Spin-wave Research Articles

Influence of local Dzyaloshinskii–Moriya interaction on spin waves in symmetrical heavy metal/ferromagnet/heavy metal multilayer

Abstract In a heavy metal/ferromagnet/heavy metal multilayer with inversion symmetry, the total interlayer Dzyaloshinskii–Moriya interaction (iDMI) is expected to be absent while the local iDMI constants for the top and bottom FM atomic layers can still be nonzero. This study investigates the influence of local iDMI on spin waves in this structure, in which the FM medium consists of two atomic layers with opposite local iDMI at the bottom HM/FM interface and the top FM/HM interface. Theoretical analysis and simulations are conducted to examine the spin-wave dynamics and propagation in the symmetrical sandwich structure. The results show that the local iDMI decreases the characteristic frequency of spin waves, indicating a regulation of spin wave propagation by iDMI and shows the asymmetry of spin-wave propagation between the two FM atomic layers due to iDMI. This work paves the way for deep exploration of spin waves in such structures and offers valuable insights for subtle magnetic interactions in other symmetrical multilayers.

Micromagnetic study of the dynamics of toron chains

This work analyzes the nucleation and stabilization of torons in cylindrical nanopillars with bulk DMI and easy-plane anisotropy. A micromagnetic study reveals the dependence of toron chains on the nanopillar length and the different behaviors when nucleating an even or odd number of torons in a magnetic nanopillar. Spin wave resonant modes in these systems are explored, evidencing differences according to the number of torons. The interplay between torons, chiral bobbers, and their associated spin wave modes is analyzed, which is relevant for applications demanding dynamical modes in the range of gigahertz.

Magnetic ordering and dynamics in monolayers and bilayers of chromium trihalides: atomistic simulations approach

We analyze magnetic properties of monolayers and bilayers of chromium iodide, \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {CrI}_3$$\\end{document}, in two different stacking configurations: AA and rhombohedral ones. Our main focus is on the corresponding Curie temperatures, hysteresis curves, equilibrium spin structures, and spin wave excitations. To obtain all these magnetic characteristic, we employ the atomistic spin dynamics and Monte Carlo simulation techniques. The model Hamiltonian includes isotropic exchange coupling, magnetic anisotropy, and Dzyaloshinskii-Moriya interaction. As the latter interaction is relatively weak in pristine \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {CrI}_3$$\\end{document}, we consider a more general case, when the Dzyaloshinskii-Moriya interaction is enhanced externally (e.g. due to gate voltage, mechanical strain, or proximity effects). An important issue of the analysis is the correlation between hysteresis curves and spin configurations in the system, as well as formation of the skyrmion textures.

Nonreciprocal transmission of surface acoustic waves induced by magneotoelastic coupling with an anti-magnetostrictive bilayer

In this study, we report giant nonreciprocal transmission of shear-horizontal surface acoustic waves (SH-SAWs) in a ferromagnetic bilayer structure with negative–positive magnetostriction configuration. Although the directions of magnetization in the neighboring layers are parallel, SH-SAWs can excite optical-mode spin waves (SWs) via magnetoelastic coupling at relatively low frequencies, which is much stronger than acoustic-mode SWs at high frequencies. The measured magnitude nonreciprocity or isolation of SH-SAWs exceeds 40 dB (or 80 dB/mm) at 2.333 GHz. In addition, maximum nonreciprocal phase accumulation reaches 188° (376°/mm). Our theoretical model and calculations provide an insight into the observed phenomena and demonstrate a pathway for further improving nonreciprocal acoustic devices toward highly compact microwave isolators and circulators.

Time-resolved x-ray imaging of nanoscale spin-wave dynamics at multi-GHz frequencies using low-alpha synchrotron operation

Time-resolved x-ray microscopy is used in a low-alpha synchrotron operation mode to image spin dynamics at an unprecedented combination of temporal and spatial resolution. Thereby, nanoscale spin waves with wavelengths down to 70 nm and frequencies up to 30 GHz are directly observed in ferromagnetic thin film microelements with spin vortex ground states. In an antiparallel ferromagnetic bilayer system, we detect the propagation of both optic and acoustic modes, the latter exhibiting even a strong non-reciprocity. In single-layer systems, quasi-uniform spin waves are observed together with modes of higher order (up to the 4th order), bearing precessional nodes over the thickness of the film. Furthermore, the effects of magnetic material properties, film thickness, and magnetic fields on the spin-wave spectrum are determined experimentally. Our experimental results are consistent with numerical calculations from a micromagnetic theory even on these so-far unexplored time- and length scales.

Skyrmion motion monitoring based on a ferromagnetic nanodot chain

As one of the most promising information carriers, the generation, manipulation, and detection of magnetic skyrmions have emerged as a hot topic in the field of spintronics. However, a major bottleneck to their practical application lies in the existing limitations of detection technology, which fails to accurately locate skyrmions or monitor their real-time motion behavior. In this work, we propose a patterned heterostructure scheme comprising a nanodot chain (NDC) layer and a skyrmion nano-racetrack layer for precise monitoring of skyrmion motion. By exploiting the stray field generated by the moving skyrmion within the racetrack layer, magnetization changes are induced in nanodots within the NDC layer. These changes then translate into high-frequency magnetization oscillation signals that encode valuable information about the dynamic characteristics of driven skyrmions, such as speed and acceleration of the skyrmion, either by spin waves or spin currents. This scheme holds great potential for advancing spintronic devices based on a profound understanding of skyrmion dynamics.

Hydrodynamic equations for a U(N) invariant superfluid

Abstract In this paper, we develop an appropriate set of hydrodynamic equations for a U(N) invariant superfluid that couple the dynamics of superflow and magnetization. In the special case when both the superfluid and normal velocities are zero, the hydrodynamic equations reduce to a generalized version of Landau-Lifshitz equation for ferromagnetism with U(N) symmetry. When both velocities are non-zero, there appears couplings between the superflow and magnetization dynamics, and the superfluid velocity no longer satisfies the irrotational condition. On the other hand, the magnitude of magnetization is no longer a constant of motion as was the case for the standard Landau-Lifshitz theory. In comparison with the simple superfluid, the first and second sounds are modified by a non-zero magnetization through various thermodynamic functions. For U(2) invariant superfluid, we get both (zero-) sound wave and a spin wave at zero temperature. It is found that the dispersion of spin wave is always quadratic, which is consistent with microscopic analysis. In the Appendix, we
show that the hydrodynamic equation for a U(N) invariant superfluid can be obtained from the general hydrodynamic equation with arbitrary internal symmetries.

Parametric Excitation and Instabilities of Spin Waves Driven by Surface Acoustic Waves

AbstractThe parametric excitation of spin waves by coherent surface acoustic waves in metallic magnetic thin film structures is demonstrated experimentally using Brillouin light scattering spectroscopy. Complementary micromagnetic simulations and analytical modeling reveal that, depending on the experimental conditions, the spin‐wave instabilities originate from different phonon‐magnon and magnon‐magnon scattering processes. This opens novel ways to create micro‐scaled nonlinear magnonic systems for logic and data processing that profit from the efficient excitation of phonons using piezoelectricity.

Control of spin-wave polarity and velocity using a ferrimagnetic domain wall

We present a theoretical study of the scattering of spin waves by a domain wall (DW) in a ferrimagnetic (FiM) spin chain in which two sublattices carry spins of unequal magnitudes. We find that a narrow but atomically smooth FiM DW exhibits a different behavior in comparison with similarly smooth ferromagnetic and antiferromagnetic DWs due to the inequivalence of the two sublattices. Specifically, for sufficiently weak anisotropy, the smaller spin at the center of the DW is found to become precisely normal to the easy axis, selecting an arbitrary direction in the xy plane and thereby breaking the U(1) spin-rotational symmetry spontaneously. This particular form of a FiM DW does not occur in antiferromagnetic systems and is shown to lead to a strong dependence of spin-wave scattering pattern on the state of polarization of the spin wave, which can be either right-handed or left-handed, suggesting the utilization of such a narrow DW as a spin-wave filter. Moreover, we find that in the case of an atomically sharp DW, where all the spins point either up or down due to strong easy-axis anisotropy and therefore the polarization of the spin wave is conserved upon transmission, the wave vector of the spin-wave changes after passing through the DW leading to a change in the group velocity of the spin wave. This change of the wave vector indicates the acceleration or deceleration of the spin waves and thus a sharp FiM DW could serve as a spin-wave accelerator or decelerator in spintronics devices, offering a functionality absent in a ferromagnetic and an antiferromagnetic counterpart. Our results indicate that FiM spin textures can interact with spin waves distinctly from ferromagnetic and antiferromagnetic counterparts, suggesting that they may offer spin-wave functionalities that are absent in more conventional magnets. Published by the American Physical Society 2024

Magnetic-field controlled on-off switchable non-reciprocal negative refractive index in non-Hermitian photon-magnon hybrid systems

Photon-magnon coupling, where electromagnetic waves interact with spin waves, and negative refraction, which bends the direction of electromagnetic waves unnaturally, constitute critical foundations and advancements in the realms of optics, spintronics, and quantum information technology. Here, we explore a magnetic-field-controlled, on-off switchable, non-reciprocal negative refractive index within a non-Hermitian photon-magnon hybrid system. By integrating an yttrium iron garnet film with an inverted split-ring resonator, we discover pronounced negative refractive index driven by the system’s non-Hermitian properties. This phenomenon exhibits unique non-reciprocal behavior dependent on the signal’s propagation direction. Our analytical model sheds light on the crucial interplay between coherent and dissipative coupling, significantly altering permittivity and permeability’s imaginary components, crucial for negative refractive index’s emergence. This work pioneers new avenues for employing negative refractive index in photon-magnon hybrid systems, signaling substantial advancements in quantum hybrid systems.

Antiferromagnetic standing spin waves generated in NiO thin films by short strain pulses

Antiferromagnetic standing spin waves generated in NiO thin films by short strain pulses

Characterizing the diffuse continuum excitations in the classical spin liquid h−YMnO3

We extend previous inelastic neutron scattering results on the geometrically frustrated antiferromagnet hexagonal-YMnO3, which has been suggested to belong to the class of classical spin liquids. We expand the energy transfer range of the diffuse signal up to 6.9 meV within a wide temperature range around the ordering temperature, TN. The two distinct diffuse signals in the a−b plane, the signal localized at Γ′ and the scattering intensity connecting Γ′ points over the M′, are shown to be only weakly energy dependent. In addition, an external magnetic field of up to 10.5 T applied along c is shown to have no effect on the diffuse signal. In the orthogonal scattering plane, the signals are shown to be dependent on l only through the magnetic form factor, showing that the correlations are purely two-dimensional, and supporting the origin of this to be the frustrated Mn3+ triangles. This result is corroborated by atomistic spin dynamics simulations showing similar scattering vector and temperature behaviors. Lastly, data for the spin wave scattering in the (h,0,l) plane allow for a discussion of the magnetic ground state where better agreement is found between the data and an ordered structure of the Γ1 or Γ3 symmetry, albeit crystal electric field arguments dismiss the Γ1 as a possibility. Published by the American Physical Society 2024

Spin waves and orbital contribution to ferromagnetism in a topological metal

Honeycomb and kagome lattices can host propagating excitations with non-trivial topology as defined by their evolution along closed paths in momentum space. Excitations on such lattices can also be momentum-independent, and the associated flat bands are of interest due to strong interactions between heavy quasiparticles. Here, we report the discovery — using circularly polarized X-rays for the unambiguous isolation of magnetic signals — of a nearly flat spin-wave band and large (compared to elemental iron) orbital moment in the metallic ferromagnet Fe3Sn2 with compact AB-stacked kagome bilayers. As a function of out-of-plane momentum, the nearly flat optical mode and the global rotation symmetry-restoring acoustic mode are out of phase, consistent with a bilayer exchange coupling that is larger than the already large in-plane couplings. The defining units of this topological metal are therefore triangular lattices of octahedral iron clusters rather than weakly coupled kagome planes. The spin waves are strongly damped when compared to elemental iron, opening the topic of topological boson–fermion interactions for deeper exploration within this material platform.

Role of voltage-controlled magnetic anisotropy in the recent development of magnonics and spintronics

With significant recent progress in the thin film deposition and nanofabrication technology, a number of physical phenomena occur at the interfaces of magnetic thin films, and their heterostructures have been discovered. Consequently, the electric field-induced modulation of those interfacial properties mediated through spin–orbit coupling promises to develop magnetic material based smarter, faster, miniaturized, energy efficient spintronic devices. Among them, the electric field-induced modification of interfacial magnetic anisotropy, popularly termed as voltage-controlled magnetic anisotropy (VCMA), has attracted special attention because of its salient features. This article is devoted to reviewing the recent development of magnonics, which deals with collective precessional motion of ordered magnetic spins, i.e., spin waves (SWs), and skyrmions with chiral spin textures, with VCMA, including the perspectives of this research field. Starting with a broad introduction, the key features of VCMA and its advantages over other electric field-induced methods are highlighted. These are followed by describing the state-of-the-art of VCMA, and various other direct and indirect electric field-induced methods for magnetization reversal; controlling skyrmion dynamics; excitation, manipulation, and channeling of SWs; and tailoring magnonic bands. The critical challenges, their possible solutions, and future perspectives of this field are thoroughly discussed throughout the article.

Detection of geometric phases in spin waves using nitrogen vacancy centers

Detection of geometric phases in spin waves using nitrogen vacancy centers

Bragg resonances in the yttrium iron garnet – platinum – yttrium iron garnet layered structure

We studied theoretically the interaction between the spin current in a conductor with a strong spin-orbit coupling (platinum, Pt) and the spin wave in yttrium iron garnet ferromagnetic layers (YIG) with periodic thickness modulation under conditions of Bragg resonances and interlayer coupling. It is shown that in the YIG/Pt/YIG sandwich structure the conditions for two Bragg resonances in the first Brillouin area in the spin wave spectrum are fulfilled. The spin current in Pt allows frequency tuning of the resonances and control the depth of the spin wave band gap corresponding to the resonance conditions.

Magnon confinement in a nanomagnonic waveguide by a magnetic Moiré superlattice

The study of moiré superlattices has revealed intriguing phenomena in electronic systems, including unconventional superconductivity and ferromagnetism observed in magic-angle bilayer graphene. This approach has recently been adapted to the field of magnonics. In this Letter, we investigate the confinement of spin waves in a nanomagnonic waveguide integrated on top of a magnetic moiré superlattice. Our numerical analysis reveals a magnonic flatband at the center of the Brillouin zone, created by a 3.5° twist in the moiré superlattice. The flatband, characterized by a high magnon density of states and a zero group velocity, allows for the confinement of magnons within the AB stacking region. The flatband results from the mode anticrossing of several different magnon bands, covering a wavevector range of nearly 40 rad/μm and a 166 nm wide spatial distribution of the magnon trapping in the waveguide. Our results pave the way for nanomagnonic devices and circuits based on spin-wave trapping in magnon waveguides.

Breaking Surface-Plasmon Excitation Constraint via Surface Spin Waves.

Surface plasmons in two-dimensional (2D) electron systems have attracted great attention for their promising light-matter applications. However, the excitation of a surface plasmon, in particular, transverse-electric (TE) surface plasmon, remains an outstanding challenge due to the difficulty to conserve energy and momentum simultaneously in the normal 2D materials. Here we show that the TE surface plasmons ranging from gigahertz to terahertz regime can be effectively excited and manipulated in a hybrid dielectric, 2D material, and magnet structure. The essential physics is that the surface spin wave supplements an additional freedom of surface plasmon excitation and thus greatly enhances the electric field in the 2D medium. Based on widely used magnetic materials like yttrium iron garnet and manganese difluoride, we further show that the plasmon excitation manifests itself as a measurable dip in the reflection spectrum of the hybrid system while the dip position and the dip depth can be well controlled by an electric gating on the 2D layer and an external magnetic field. Our findings should bridge the fields of low-dimensional physics, plasmonics, and spintronics and open a novel route to integrate plasmonic and spintronic devices.

Spin Wave Chiral Scattering by Skyrmion Lattice in Ferromagnetic Nanotubes

Spin Wave Chiral Scattering by Skyrmion Lattice in Ferromagnetic Nanotubes

Spin-wave mode coupling in the presence of the demagnetizing field in cobalt-permalloy magnonic crystals

We present the results of studies on the non-uniform frequency shift of spin wave spectrum in a two-dimensional magnonic crystal of cobalt/permalloy under the influence of external magnetic field changes. We investigate the phenomenon of coupling of modes and, as a consequence, their hybridization. By taking advantage of the fact that compressing the crystal structure along the direction of the external magnetic field leads to an enhancement of the demagnetizing field, we analyse its effect on the frequency shift of individual modes depending on their concentration in Co. We show that the consequence of this enhancement is a shift in the coupling of modes towards higher magnetic fields. This provides a potential opportunity to design which pairs of modes and in what range of fields hybridization will occur.