Nonlinear phase shifts induced by pumping spin waves in magnonic crystals

Magnonics rely on the wave nature of the magnetic excitations to process information, an approach that is common to many fields such as photonics, phononics, and plasmonics. Nevertheless, magnons, the quanta of spin-wave excitations, have the unique advantage to be at frequencies that are lying between a few GHz to tens of GHz, that is, in the technologically relevant radio-frequency bands for 4G and 5G telecommunications. Furthermore, their typical wavelengths are compatible with on-chip integration. Here, we demonstrate radio-frequency signal filtering by a micron-scale magnonic crystal (MC) based on a nanopatterned 20 nm-thick film of yttrium iron garnet with a minimum feature size of 100 nm where the Bragg vector is set to be kB = 2.1 μm–1. We map the intensity and the phase of spin waves (SWs) propagating in the periodic magnetic structure using phase-resolved microfocus Brillouin light-scattering spectroscopy. Based on these maps, we obtain the SW dispersion and the attenuation characteristics. Efficient filtering is obtained with a frequency selectivity of 20 MHz at an operating frequency of 4.9 GHz. The results are analyzed by performing time- and frequency-resolved full-scale micromagnetic simulations of the MC that reproduce quantitatively the complexity of the harmonic response across the magnonic band gap and allow the identification of the relevant SW-quantized modes, thereby providing an in-depth insight into the physics of SW propagation in periodically modulated nanoscale structures.

In recent years much research has been directed towards the use of spin waves (SWs) for signal processing at microwave and subterahertz frequencies due to the possibility to carry the information signal without the transmission of a charge current [1, 2]. Magnonic crystal (MCs) [1, 2] have attracted a significant attention due to their wide range of application and numerous ways to fabrication. MCs are used in linear and nonlinear magnonics as a building block of magnonic networks. In the majority of the previously realized devices the MCs have been used as the high-Q tunable rejection filter due to the most important features of the MCs - the magnonic forbidden (rejection) band or magnonic band gap in the spin-wave spectra. However, the possible use of the magnonic forbidden band to fabricate the drop filters in magnonic integrated circuits can extend the application of MCs, in particular, for the magnonic logic [1, 2, 3]. In the asymmetric MC the spin waves have non-equal propagation constants and thus the phase-matching condition is violated. While phase mismatch might seem on the surface to decrease the coupling efficiency, the unusual increase of the spin-wave coupling can be observed at the frequencies in the vicinity of the magnonic band gap. This idea can underpin the experimental and theoretical studies regarding the side-coupled magnonic crystals, which can also act as frequency selective multiplexers. Here we report the experimental observation of the spin-wave coupling in the asymmetric adjacent magnonic crystals based on multimode yttrium iron garnet (YIG) waveguides [4– 8]. We show, that the combination of frequency and spatial filtering features of the MC and spin-wave coupling in the adjacent magnetic waveguide leads to the realization of the magnonic drop filter ( Fig.1). We also identify the mechanism of the efficient spin-wave power transmission between the magnonic crystals. As a major result, we have demonstrated by the means of the space-resolved Brillouin light scattering (BLS) technique, that non-identical MCs within close proximity demonstrates the efficient spin-wave coupling at the frequency of the magnonic forbidden gap of one of the MCs. Thus MCs can be used not only to achieve the spatial and frequency filtering of spin-wave signal but also to provide the phase condition with an efficient spin-wave power transfer from the input to drop port of magnonic coupler. The obtained results open new perspectives for the future-generation electronics using integrated magnonic networks. This work was supported partially by the grant from Russian Science Foundation (No. 16-19-10283).

It is experimentally shown that metal cladding of the surface of a one-dimensional magnonic crystal destroys the Bragg band gaps in microwave transmission spectra of propagating magnetostatic surface spin waves in magnonic crystal. This is a consequence of violating a phase synchronism condition of forward and reflected by a magnonic crystal magnetostatic surface wave. When a magnetostatic surface wave propagates in a layered structure, ferromagnetic film with a magnonic crystal-dielectric layer-metal cladding this synchronism condition can also be fulfilled, not depending on the thickness of a dielectric layer.

Yttrium iron garnet (YIG) films with various thicknesses have been fabricated by liquid-phase epitaxy (LPE) on gadolinium gallium garnet (GGG) (1 1 1) substrates showing unusual magnetic properties. In this work, we studied YIG films with thicknesses between 100 nm and 1000 nm and compared them with La3+-doped YIG films (La:YIG) over a similar scale. The relaxation of tensile stress at the GGG/YIG interface, which caused a magnetic hysteresis loop, showed an easy magnetization phase for thin YIG films (⩽400 nm), while a hard magnetization phase gradually appeared with further increases to the film thickness. The stable magnetic properties of La:YIG at different thicknesses and the not so perfect magnetic hysteresis loop are caused by the intrinsic distortion of the sublattice from La3+ partially replacing Y3+. The saturation magnetization field (4πMs) obtained from broadband ferromagnetic resonance measurement of the YIG films increased with increasing Ha and stayed around the theoretical value. This work helps us to understand the properties of LPE-based YIG films and devices at the sub-microscale.

The linear and nonlinear interactions between spin waves (magnons) and acoustic waves (phonons) in magnetostrictive materials provide an exciting opportunity for realizing novel microwave signal processing devices and spintronic circuits. Here, we demonstrate the parametric pumping of spin waves by acoustic waves in a ferrimagnet, the possibility of which has long been theoretically anticipated but never experimentally realized. Spin waves propagating in a thin film of yttrium iron garnet—a magnetostrictive ferrimagnet with low spin and acoustic wave damping—are pumped using an acoustic resonator driven at frequencies near twice the spin wave frequency. The observation of a counterpropagating idler wave and a distinct pump threshold that increases quadratically with frequency nondegeneracy are evidence of a nonlinear parametric pumping process consistent with classical theory. This demonstration of acoustic parametric pumping lays the groundwork for developing new spintronic and microwave signal processing devices based on amplification and manipulation of spin waves by efficient, spatially localized acoustic transducers.

An insulator‐based spin‐thermoelectric (STE) generator is composed of a thin paramagnetic metal (PM: Pt) layer for generating the STE voltage VSTE via the inverse spin Hall effect and a ferrimagnetic insulator (FMI) film or slab used for producing spin‐wave spin currents due to a temperature gradient . Yttrium iron garnet (YIG) is a key material used for the FMI of STE generators. We examined in detail the ferromagnetic resonance (FMR) properties of YIG (Y3Fe5O12), Bi‐substituted YIG (Y3‐xBixFe5O12) and Bi‐ and Al‐substituted YIG (Y3‐xBixFe5‐yAlyO12) films grown by liquid phase epitaxy (LPE). Bi‐substituted YIG films exhibited the large FMR damping and high STE voltage when the films were incorporated in the STE generator. The LPE‐grown Y2.35Bi0.65Fe5O12 film exhibited the FMR linewidth ΔH of 30 mT and the VSTE of 70 μV for the ∇T of . This paper comprehensively presents the origin of the STE generation in the insulator‐based generators on the basis of the results of FMR and STE measurements. To clarify the origin of STE generation in the generators fabricated from single crystal YIG (YIG, Bi‐substituted YIG and Bi‐ and Al‐substituted YIG) films grown by LPE, three principal factors are explained: (a) thermal energy transfer from the phonon system to the spin system, which strengthens the heat excitation of spin precession, through the spin–orbit coupling enhanced by the growth‐induced magnetic anisotropy of LPE‐grown YIG films, (b) in the YIG films incorporated in the STE generators, the generation of spin‐wave spin currents owing its origin to the ferromagnetic spin exchange interaction acting between neighboring spins due to , and (c) how the spin currents pumped into the Pt layer from the YIG film at the Pt/YIG bilayer interface are affected by the magnetization processes of single crystal YIG films. It is concluded that the STE generation of insulator‐based STE generators can be explained from a viewpoint of the spin dynamics under the effect of in the Pt/YIG bilayer structure. © 2024 Institute of Electrical Engineers of Japan and Wiley Periodicals LLC.

We 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.

This chapter reviews the dynamics of spin waves under the influence of bulk and interfacial Dzyaloshinskii–Moriya interaction (DMI), both in thin films and magnonic crystals. Micromagnetic simulations are a powerful numerical method for the analysis of spin dynamics in magnetic samples of a few microns. Spin waves propagating in a periodic magnetic media, such as an magnonic crystal (MC)—the magnetic counterpart of a photonic crystal—exhibit magnonic bandgaps induced by Bragg scattering, which means there are frequencies where propagation is forbidden. The influence of DMI on the behavior of spin waves in chiral lattice magnets, ultrathin ferromagnetic films capped with a heavy metal layer, and chiral MCs has been reviewed. Such anisotropic exchange coupling produces nonreciprocal features on the spin-wave spectrum of a magnetic system, a phenomenon that occurs for both bulk and interfacial DMI. The concept of a chiral MC has been introduced theoretically, where the interfacial DMI is considered as periodic along a single direction.

Band structure and Wannier-Stark ladders in the spin wave spectrum

Collective spin excitation in finite size array of patterned magnonic crystals

Yttrium iron garnet (YIG) films have been widely studied for application in magnonic and integrated photonic devices. The growth of YIG thin films on SiO 2 /Si substrate, particularly films with perpendicular magnetic anisotropy (PMA), is important for compact magnonic and nonreciprocal photonic device applications. However, the crystallization kinetics of YIG films, especially their nucleation and growth kinetics, have rarely been explored. PMA is usually ensured by rare‐earth ion doping, which inevitably causes large Gilbert damping. In this paper, we report the crystallization kinetics and control of the magnetic anisotropy of YIG thin films grown on SiO 2 substrates via localized CO 2 laser annealing. The distinct nucleation and growth activation energies of YIG films (10.54 and 3.06 eV, respectively) enabled the separate control of these two steps, enabling the fabrication of YIG films with various grain sizes and distinct magnetic anisotropies. Clear PMA was observed in YIG films with a small grain size, which are dominated by the nucleation process. This study provides a theoretical basis for understanding the nucleation and growth kinetics of YIG films and offers a technical approach for controlling their magnetic anisotropy, contributing significantly toward YIG film research and paving the way for the development of high‐performance devices for magnonic and photonic applications.

Magnonic holographic memory is a type of memory that uses spin waves for magnetic bit read-in and read-out. Its operation is based on the interaction between magnets and propagating spin waves where the phase and the amplitude of the spin wave are sensitive to the magnetic field produced by the magnet. Memory states 0 and 1 are associated with the presence/absence of the magnet in a specific location. In this work, we present experimental data showing the feasibility of magnetic bit location using spin waves. The testbed consists of four micro-antennas covered by Y3Fe2(FeO4)3 yttrium iron garnet (YIG) film. A constant in-plane bias magnetic field is provided by NdFeB permanent magnet. The magnetic bit is made of strips of magnetic steel to maximize interaction with propagating spin waves. In the first set of experiments, the position of the bit was concluded by the change produced in the transmittance between two antennas. The minima appear at different frequencies and show different depths for different positions of the bit. In the second set of experiments, two input spin waves were generated, where the phase difference between the waves is controlled by the phase shifter. The minima in the transmitted spectra appear at different phases for different positions of magnetic bit. The utilization of the structured bit enhances its interaction with propagating spin waves and improves recognition fidelity compared to a regular-shaped bit. The recognition accuracy is further improved by exploiting spin wave interference. The depth of the transmission minima corresponding to different magnet positions may exceed 30 dB. All experiments are accomplished at room temperature. Overall, the presented data demonstrate the practical feasibility of using spin waves for magnetic bit red-out. The practical challenges are also discussed.

The Landau-Lifshitz equation with a scalar damping constant predicts that the damping of spin waves propagating in an infinite homogeneous magnetic medium does not depend on the direction of propagation. This is not the case in materials with a periodic arrangement of magnetic constituents (known as magnonic crystals). In this paper, the plane wave method is extended to include damping in the calculation of the dispersion and relaxation of spin waves in three-dimensional magnonic crystals. A model material system is introduced and calculations are then presented for magnonic crystals realized in the direct and inverted structure and for two different filling fractions. The ability of magnonic crystals to support the propagation of spin waves is characterized in terms of a figure of merit, defined as the ratio of the spin wave frequency to the decay constant. The calculations reveal that in magnonic crystals with a modulated value of the relaxation constant, the figure of merit depends strongly on the frequency and wave vector of the spin waves, with the dependence determined by the spatial distribution of the spin wave amplitude within the unit cell of the magnonic crystal. Bands and directions of exceptionally long spin wave propagation have been identified. The results are also discussed in terms of the use of magnonic crystals as metamaterials with designed magnetic permeability.

Experimental study of the spin-wave resonance (SWR) spectrum of a nonuniform yttrium iron garnet (YIG) film has revealed an existence of sharp disturbances inside the spectrum. The experiments were carried out at frequency range 4–12 GHz. The observation has been attributed to excitation of acoustic waves. The interpretation follows from the facts that (1) nonuniformity of magnetic parameters across the thickness of the tested film results in effective excitation of spin waves; and (2) coincidence of phase velocities of the spin waves and the transverse acoustic waves leads to partial power, transforming from the magnetic into the elastic system.

Characterization of parametrically amplified spin wave resonances in thin YIG films