Surface Spin Waves Research Articles

Route toward semiconductor magnonics: Light-induced spin-wave nonreciprocity in a YIG/GaAs structure

We demonstrate laser-induced nonreciprocity of spin waves in the ferromagnetic-semiconductor structure. Surface spin waves in yttrium iron garnet film grown at the top of $n$-type gallium arsenide substrate were studied by means of Brillouin light-scattering spectroscopy. It is shown that spin-wave dispersion can be modified in a controlled manner by laser radiation. We observe the difference of up to 225 MHz when comparing the frequencies of counterpropagating spin waves. We attribute this frequency shift to the mutual influence of nonreciprocal spin-wave modal profiles and differences in magnetic anisotropies at two film surfaces as the result of laser-induced conductivity variation in GaAs substrate. We propose a simple model based on analytical dipole theory to describe the induced spin-wave nonreciprocity. Our results show the possibility of integration of magnonics and semiconductor electronics on the base of YIG/GaAs structures.

Phase Transition in Frustrated Magnetic Thin Film-Physics at Phase Boundaries.

In this review, we outline some principal theoretical knowledge of the properties of frustrated spin systems and magnetic thin films. The two points we would like to emphasize: (i) the physics in low dimensions where exact solutions can be obtained; (ii) the physics at phase boundaries where interesting phenomena can occur due to competing interactions of the two phases around the boundary. This competition causes a frustration. We will concentrate our attention on magnetic thin films and phenomena occurring near the boundary of two phases of different symmetries. Two-dimensional (2D) systems are in fact the limiting case of thin films with a monolayer. Naturally, we will treat this case at the beginning. We begin by defining the frustration and giving examples of frustrated 2D Ising systems that we can exactly solve by transforming them into vertex models. We will show that these simple systems already contain most of the striking features of frustrated systems such as the high degeneracy of the ground state (GS), many phases in the GS phase diagram in the space of interaction parameters, the reentrance occurring near the boundaries of these phases, the disorder lines in the paramagnetic phase, and the partial disorder coexisting with the order at equilibrium. Thin films are then presented with different aspects: surface elementary excitations (surface spin waves), surface phase transition, and criticality. Several examples are shown and discussed. New results on skyrmions in thin films and superlattices are also displayed. By the examples presented in this review we show that the frustration when combined with the surface effect in low dimensions gives rise to striking phenomena observed in particular near the phase boundaries.

Нелинейное затухание и нелинейный фазовый набег интенсивных спиновых волн в экранированных ферритовых пленках

AbstractWe analyze for the first time nonlinear damping and nonlinear phase shift of intense surface spin waves in screened yttrium–iron garnet films. Nonlinear damping coefficients are determined using the phenomenological model of spin wave propagation in such films. It is found that nonlinear phase shift in screened films is a weak effect as compared with free films and amounts to tens of degrees. At the same time, nonlinear damping in screened films is comparable with the corresponding value for free films. It is shown that for micrometer-thick films, the losses introduced upon an increase of power to 15 dBm become larger by 2–5 dB.

Modified dispersion law for spin waves coupled to a superconductor

In this work, we consider dispersion laws of spin waves that propagate in a ferromagnet/superconductor bilayer, specifically in a ferromagnetic film coupled inductively to a superconductor. The coupling is viewed as an interaction of a spin wave in a ferromagnetic film with its mirrored image generated by the superconductor. We show that, in general, the coupling enhances substantially the phase velocity of magnons in in-plane spin wave geometries. In addition, a heavy nonreciprocity of the dispersion law is observed in the magnetostatic surface spin wave geometry where the phase velocity depends on the direction of the wave propagation.

Spin wave propagation in three-dimensional magnonic crystals and coupled structures

We present three-dimensional (3D) model of periodic meander-shaped ferromagnetic films. Spin wave propagation in such films and vertically coupled structures was studied using micromagnetic modeling and theoretical plane wave method. Spin waves in these structures essentially propagate in film’s segments located at right angles with respect to each other. This makes really possible for the wave to propagate in three dimensions. We calculated internal effective magnetic fields and obtained spin wave dispersion in single and vertically coupled structures. Additionally, comparison of surface and volume spin waves propagation in such meander films was provided. Our results can be useful for magnonic logic elements development.

Features of Dispersion Characteristics of Surface Spin Waves in Coupled Antiferromagnetic Films with Easy-Axis Anisotropy

Propagation of magnetostatic spin waves in a layered structure consisting of two vertically coupled via a nonmagnetic dielectric layer antiferromagnetic (AFM) films has been studied. The results of calculation of the dispersion dependence for the AFM films with an easy-axis anisotropy are presented. It has been shown that splitting of coupled modes occurs in the region of AFM resonance frequencies. The behavior of coupled modes as a function of the parameter of coupling between films has been studied.

Surface spin waves in coupled easy-axis antiferromagnetic films

Abstract We present results of studying properties of surface spin waves in vertically coupled antiferromagnetic films with easy-axis anisotropy. The dispersion curves and frequency region of the existence of the surface magnetostatic waves are obtained using theoretical model for different antiferromagnetic materials. This study was carried out by solving the dispersion relation for the surface magnetostatic waves. Behavior of two types of dispersion curves on the value of coupling parameter is investigated. We conclude, that results have potential applications for antiferromagnetic THz spintronics.

Raman fingerprint of two terahertz spin wave branches in a two-dimensional honeycomb Ising ferromagnet

Two-dimensional (2D) magnetism has been long sought-after and only very recently realized in atomic crystals of magnetic van der Waals materials. So far, a comprehensive understanding of the magnetic excitations in such 2D magnets remains missing. Here we report polarized micro-Raman spectroscopy studies on a 2D honeycomb ferromagnet CrI3. We show the definitive evidence of two sets of zero-momentum spin waves at frequencies of 2.28 terahertz (THz) and 3.75 THz, respectively, that are three orders of magnitude higher than those of conventional ferromagnets. By tracking the thickness dependence of both spin waves, we reveal that both are surface spin waves with lifetimes an order of magnitude longer than their temporal periods. Our results of two branches of high-frequency, long-lived surface spin waves in 2D CrI3 demonstrate intriguing spin dynamics and intricate interplay with fluctuations in the 2D limit, thus opening up opportunities for ultrafast spintronics incorporating 2D magnets.

Time-Resolved Measurements of Surface Spin-Wave Pulses at Millikelvin Temperatures

In this work, we experimentally investigate the propagation of pulsed magnetostatic surface spin-wave (magnon) signals in an yttrium iron garnet (YIG) waveguide at millikelvin temperatures. Our measurements are performed in a dilution refrigerator at microwave frequencies. The excellent signal-to-noise ratio afforded by the low-temperature environment allows the propagation of the pulses to be observed in detail. The work gives insight both into low-temperature magnon dynamics in YIG and the potential application of systems of propagating magnons to solid-state quantum information processing.

Surface anisotropy and spin wave resonance in submicron amorphous wires

Abstract The influence of surface magnetic anisotropy on ferromagnetic resonance (FMR) in submicron wires is investigated both theoretically and experimentally. An analytical formula for resonance fields of radial spin waves in a long axially magnetized circular cylinder is obtained. Three types of surface anisotropy with the easy direction along one of the cylindrical coordinates are considered. For sufficiently strong surface anisotropy with hard direction parallel to the cylinder axis the surface spin wave mode is observed. The theoretical results are verified by FMR measurements at six microwave frequencies from 9 to 69 GHz on glass-covered amorphous FeCrSiB wires with diameters from 541 to 1032 nm. From the bulk spin wave resonances the exchange stiffness constant about 6.5 10−7 erg/cm is obtained. The frequency dependence of the surface mode resonance field indicates that a perpendicular surface anisotropy with anisotropy constant Ks = 6.2 erg/cm2 is present at the metal/glass interface.

Characteristics of Spin Surface Waves in a Tangentially Magnetized Metal–Dielectric–Ferrite–Dielectric–Metal Structure

The dispersion and isofrequency characteristics of surface spin waves in a tangentially magnetized metal–dielectric–ferrite–dielectric–metal structure are studied. It is found that the dispersion surface of spin waves in this structure goes from the frequency axis in a wide frequency interval, in a part of which the isofrequency dependences of the waves have the form of a closed loop starting at k → 0. It is found that the dispersion surface can simultaneously have bell-shaped regions, in which the isofrequency dependences near the vertex are similar to ovals, and saddle-like regions, in which the isofrequency dependences near the saddle point are similar to hyperbolas. It is established that the range of possible orientations of the wave vector at a given frequency can be wider than the interval limited by the cutoff angles. It was found that the cross section of the dispersion surface by an angle exceeding the maximum cutoff angle is a dispersion relation with a limited interval of wavenumbers.

Magnetic domain walls as broadband spin wave and elastic magnetisation wave emitters

We report on the direct observation of spin wave and elastic wave emission from magnetic domain walls in ferromagnetic thin films. Driven by alternating homogeneous magnetic fields the magnetic domain walls act as coherent magnetisation wave sources. Directional and low damped elastic waves below and above the ferromagnetic resonance are excited. The wave vector of the magnetoelastically induced acoustic waves is tuned by varying the excitation frequency. The occurrence of elastic wave emission is proved by a combination of micromagnetic and mechanical finite element simulations. Domain wall emitted magnetostatic surface spin waves occur at higher frequencies, which characteristics are confirmed by micromagnetic simulations. The distinct modes of magnetisation wave excitation from micromagnetic objects are a general physical phenomenon relevant for dynamic magnetisation processes in structured magnetic films. Magnetic domain walls can act as reconfigurable antennas for spin wave and elastic wave generation. The wave orientation can be controlled separately via the domain wall orientation for elastic waves and via magnetization orientation for magnetostatic surface spin waves.

The Influence of Dissipation on the Dispersion Characteristics of Surface Spin Waves

Dissipation of surface spin waves propagating in arbitrary directions in a magnetized ferromagnetic film is studied. It is shown that the allowance for dissipation restricts the domain of possible wave vectors to a finite region and leads to the appearance above this region of the continuation of a dispersion surface in the form of an inverted cap with a negative slope and a large damping of the waves. It is noted that the solution gives an infinite number of almost coincident dispersion surfaces with a sharp increase in the damping coefficient with each next surface, so that the waves become practically nonpropagating. The analysis of the dispersion surfaces shows that the lower part has a positive slope and describes forward waves, the damping of which is proportional to a small dissipation constant but, in the upper part of the surface, the damping coefficient sharply increases. It is found that the directions of the group velocities of the waves substantially change in comparison with a dissipationless medium and some frequency and angular restrictions on the propagation of surface waves also become different.

Applicability of the Group Velocity Concept in Describing a Surface Spin Wave

The conditions under which a useful signal modulating a surface spin wave is not distorted and its propagation in a ferrite film can be characterized using the group velocity concept is studied. It is shown that the greater the ratio of the first and second derivatives of the dispersion dependence for the spin wave, the greater the distance this signal can pass without distortion. It is found that for a surface spin wave, the best conditions for the transmission of the useful signal are observed for a small region of wave numbers in the initial part of the wave spectrum, where this distance can be as great as several millimeters.

Superdirectional beam of surface spin wave

The visualized diffraction patterns of a surface spin wave excited by an arbitrarily oriented linear transducer are investigated experimentally in the plane of a tangentially magnetized ferrite film for the case in which the transducer length D is much larger than the wavelength . It is shown experimentally and theoretically that the angular width of a diffracted surface spin wave beam in an anisotropic ferrite film can take values greater or less than and can also be zero. For the last case a superdirectional (non-expanding) beam of the surface spin wave is observed experimentally: the smearing of the beam energy along the film plane is absent and the length of the beam trajectory is maximal . It is found that the well-known Rayleigh criterion used in isotropic media cannot be used to estimate the angular width of spin wave beams. The experimental results are in good agreement with theoretical investigations, predictions, calculations and formulas obtained recently.

Cutoff Angles of the Backward Spin Wave in a Tangentially Magnetized Ferrite Plate

The cutoff angles for the wave vector and the group velocity vector of a backward spin wave propagating in a tangentially magnetized ferrite plate are calculated. It is established that the expressions for these angles do not depend on the mode number and coincide with the analogous expressions for the corresponding cutoff angles of a spin wave in an unbounded ferromagnet. It is found that the regions corresponding to all possible orientations of the wave vector for the backward and surface spin waves in a ferrite slab and for a spin wave in an unbounded ferromagnet are adjacent and nonintersecting. It is shown that, if the isofrequency dependence of the wave has inflection points, then the range of all possible orientations of the group velocity vector of the wave can be wider than the angular interval enclosed between the cutoff angles for the group velocity vector.

Filtration of Surface Magnetostatic Waves in Yttrium Iron Garnet Films of Variable Width Excited by Focusing Transducers

We have studied the filtration of surface spin waves in yttrium iron garnet (YIG) films of variable width excited by focusing transducers integrated with the film.

Effect of Losses on the Group Velocity of Surface Spin Waves

It is found that considering the dissipation of surface spin waves propagating in arbitrary directions in a magnetized ferromagnetic film results in a substantial change in the dispersion surface: it is transformed from an infinite surface into a closed and bounded one consisting of an upper part with strong attenuation and a lower part with weak attenuation that join along an inclined ellipsoidal curve. The change in the dispersion surface is shown to have a considerable effect on the directions of the group velocity of the waves.

Characteristics of Surface Spin Waves in a Metal–Dielectric–Ferrite–Dielectric–Metal Structure

The dispersion and isofrequency characteristics of surface spin waves in a tangentially magnetized metal–dielectric–ferrite–dielectric–metal structure are investigated. A loop-like change in the isofrequency dependences for spin waves is observed in a certain range of frequencies, and the origin of the loop is always located at wave number value k → 0.

Optical determination of the exchange stiffness constant in an iron garnet

Brillouin light scattering measurements were performed in the backscattering geometry on a Bi-substituted rare earth iron garnet. We observed two different peaks: One is attributed to a surface spin wave in the dipole-exchange regime while the other is referred to as a backscattering magnon mode, because the incident light in this case was scattered backward by exchange-dominated spin wave inside the material. We propose a method to estimate the exchange stiffness constant from the frequency of the backscattering magnon mode. The obtained value is comparable with the previously reported values for Y3Fe5O12.