Sources Of Spin Waves Research Articles

Consistency between domain wall oscillation modes and spin wave modes in nanostrips

Investigations on domain wall (DW) and spin wave (SW) modes in a series of nanostrips with different widths and thicknesses have been carried out using micromagnetic simulation. The simulation results show that the frequencies of SW modes and the corresponding DW modes are consistent with each other if they have the same node number along the width direction. This consistency is more pronounced in wide and thin nanostrips, favoring the DW motion driven by SWs. Further analysis of the moving behavior of a DW driven by SWs is also carried out. The average DW speed can reach a larger value of ∼140 m/s under two different SW sources. We argue that this study is very meaningful for the potential application of DW motion driven by SWs.

Patterned magnetophotonic crystal for all-optical magnetization precession generation

A magnetophotonic crystal (MPC) with a locally etched magnetic layer is proposed for all-optical excitation of magnetization precession in the confined area. When the sample is illuminated by circularly polarized monochromatic plane wave, the optical MPC mode and the effective magnetic field excited by the inverse Faraday effect are localized inside the etched area. Therefore, the optical impact on the spin system in a magnetically ordered medium becomes confined in both lateral directions on the order of tens of nanometers and along the film thickness serving as a nanoconfined source of spin waves. The dependence of the spatial distribution of the effective magnetic field and magnetization precession amplitude on the depth and dimensions of the etched area are addressed. A proposed design of the MPC can find its applications in the field of magnetization dynamics generation as a compact and stable source of the spin waves that allows compact arrangement of the sources for some complex spin waves excitations.

Self-Oscillations in a Nanogap Spin Hall Nano-Oscillator with a Perpendicularly Magnetized External Film

We study spin-current-induced magnetodynamics in a planar nanogap spin Hall nano-oscillator (SHNO) based on $\mathrm{Pt}/[\mathrm{Co}/\mathrm{Ni}{]}_{4}$ with a large perpendicular magnetic anisotropy (PMA) at in-plane field geometry. Two distinct dynamic modes are observed in the spin-torque ferromagnetic resonance (FMR) and the generated microwave spectra. The primary mode has a frequency above the FMR frequency and a significant blueshift of the frequency with the increasing current $I$, consistent with the propagating spin-wave mode. The secondary higher-frequency mode with a much weaker power intensity, discrete frequency jump, and a slight redshift of frequency is only observed at low fields below the saturation field of 2 kOe, suggesting that it is likely related to the emerged domain region with in-plane magnetization due to grain-boundary defects, which is further supported by the frequency enhancement with reducing temperature. The primary propagating mode decreases its frequency and broadens its minimum linewidth by reducing the temperature, indicating that the frequency-drift instability dominates its dynamical decoherence due to the boundary-pinning effect. Furthermore, the micromagnetic simulation reproduces the experimentally observed propagating spin-wave and in-plane domain modes, and provides their spatial characteristics. Our demonstrated propagating spin waves in a SHNO with an out-of-plane magnetization extended bilayer can facilitate long-distance mutual synchronization in the SHNO network arrays for enhancing coherence and power as spin-wave sources in magnon-based devices or spin-based neuromorphic computing.

Gyromotion of skyrmion along the nanofilm edge driven by spin waves

The dynamic behaviors of skyrmion in a nanofilm driven by spin waves have been numerically investigated by means of micromagnetic simulation. The results show that (i) the skyrmion shows a backward motion (towards the spin wave source) in the nanofilm with skyrmion Hall effect due to the spin wave scattering from the skyrmion; (ii) a fast (>100m/s) and periodic skyrmion motion can be achieved by introducing two spin wave sources to force the skyrmion move along the film edges; (iii) the velocity of the skyrmion motion depends on the amplitude and frequency of the driving magnetic fields as well as the nanofilm geometry, and the rotation sense of the skyrmion motion along the film edge depends on the topological number (Q=±1). The results might be a guidance for the design and development of future skyrmion-based devices in which skyrmions are used as information carriers.

Concept of the Optomagnonic Logic Operation

We present the concept of the optomagnonic logic gate, where interference of spin waves excited by femtosecond laser pulses at multiple points by the inverse Faraday effect in a bismuth-substituted yttrium iron garnet film plays a decisive role. The resulting spin-wave distribution is determined by constructive and destructive interference, which is controlled by the polarization of laser sources. A variable platform with a spatial light modulator is used to control the overall spin-wave distribution by manipulating the arrangement of the local spin-wave sources. We demonstrate the experimental optical excitation of backwards volume magnetostatic spin waves at multiple points and use wavelet analysis and micromagnetic numeric simulations to analyze them. Simulations provide an excellent agreement with the experimental data and are further used to model the magnon logical gate (xnor) based on optically excited spin-wave interference.

Imaging and phase-locking of non-linear spin waves

Non-linear processes are a key feature in the emerging field of spin-wave based information processing and allow to convert uniform spin-wave excitations into propagating modes at different frequencies. Recently, the existence of non-linear magnons at half-integer multiples of the driving frequency has been predicted for Ni80Fe20 at low bias fields. However, it is an open question under which conditions such non-linear spin waves emerge coherently and how they may be used in device structures. Usually non-linear processes are explored in the small modulation regime and result in the well known three and four magnon scattering processes. Here we demonstrate and image a class of spin waves oscillating at half-integer harmonics that have only recently been proposed for the strong modulation regime. The direct imaging of these parametrically generated magnons in Ni80Fe20 elements allows to visualize their wave vectors. In addition, we demonstrate the presence of two degenerate phase states that may be selected by external phase-locking. These results open new possibilities for applications such as spin-wave sources, amplifiers and phase-encoded information processing with magnons.

Self-Imaging of Spin Waves in Thin, Multimode Ferromagnetic Waveguides

Self-imaging of waves is an intriguing and spectacular effect. The phenomenon was first observed for light in 1836 by Henry Fox Talbot and to this day is the subject of research in many areas of physics, for various types of waves and in terms of different applications. This article is a Talbot-effect study for spin waves (SWs) in systems composed of a thin, ferromagnetic waveguide with a series of single-mode sources of SWs flowing into it. The proposed systems are studied with the use of micromagnetic simulations, and the SW self-imaging dependencies on many parameters are examined. We formulated conditions required for the formation of self-images and suitable for experimental realization. The results of the research form the basis for the further development of self-imaging-based magnonic devices.

Modelling nonlocal nonlinear spin dynamics in antiferromagnetic orthoferrites.

Excitation with an ultrashort light pulse is arguably the only way to control spins in antiferromagnetic materials at both the nanoscale in space and ultrafast time scale. While recent experiments highlighted tantalising opportunities for spin switching and magnonics in antiferromagnets, the theoretical description of antiferromagnetic spin dynamics driven by strongly localised and ultrashort excitation is in its infancy. Here we report a theoretical model describing the nonlocal and nonlinear spin response to the excitation by light. We show that strongly localised ultrafast excitation can drive spin switching, which propagates in space and acts as a source of spin waves. Our theoretical formalism is readily available to describe current and future ultrafast spectroscopy experiments in antiferromagnets.

Suppression of Skyrmion Hall Motion in Antiferromagnets Driven by Circularly Polarized Spin Waves

An investigation of spin waves interacting with antiferromagnetic spin textures is meaningful for future spintronic and magnonic-based memory and logic applications. In this work, we numerically study the skyrmion dynamics driven by circularly polarized spin waves in antiferromagnets and propose a method of suppressing the Hall motion. It is demonstrated that the application of two circularly polarized spin waves with opposite chirality allows the skyrmion motion straightly along the intersection line of the two spin wave sources. The skyrmion speed depending on these parameters of the spin waves and system is estimated, and a comparison with other methods is provided. Furthermore, two depinning behaviors of the skyrmion related to the strengths of the defect are also observed in the simulations. Thus, the proposed method could be used in precisely modulating the skyrmion dynamics, contributing to skyrmion-based memory device design.

Spin-Wave Driven Bidirectional Domain Wall Motion in Kagome Antiferromagnets.

We predict a mechanism to controllably manipulate domain walls in kagome antiferromagnets via a single linearly polarized spin-wave source. We show by means of atomistic spin dynamics simulations of antiferromagnets with kagome structure that the speed and direction of the domain wall motion can be regulated by only tuning the frequency of the applied spin wave. Starting from microscopics, we establish an effective action and derive the corresponding equations of motion for the spin-wave-driven domain wall. Our analytical calculations reveal that the coupling of two spin-wave modes inside the domain wall explains the frequency-dependent velocity of the spin texture. Such a highly tunable spin-wave-induced domain wall motion provides a key component toward next-generation fast, energy-efficient, and Joule-heating-free antiferromagnetic insulator devices.

Collimated Bidirectional Propagating Spin Wave Generated by a Nonlocal Spin-Current Nano-oscillator

Pure spin currents, generated by nonlocal spin injection and spin-orbit effects, have been widely used to control magnetization reversal and dynamics by current without charge transfer and its side effects in the device's actual working region. Here we experimentally demonstrate that a single coherent spin-wave mode can be excited by the pure spin current in a nonlocal spin-injection spin-valve device. The microwave spectra show that the observed spin-wave frequency is higher than the ferromagnetic resonance frequency, and almost does not change with the excitation current at moderate magnetic fields, indicating that the observed dynamical mode is a linear propagating spin-wave mode. Furthermore, micromagnetic simulations based on our device geometry generally reproduce the experimentally observed field-dependent and current-dependent oscillation characteristics, and provide us with the additional spatial information that the spin-wave mode exhibits collimated and bidirectional propagation paths in the direction perpendicular to the applied magnetic field. Our simulation results also show that the current-local Oersted field in our nonlocal device is much smaller than in conventional nanocontact magnetic oscillators, and has a minimal impact on the spin-wave dynamics. A near-symmetrically-collimated and bidirectional propagating spin-wave beam with magnetic field--controllable beam direction and current-independent frequency, achieved in our demonstrated nonlocal spin-current nano-oscillator, can be used as a local spin-wave source for magnonic logic devices and can be used to build daisy-chaining oscillatory neural networks with mutual synchronization.

Unidirectional emission and reconfigurability of channeled spin waves from a vortex core in a teardrop-shaped nanopatch

We propose the design of a teardrop-shaped magnetic patch as a unidirectional magnetically driven spin wave emitter capable of operating in a wide range of frequencies. We explore its potential through micromagnetic studies in line with vibrational sample magnetometry measurements and ferromagnetic resonance experiments. The proposed system is based on the excitation of a vortex core, acting as a source of spin waves, and a single Bloch domain wall, as a channel for the confinement and propagation of the mode in a sufficiently thick magnetic patch in the single magnetic vortex (SMV) state. The novelty consists in the reconfigurability and simplicity of the system, that is operational without the need of external saturating fields, retaining a single Bloch domain wall and a movable single vortex core. This allows significant suppression of the wave emission by means of an external bias field, which in turn allows a controllable valvelike effect. Following our proposed strategy for cultivating a single vortex core in the shape, and after a thorough micromagnetic study of the most prominent magnetization dynamics in the patch, we show that the SMV state can be obtained in a thick enough (80 nm) teardrop-shaped patch. Micromagnetic results show the potential of this simple structure as a tunable and unidirectional spin wave emitter. Experimental results also suggest that the required magnetic configuration has been experimentally obtained in the structure, in good agreement with micromagnetic simulations.

Direct observation of spin-wave focusing by a Fresnel lens

Spin waves are discussed as promising information carrier for beyond complementary metal-oxide semiconductor data processing. One major challenge is guiding and steering of spin waves in a uniform film. Here, we explore the use of diffractive optics for these tasks by nanoscale real-space imaging using x-ray microscopy and careful analysis with micromagnetic simulations. We discuss the properties of the focused caustic beams that are generated by a Fresnel-type zone plate and demonstrate control and steering of the focal spot. Thus, we present a steerable and intense nanometer-sized spin-wave source. Potentially, this could be used to selectively illuminate magnonic devices like nano-oscillators.

Spin waves and transverse domain walls driven by spin waves: Role of damping**Project supported by the National Natural Science Foundation of China (Grant Nos. 61774001 and 51972103), the Natural Science Foundation of Hebei Province of China (Grant No. F2019202141), and the Fund of the State Key Laboratory of Quantum Optics and Quantum Optics Devices, Shanxi University, China (Grant No. KF201906).

Based on the uniform, helical and spiral domain-wall magnetic configurations, the excited spin waves are studied with emphasis on the role of damping. We find that the damping closes the gap of dispersion, and greatly influences the dispersion in the long-wave region for the spin waves of spiral wall and helical structure. For the uniform configuration, the Dzyaloshinskii–Moriya interaction determines the modification of dispersion by the damping. Furthermore, we investigate the interaction between spin waves and a moving spiral domain wall. In the presence of damping, the amplitude of spin wave can increase after running across the wall for small wave numbers. Driving by the spin waves, the wall propagates towards the spin-wave source with an increasing velocity. Unlike the case without damping, the relation between the wall velocity and the spin-wave frequency depends on the position of wall.

Synchronized excitation of magnetization dynamics via spin waves in Bi-YIG thin film by slot line waveguide

We have studied magnetization dynamics of single Bi doped yttrium iron garnet single crystal (Bi-YIG) thin films by means of the high frequency power response induced by a slot line waveguide. Multiple absorption peaks that correspond to excitement states in magnetization dynamics appeared without the ferromagnetic resonance (FMR) condition. The peaks were strongly influenced by a waveguide linewidth and a distance between the lines. Micromagnetics simulation reveals that each line induces a local magnetization dynamics oscillation and generates spin waves. The spin wave that propagates from one of the lines interacts with the other side of local magnetization dynamics oscillation around the other line, resulting in an amplification of the oscillation when they are in synchronization with each other. This amplification occurs at both sides of the lines by interference. Thus, the possible mechanism of the excitation in the magnetization dynamics oscillation is the synchronization of mutual magnetization dynamics oscillation via spin waves. This technique resonantly excites the local magnetization dynamics without the FMR condition, which is applicable as a highly coherent spin wave source.

Propagating Magnetic Droplet Solitons as Moveable Nanoscale Spin-Wave Sources with Tunable Direction of Emission

Magnetic droplets are strongly nonlinear and localized spin-wave solitons that can be formed in current-driven nanocontacts. Here, we propose a simple way to launch droplets in an inhomogeneous nanoscopic waveguide. We use the drift motion of a droplet and show that in a system with broken translational symmetry, the droplet acquires a linear momentum and propagates. We find that the droplet velocity can be tuned via the strength of the break in symmetry and the size of the nanocontact. In addition, we demonstrate that the launched droplet can propagate up to several micrometers in a realistic system with reasonable damping. Finally, we demonstrate how an annihilating droplet delivers its momentum to a highly nonreciprocal spin-wave burst with a tunable wave vector with nanometer wavelengths. Such a propagating droplet can be used as a moveable spin-wave source in nanoscale magnonic networks. The presented method enables full control of the spin-wave emission direction, which can largely extend the freedom to design integrated magnonic circuits with a single spin-wave source.

Bidirectional spin-wave-driven domain wall motion in ferrimagnets

We investigate ferrimagnetic domain wall dynamics induced by circularly polarized spin waves theoretically and numerically. We find that the direction of domain wall motion depends on both the circular polarization of spin waves and the sign of net spin density of ferrimagnet. Below the angular momentum compensation point, left- (right-) circularly polarized spin waves push a domain wall towards (away from) the spin-wave source. Above the angular momentum compensation point, on the other hand, the direction of domain wall motion is reversed. This bidirectional motion originates from the fact that the sign of spin-wave-induced magnonic torque depends on the circular polarization and the subsequent response of the domain wall to the magnonic torque is governed by the net spin density. Our finding provides a way to utilize a spin wave as a versatile driving force for bidirectional domain wall motion.

Motion of a skyrmionium driven by spin wave

A skyrmionium is composed of two skyrmions with opposite skyrmion numbers and different sizes in the same track. In recent years, the motion of a skyrmionium driven by spin-polarized current has been investigated. However, the motion of a skyrmionium driven by a spin wave has not been reported. In this paper, we report our work concerning the numerical analysis of spin wave-driven motion of a skyrmionium in a nanotrack. The results show that the motion of a skyrmionium was significantly influenced by varying the frequency and amplitude of the AC magnetic field for exciting a spin wave, the distance between the spin wave source and the skyrmionium, the damping coefficient of the ferromagnetic track, and the track width. We found skyrmionium deformation during its initial motion process, but its shape could be recovered as it moved farther away from the spin wave source. Additionally, a series of velocity peaks were observed in the frequency range between 25 GHz and 175 GHz. When compared to a skyrmion, the skyrmionium could be driven by a spin wave to move in a wider frequency range at a higher velocity, and the velocity of the skyrmionium kept increasing with the increase in the track width till the track edge was far away from the skyrmionium. The result offers skyrmionium potential applications in wide-frequency spintronic devices.

Broadband conversion of microwaves into propagating spin waves in patterned magnetic structures

We have used time-resolved scanning Kerr microscopy and micromagnetic simulations to demonstrate that, when driven by the spatially uniform microwave field, the edges of patterned magnetic samples represent both efficient and highly tunable sources of propagating spin waves. The excitation is due to the local enhancement of the resonance frequency induced by the non-uniform dynamic demagnetizing field generated by precessing magnetization aligned with the edges. Our findings represent a crucial step forward in the design of nanoscale spin-wave sources for magnonic architectures and are also highly relevant to the understanding and interpretation of magnetization dynamics driven by spatially uniform magnetic fields in patterned magnetic samples.

Shape dependent resonant modes of skyrmions in magnetic nanodisks

Resonant modes of a single Néel type skyrmion in confined nanodisks with varying aspect ratios (AR) are investigated using micromagnetic simulations. The AR of the skyrmion has a non-linear dependence on that of the nanodisk. The power spectra of skyrmions in nanodisks with AR ranging from 1.0 to 2.0 are calculated. With the increase of disk AR, multiple new modes emerge in the power spectrum, which originate from the broken rotational symmetry of both the nanodisk and the skyrmion. All of the spin wave modes are resolved by spatial maps of the real time magnetization fluctuations. New mixed modes such as rotation modes and oscillation modes with different azimuthal and radial components are identified in the elliptical nanodisk with AR=1.8. The new emergent modes may provide new approaches to skyrmion-based oscillators and spin wave sources in confined structures.