Roman Khymyn, Johan Akerman, Nian X. Sun, Boris A. Ivanov

Abstract: A theory of magnetization dynamics in ferrimagnetic materials with antiparallel aligned spin sub-lattices under action of spin-transfer torques (STT) is developed. We consider magnetization dynamics in GdFeCo layers in two cases of magnetic anisotropy: easy plane and easy axis. We demonstrate that, (i) for the easy plane anisotropy the precession of the Néel vector is conical and the cone angle de… ▽ More A theory of magnetization dynamics in ferrimagnetic materials with antiparallel aligned spin sub-lattices under action of spin-transfer torques (STT) is developed. We consider magnetization dynamics in GdFeCo layers in two cases of magnetic anisotropy: easy plane and easy axis. We demonstrate that, (i) for the easy plane anisotropy the precession of the Néel vector is conical and the cone angle depends on the STT strength and the value of spin non-compensation, while the frequency of precession can reach sub-THz frequencies; (ii) for the easy axis anisotropy two regimes are possible: deterministic switching of the net magnetization and a conical sub-THz precession depending on the STT strength. △ Less

Submitted 19 April, 2019; originally announced April 2019.

Comments: 5 pages, 5 figures

Journal ref: Phys. Rev. B 100, 100409 (2019)

Spin-Orbit-Torque Driven Propagating Spin Waves

Authors: Himanshu Fulara, Mohammad Zahedinejad, Roman Khymyn, Ahmad Awad, Shreyas Muralidhar, Mykola Dvornik, Johan Åkerman

Abstract: 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 non-linear 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 dipo… ▽ More 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 non-linear 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 nano-constrictions 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 neuro-morphic computing, directly compatible with CMOS technology. △ Less

Submitted 15 April, 2019; originally announced April 2019.

Journal ref: Sci. Adv. 5, eaax8467 (2019)

Two-dimensional mutual synchronization in spin Hall nano-oscillator arrays

Authors: Mohammad Zahedinejad, Ahmad A. Awad, Shreyas Muralidhar, Roman Khymyn, Himanshu Fulara, Hamid Mazraati, Mykola Dvornik, Johan Åkerman

Abstract: Spin Hall nano-oscillators (SHNOs) utilize pure spin currents to 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 sometimes mutually synchronize, in pairs or in short linear chains. Here we demonstrate robust mutual synchronization of two-dimensional SHNO arrays ranging from… ▽ More Spin Hall nano-oscillators (SHNOs) utilize pure spin currents to 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 sometimes mutually synchronize, in pairs or in short linear chains. Here we demonstrate robust mutual synchronization of two-dimensional SHNO arrays ranging from 2 x 2 to 8 x 8 nano-constrictions, observed both electrically and using micro-Brillouin Light Scattering microscopy. The signal quality factor, $Q=f/Δf$, increases linearly with number of mutually synchronized nano-constrictions ($N$), reaching 170,000 in the largest arrays. While the microwave peak power first increases as $N^2$, it eventually levels off, indicating a non-zero relative phase shift between nano-constrictions. Our demonstration will enable the use of SHNO arrays in two-dimensional oscillator networks for high-quality microwave signal generation and neuromorphic computing. △ Less

Submitted 22 December, 2018; originally announced December 2018.

Tuning spin torque nano-oscillator nonlinearity using He+ irradiation

Authors: Sheng Jiang, Roman Khymyn, Sunjae Chung, Quang Tuan Le, Liza Herrera Diez, Afshin Houshang, Mohammad Zahedinejad, Dafine Ravelosona, Johan Åkerman

Abstract: We use He$^+$ irradiation to tune the nonlinearity, $\mathcal{N}$, of all-perpendicular spin-torque nano-oscillators (STNOs) using the He$^+$ fluence-dependent perpendicular magnetic anisotropy (PMA) of the [Co/Ni] free layer. Employing fluences from 6 to 20$\times10^{14}$~He$^{+}$/cm$^{2}$, we are able to tune $\mathcal{N}$ in an in-plane field from strongly positive to moderately negative. As th… ▽ More We use He$^+$ irradiation to tune the nonlinearity, $\mathcal{N}$, of all-perpendicular spin-torque nano-oscillators (STNOs) using the He$^+$ fluence-dependent perpendicular magnetic anisotropy (PMA) of the [Co/Ni] free layer. Employing fluences from 6 to 20$\times10^{14}$~He$^{+}$/cm$^{2}$, we are able to tune $\mathcal{N}$ in an in-plane field from strongly positive to moderately negative. As the STNO microwave signal properties are mainly governed by $\mathcal{N}$, we can in this way directly control the threshold current, the current tunability of the frequency, and the STNO linewidth. In particular, we can dramatically improve the latter by more than two orders of magnitude. Our results are in good agreement with the theory for nonlinear auto-oscillators, confirm theoretical predictions of the role of nonlinearity, and demonstrate a straightforward path towards improving the microwave properties of STNOs. △ Less

Submitted 20 December, 2018; originally announced December 2018.

Comments: 5 pages, 4 figures

An Intrinsic Spin Orbit Torque Nano-Oscillator

Authors: M. Haidar, A. A. Awad, M. Dvornik, R. Khymyn, A. Houshang, J. Akerman

Abstract: Spin torque and spin Hall effect nanooscillators generate high intensity spin wave auto oscillations on the nanoscale enabling novel microwave applications in spintronics, magnonics, and neuromorphic computing. For their operation, these devices require externally generated spin currents either from an additional ferromagnetic layer or a material with a high spin Hall angle. Here we demonstrate hi… ▽ More Spin torque and spin Hall effect nanooscillators generate high intensity spin wave auto oscillations on the nanoscale enabling novel microwave applications in spintronics, magnonics, and neuromorphic computing. For their operation, these devices require externally generated spin currents either from an additional ferromagnetic layer or a material with a high spin Hall angle. Here we demonstrate highly coherent field and current tunable microwave signals from nanoconstrictions in single 15 and 20 nm thick permalloy layers. Using a combination of spin torque ferromagnetic resonance measurements, scanning microBrillouin light scattering microscopy, and micromagnetic simulations, we identify the autooscillations as emanating from a localized edge mode of the nanoconstriction driven by spin orbit torques. Our results pave the way for greatly simplified designs of auto oscillating nanomagnetic systems only requiring a single ferromagnetic layer. △ Less

Submitted 28 August, 2018; originally announced August 2018.

Comments: 11 pages, 2 figures

Micromagnetic modeling of Terahertz oscillations in an antiferromagnetic material driven by spin-Hall effect

Authors: Vito Puliafito, Roman Khymyn, Mario Carpentieri, Bruno Azzerboni, Vasyl Tiberkevich, Andrei Slavin, Giovanni Finocchio

Abstract: The realization of THz sources is a fundamental aspect for a wide range of applications. Over different approaches, compact THz oscillators can be realized taking advantage of dynamics in antiferromagnetic (AFMs) thin films driven by spin-Hall effect. Here we perform a systematic study of these THz oscillators within a full micromagnetic solver based on the numerical solution of two coupled Landau… ▽ More The realization of THz sources is a fundamental aspect for a wide range of applications. Over different approaches, compact THz oscillators can be realized taking advantage of dynamics in antiferromagnetic (AFMs) thin films driven by spin-Hall effect. Here we perform a systematic study of these THz oscillators within a full micromagnetic solver based on the numerical solution of two coupled Landau-Lifshitz-Gilbert-Slonczewski equations, for the case of ultra-thin films, i.e. when the Néel temperature of an AFM is substantially reduced. We have found two different dynamical modes depending on the strength of the Dzyaloshinskii-Moriya interaction (DMI). At low DMI, a large amplitude precession is excited where both the magnetizations of the sublattices are in a uniform state and rotate in the same direction. At large enough DMI, the ground state of the AFM becomes non-uniform and the antiferromagnetic dynamics is characterized by ultrafast domain wall motion. △ Less

Submitted 11 August, 2018; originally announced August 2018.

Comments: manuscript 15 pages 7 figures - supplemental material 3 pages 2 figures

Journal ref: Phys. Rev. B 99, 024405 (2019)

Ultra-fast artificial neuron: generation of picosecond-duration spikes in a current-driven antiferromagnetic auto-oscillator

Authors: Roman Khymyn, Ivan Lisenkov, Jamie Voorheis, Olga Sulymenko, Oleksandr Prokopenko, Vasil Tiberkevich, Johan Akerman, Andrei Slavin

Abstract: We demonstrate analytically and numerically, that a thin film of an antiferromagnetic (AFM) material, having biaxial magnetic anisotropy and being driven by an external spin-transfer torque signal, can be used for the generation of ultra-short "Dirac-delta-like" spikes. The duration of the generated spikes is several picoseconds for typical AFM materials and is determined by the in-plane magnetic… ▽ More We demonstrate analytically and numerically, that a thin film of an antiferromagnetic (AFM) material, having biaxial magnetic anisotropy and being driven by an external spin-transfer torque signal, can be used for the generation of ultra-short "Dirac-delta-like" spikes. The duration of the generated spikes is several picoseconds for typical AFM materials and is determined by the in-plane magnetic anisotropy and the effective damping of the AFM material. The generated output signal can consist of a single spike or a discrete group of spikes ("bursting"), which depends on the repetition (clock) rate, amplitude, and shape of the external control signal. The spike generation occurs only when the amplitude of the control signal exceeds a certain threshold, similar to the action of a biological neuron in response to an external stimulus. The "threshold" behavior of the proposed AFM spike generator makes possible its application not only in the traditional microwave signal processing but also in the future neuromorphic signal processing circuits working at clock frequencies of tens of gigahertz. △ Less

Submitted 27 February, 2018; originally announced February 2018.

Faster, farther, stronger: spin transfer torque driven high order propagating spin waves in nano-contact magnetic tunnel junctions

Authors: A. Houshang, R. Khymyn, M. Dvornik, M. Haidar, S. R. Etesami, R. Ferreira, P. P. Freitas, R. K. Dumas, J. Åkerman

Abstract: Short wave-length exchange-dominated propagating spin waves will enable magnonic devices to operate at higher frequencies and higher data transmission rates.1 While GMR based magnetic nano-contacts are highly efficient injectors of propagating spin waves2,3, the generated wave lengths are 2.6 times the nano-contact diameter4, and the electrical signal strength remains much too weak for practical a… ▽ More Short wave-length exchange-dominated propagating spin waves will enable magnonic devices to operate at higher frequencies and higher data transmission rates.1 While GMR based magnetic nano-contacts are highly efficient injectors of propagating spin waves2,3, the generated wave lengths are 2.6 times the nano-contact diameter4, and the electrical signal strength remains much too weak for practical applications. Here we demonstrate nano-contact based spin wave generation in magnetic tunnel junction stacks, and observe large discrete frequency steps consistent with the hitherto ignored possibility of second and third order propagating spin waves with wave lengths of 120 and 74 nm, i.e. much smaller than the 150 nm nano-contact. These higher-order propagating spin waves will not only enable magnonic devices to operate at much higher frequencies, but also greatly increase their transmission rates and spin wave propagating lengths, both proportional to the much higher group velocity. △ Less

Submitted 5 December, 2017; v1 submitted 4 December, 2017; originally announced December 2017.

Low Power Microwave Signal Detection With a Spin-Torque Nano-Oscillator in the Active Self-Oscillating Regime

Authors: Steven Louis, Vasyl Tyberkevych, Jia Li, Ivan Lisenkov, Roman Khymyn, Elena Bankowski, Thomas Meitzler, Ilya Krivorotov, Andrei Slavin

Abstract: A spin-torque nano-oscillator (STNO) driven by a ramped bias current can perform spectrum analysis quickly over a wide frequency bandwidth. The STNO spectrum analyzer operates by injection locking to external microwave signals and produces an output DC voltage $V_{\rm dc}$ that temporally encodes the input spectrum. We found, via numerical analysis with a macrospin approximation, that an STNO is a… ▽ More A spin-torque nano-oscillator (STNO) driven by a ramped bias current can perform spectrum analysis quickly over a wide frequency bandwidth. The STNO spectrum analyzer operates by injection locking to external microwave signals and produces an output DC voltage $V_{\rm dc}$ that temporally encodes the input spectrum. We found, via numerical analysis with a macrospin approximation, that an STNO is able to scan a $10~\rm GHz$ bandwidth in less than $100~\rm ns$ (scanning rate $R$ exceeds $100~\rm MHz/ns$). In contrast to conventional quadratic microwave detectors, the output voltage of the STNO analyzer is proportional to the amplitude of the input microwave signal $I_{\rm rf}$ with sensitivity $S = dV_{\rm dc}/dI_{\rm rf} \approx 750~\rm mV/mA$. The minimum detectable signal of the analyzer depends on the scanning rate $R$ and, at low $R \approx 1~\rm MHz/ns$, is about $1~\rm pW$. △ Less

Submitted 11 April, 2017; originally announced April 2017.

Comments: 5 pages, 5 figures

Antiferromagnetic THz-frequency Josephson-like Oscillator Driven by Spin Current

Authors: Roman Khymyn, Ivan Lisenkov, Vasyl Tiberkevich, Boris A. Ivanov, Andrei Slavin

Abstract: The development of compact and tunable room temperature sources of coherent THz-frequency signals would open a way for numerous new applications. The existing approaches to THz-frequency generation based on superconductor Josephson junctions (JJ), free electron lasers, and quantum cascades require cryogenic temperatures or/and complex setups, preventing the miniaturization and wide use of these de… ▽ More The development of compact and tunable room temperature sources of coherent THz-frequency signals would open a way for numerous new applications. The existing approaches to THz-frequency generation based on superconductor Josephson junctions (JJ), free electron lasers, and quantum cascades require cryogenic temperatures or/and complex setups, preventing the miniaturization and wide use of these devices. We demonstrate theoretically that a bi-layer of a heavy metal (Pt) and a bi-axial antiferromagnetic (AFM) dielectric (NiO) can be a source of a coherent THz signal. A spin-current flowing from a DC-current-driven Pt layer and polarized along the hard AFM anisotropy axis excites a non-uniform in time precession of magnetizations sublattices in the AFM, due to the presence of a weak easy-plane AFM anisotropy. The frequency of the AFM oscillations varies in the range of 0.1-2.0 THz with the driving current in the Pt layer from $10^8\text{A}/\text{cm}^2$ to $10^9\text{A}/\text{cm}^2$. The THz-frequency signal from the AFM with the amplitude exceeding 1 V/cm is picked up by the inverse spin-Hall effect in Pt. The operation of a room-temperature AFM THz-frequency oscillator is similar to that of a cryogenic JJ oscillator, with the energy of the easy-plane magnetic anisotropy playing the role of the Josephson energy. △ Less

Submitted 30 September, 2016; originally announced September 2016.

Comments: 19 pages, 5 figures

Transformation of spin current by antiferromagnetic insulators

Authors: Roman Khymyn, Ivan Lisenkov, Vasil S. Tiberkevich, Andrei N. Slavin, Boris A. Ivanov

Abstract: It is demonstrated theoretically that a thin layer of an anisotropic antiferromagnetic (AFM) insulator can effectively conduct spin current by excitation of a pair of evanescent AFM spin wave modes. The spin current flowing through the AFM is not conserved due to the interaction between the excited AFM modes and the AFM lattice, and, depending on the excitation conditions, can be either attenuated… ▽ More It is demonstrated theoretically that a thin layer of an anisotropic antiferromagnetic (AFM) insulator can effectively conduct spin current by excitation of a pair of evanescent AFM spin wave modes. The spin current flowing through the AFM is not conserved due to the interaction between the excited AFM modes and the AFM lattice, and, depending on the excitation conditions, can be either attenuated or enhanced. When the phase difference between the excited evanescent modes is close to $π/2$, there is an optimum AFM thickness for which the output spin current reaches a maximum, that can significantly exceed the magnitude of the input spin current. The spin current transfer through the AFM depends on the ambient temperature and increases substantially when temperature approaches the Neel temperature of the AFM layer. △ Less

Submitted 16 March, 2016; v1 submitted 18 November, 2015; originally announced November 2015.

Journal ref: Phys. Rev. B 93, 224421 (2016)

Self-organization of topological defects for a triangular-lattice magnetic dots array subject to a perpendicular magnetic field

Authors: R. Khymyn, V. Kireev, B. Ivanov

Abstract: The regular array of magnetic particles (magnetic dots) of the form of a two-dimensional triangular lattice in the presence of external magnetic field demonstrates complicated magnetic structures. The magnetic symmetry of the ground state for such a system is lower than that for the underlying lattice. Long range dipole-dipole interaction leads to a specific antiferromagnetic order in small fields… ▽ More The regular array of magnetic particles (magnetic dots) of the form of a two-dimensional triangular lattice in the presence of external magnetic field demonstrates complicated magnetic structures. The magnetic symmetry of the ground state for such a system is lower than that for the underlying lattice. Long range dipole-dipole interaction leads to a specific antiferromagnetic order in small fields, whereas a set of linear topological defects appears with the growth of the magnetic field. Self-organization of such defects determines the magnetization process for a system within a wide range of external magnetic fields. △ Less

Submitted 18 November, 2014; originally announced November 2014.

Comments: 10 pages, 4 figures. arXiv admin note: substantial text overlap with arXiv:1201.1747

Journal ref: Condens. Matter Phys., 2014, vol. 17, No. 3, 33701

Ground states and magnetization process for an triangular lattice array of magnetic dots with perpendicular anisotropy

Authors: V. E. Kireev, R. S. Khymyn, B. A. Ivanov, C. E. Zaspel

Abstract: We analyzed the ground state of the array of magnetic particles (magnetic dots) which form a two-dimensional triangular lattice, and magnetic moment of which is perpendicular to the plane of the lattice, in the presence of external magnetic field. In the small fields long range dipole-dipole interaction leads to the specific antiferromagnetic order, where two out of six nearest neighbors of the pa… ▽ More We analyzed the ground state of the array of magnetic particles (magnetic dots) which form a two-dimensional triangular lattice, and magnetic moment of which is perpendicular to the plane of the lattice, in the presence of external magnetic field. In the small fields long range dipole-dipole interaction leads to the specific antiferromagnetic order, where two out of six nearest neighbors of the particle have the same direction of magnetization moment and four - the opposite one. It is shown that magnetization process in such array of particles as opposed to the rectangular lattices results from the formation of the magnetized topological defects (dislocations) in the shape of the domain walls. △ Less

Submitted 9 January, 2012; originally announced January 2012.

Comments: 14 pages, 14 figures

MSC Class: 65Z05 ACM Class: F.2.2; I.2.7; F.2.2; I.2.7

Core-Core Dynamics in Spin Vortex Pairs

Authors: S. S. Cherepov, B. C. Koop, V. Korenivski, D. C. Worledge, A. Yu. Galkin, R. S. Khymyn, B. A. Ivanov

Abstract: We investigate magnetic nano-pillars, in which two thin ferromagnetic nanoparticles are separated by a nanometer thin nonmagnetic spacer and can be set into stable spin vortex-pair configurations. The 16 ground states of the vortex-pair system are characterized by parallel or antiparallel chirality and parallel or antiparallel core-core alignment. We detect and differentiate these individual vorte… ▽ More We investigate magnetic nano-pillars, in which two thin ferromagnetic nanoparticles are separated by a nanometer thin nonmagnetic spacer and can be set into stable spin vortex-pair configurations. The 16 ground states of the vortex-pair system are characterized by parallel or antiparallel chirality and parallel or antiparallel core-core alignment. We detect and differentiate these individual vortex-pair states experimentally and analyze their dynamics analytically and numerically. Of particular interest is the limit of strong core-core coupling, which we find can dominate the spin dynamics in the system. We observe that the 0.2 GHz gyrational resonance modes of the individual vortices are replaced with 2-6 GHz range collective rotational and vibrational core-core resonances in the configurations where the cores form a bound pair. These results demonstrate new opportunities in producing and manipulating spin states on the nanoscale and may prove useful for new types of ultra-dense storage devices where the information is stored as multiple vortex-core configurations. △ Less

Submitted 19 October, 2011; originally announced October 2011.

Journal ref: Phys. Rev. Lett. 109, 097204 (2012)

Non-linear dynamics and two-dimensional solitons for spin $ S=1$ ferromagnets with biquadratic exchange

Authors: B. A. Ivanov, A. Yu. Galkin, R. S. Khymyn, A. Yu. Merkulov

Abstract: We develop a consistent semiclassical theory of spin dynamics for an isotropic ferromagnet with a spin $ S=1$ taking into consideration both bilinear and biquadratic over spin operators exchange interaction. For such non-Heisenberg magnets, a peculiar class of spin oscillations and waves, for which the quantum spin expectation value $ {\rm {\bf m}}=<{\rm {\bf S}}>$ does not change it direction,… ▽ More We develop a consistent semiclassical theory of spin dynamics for an isotropic ferromagnet with a spin $ S=1$ taking into consideration both bilinear and biquadratic over spin operators exchange interaction. For such non-Heisenberg magnets, a peculiar class of spin oscillations and waves, for which the quantum spin expectation value $ {\rm {\bf m}}=<{\rm {\bf S}}>$ does not change it direction, but changes in length, is presented. Such ``longitudinal'' excitations do not exist in regular magnets, dynamics of which are described in terms of the Landau-Lifshitz equation or by means of the spin Heisenberg Hamiltonian. We demonstrate the presence of non-linear uniform oscillations and waves, as well as self-localized dynamical excitations (solitons) with finite energy. A possibility of excitation of such oscillations by ultrafast laser pulse is discussed. △ Less

Submitted 27 November, 2007; originally announced November 2007.

Comments: 11 pages, 7 figures, MikTEX

Pairing of Solitons in Two-Dimensional S=1 Magnets

Authors: B. A. Ivanov, R. S. Khymyn, A. K. Kolezhuk

Abstract: We discuss the structure of topological solitons in a general non-Heisenberg model of isotropic two-dimensional magnet with spin S=1, in the vicinity of a special point where the model symmetry is enhanced to SU(3). It is shown that upon perturbing the SU(3) symmetry, solitons with odd topological charge become unstable and bind into pairs. We discuss the structure of topological solitons in a general non-Heisenberg model of isotropic two-dimensional magnet with spin S=1, in the vicinity of a special point where the model symmetry is enhanced to SU(3). It is shown that upon perturbing the SU(3) symmetry, solitons with odd topological charge become unstable and bind into pairs. △ Less

Submitted 24 October, 2007; originally announced October 2007.

Comments: 4 pages, 1 figure

Journal ref: Phys. Rev. Lett. 100, 047203 (2008)