Magnonic Devices Research Articles

Enhancement of Low‐k Spin‐Wave Transmission Efficiency with a Record‐High Group Velocity in YIG/Nonmagnetic Metal Heterojunctions

AbstractImprovement of transmission efficiency of spin waves in magnonic devices is crucial for high‐efficiency magnon‐based operations. However, existing techniques for enhancing spin‐wave detection signals, which either improve waveguide material quality or convert other forms of external energy into magnon energy, are still not efficient and convenient enough. Here, it is experimentally demonstrated that low‐k spin waves (where k is the wavenumber) transmission efficiency can be enhanced through yttrium iron garnet (YIG) surface metallization owing to the significantly increased group velocity and low spin‐pumping efficiency at low k. Given that the shielding effect of the conductive metal on YIG increases the spin‐wave group velocity from 55 to 132 km s−1, the weak magnon‐dissipation mechanism of low‐k spin waves provides the foundation for signal enhancement. Through a detailed theoretical analysis on dispersion relations and spin‐wave amplitude, the detected signal strength is increased to 2 times for copper and 2.24 times for gold is verified. In detail the competition between the spin‐pumping effect is discussed and the enhanced group velocity is introduced by the metal decoration. This competition describes the essence of partial signal enhancement in the spin‐wave transmission spectrum. The work provides a practical way to achieve spin‐wave manipulation and low‐loss transmission.

Nonreciprocal Spin-Wave Propagation in a Ferromagnet With Stepwise Interfacial Dzyaloshinskii–Moriya Interaction

In this article, boundary conditions for Landau–Lifshitz equations at the interface between two ferromagnets with different interfacial Dzyaloshinskii–Moriya interactions (iDMIs) are derived. We calculated and verified the analytical expression for the energy flux density continuity for the spin-wave (SW) propagation through the ferromagnet with stepwise iDMI considering the boundary conditions mentioned. Analytical expressions for reflection, transmission coefficients, and the nonreciprocity factor are calculated in the case of SW propagation through a ferromagnet with the stepwise iDMI. Two different types of nonreciprocity effects for SW propagation through a ferromagnet with the stepwise interface Dzyaloshinskii–Moriya interaction are revealed for opposite orientations of Dzyaloshinskii–Moriya vector. The material parameters of a ferromagnet and the stepwise iDMIs are found for which the extremely high nonreciprocity factor (>10) is expected according to the results of calculations. The results of this article deepen the knowledge about the SW propagation control in magnonic devices.

Optomagnonic frequency combs

Optical frequency combs are fundamentally important in precision measurement physics, bringing unprecedented capabilities of measurements for time keeping, metrology, and spectroscopy. In this work, we investigate theoretically the formation of a form of frequency combs in cavity optomagnonics, in which a ferrimagnetic insulator sphere supports optical whispering gallery modes for both light photons and magnons. Numerical simulations of the optomagnonic dynamics show that a robust frequency comb can be obtained at low power under the bichromatic pumping drive, and the comb spacing is adjustable. Furthermore, the optomagnonic frequency comb structure has abundant non-perturbative features, suggesting that the magnon-induced Brillouin light scattering process in cavity optomagnonics may also exhibit phenomena similar to those in atomic–molecular systems. In addition to providing insight into optomagnonic nonlinearity, optomagnonic frequency combs may also provide the feasibility of implementing frequency combs based on spintronic platforms and may find applications for precision metrology based on magnonic devices.

Unidirectional Chiral Magnonics in Cylindrical Synthetic Antiferromagnets

Progress in spintronics continues briskly, with many different approaches and physical phenomena underlying candidate technologies. This study uses modern spin-wave theory to predict the magnonic spectrum of a cylindrical synthetic antiferromagnet consisting of two coaxial nanotubes. The proposed device incorporates two coupled ferromagnetic nanotubes hosting oppositely magnetized vortices. Nonreciprocal features of the proposed architecture are provided by both intra- and interlayer dipolar coupling. Unidirectional spin-wave behavior is found, which may impact the realization of low-power magnonic devices operating at nano- to micrometer scales.

All-Optical study of Gilbert damping and spin orbit torque in Ta/CoFeB/SiO2 heterostructures

Pure spin-current induced spin–orbit torque in a magnetic layer can externally control its magnetization dynamics including Gilbert damping. Here, we demonstrate control over Gilbert damping (Δα) in Substrate/β-Ta(t)/CoFeB(3 nm)/SiO2(2 nm) heterostructures with widely varying Ta thickness (3 ≤ t ≤ 20 nm) having stable β-phase using all-optical time-resolved magneto-optical Kerr effect. The pure spin-current due to the spin Hall effect in β-Ta exerts both damping-like and field-like spin–orbit torque to the CoFeB layer. We estimate the damping-like-torque efficiency over a wide range of t with and without Joule heating. In both cases, we obtained a remarkably high-value of damping-like-torque efficiency (ξDL) ∼ 0.40, suggesting that Joule heating has a negligible effect on the Δα. However, the modulation of precession frequency mainly originates from the Joule heating where contributions from field-like-torque and Oersted field are negligible. The high-value of ξDL and its stability over a wide-range of t will be key ingredients for future energy-efficient spintronic and magnonic devices.

Giant stress response of terahertz magnons in a spin-orbit Mott insulator

Magnonic devices operating at terahertz frequencies offer intriguing prospects for high-speed electronics with minimal energy dissipation However, guiding and manipulating terahertz magnons via external parameters present formidable challenges. Here we report the results of magnetic Raman scattering experiments on the antiferromagnetic spin-orbit Mott insulator Sr2IrO4 under uniaxial stress. We find that the energies of zone-center magnons are extremely stress sensitive: lattice strain of 0.1% increases the magnon energy by 40%. The magnon response is symmetric with respect to the sign of the applied stress (tensile or compressive), but depends strongly on its direction in the IrO2 planes. A theory based on coupling of the spin-orbit-entangled iridium magnetic moments to lattice distortions provides a quantitative explanation of the Raman data and a comprehensive framework for the description of magnon-lattice interactions in magnets with strong spin-orbit coupling. The possibility to efficiently manipulate the propagation of terahertz magnons via external stress opens up multifold design options for reconfigurable magnonic devices.

Fractalized magnon transport on a quasicrystal with enhanced stability

Magnonics has been receiving significant attention in magnetism and spintronics because of its premise for devices using spin current carried by magnons, quanta of spin-wave excitations of a macroscopically ordered magnetic media. Although magnonics has clear energy-wise advantage over conventional electronics due to the absence of the Joule heating, the inherent magnon-magnon interactions give rise to finite lifetime of the magnons which has been hampering the efficient realizations of magnonic devices. To promote magnonics, it is imperative to identify the delocalized magnon modes that are minimally affected by magnon-magnon interactions and thus possess a long lifetime and use them to achieve efficient magnon transport. Here, we suggest that quasicrystals may offer the solution to this problem via the critical magnon modes that are neither extended nor localized. We find that the critical magnon exhibits fractal characteristics that are absent in conventional magnon modes in regular solids such as a unique power-law scaling and a self-similar distribution of distances showing perfect magnon transmission. Moreover, the critical magnons have longer lifetimes compared to the extended ones in a periodic system, by suppressing the magnon-magnon interaction decay rate. Such enhancement of the magnon stability originates from the presence of the quasi-periodicity and intermediate localization behavior of the critical magnons. Thus, we offer the utility of quasicrystals and their critical spin wave functions in magnonics as unique fractal transport characteristics and enhanced stability.

Topological magnons in one-dimensional ferromagnetic Su–Schrieffer–Heeger model with anisotropic interaction

Topological magnons in a one-dimensional (1D) ferromagnetic Su–Schrieffer–Heeger (SSH) model with anisotropic exchange interactions are investigated. Apart from the intercellular isotropic Heisenberg interaction, the intercellular anisotropic exchange interactions, i.e. Dzyaloshinskii–Moriya interaction and pseudo-dipolar interaction, also can induce the emergence of the non-trivial phase with two degenerate in-gap edge states separately localized at the two ends of the 1D chain, while the intracellular interactions instead unfavors the topological phase. The interplay among them has synergistic effects on the topological phase transition, very different from that in the two-dimensional (2D) ferromagnet. These results demonstrate that the 1D magnons possess rich topological phase diagrams distinctly different from the electronic version of the SSH model and even the 2D magnons. Due to the low dimensional structural characteristics of this 1D topological magnonic system, the magnonic crystals can be constructed from bottom to top, which has important potential applications in the design of novel magnonic devices.

Observation of nonlinearity and heating-induced frequency shifts in cavity magnonics

When there is a certain amount of field inhomogeneity, the biased ferrimagnetic crystal can exhibit the higher-order magnetostatic (HMS) mode in addition to the uniform-precession Kittel mode. In cavity magnonics, we show the nonlinearity and heating-induced frequency shifts of the Kittel mode and HMS mode in a yttrium–iron–garnet (YIG) sphere. When the Kittel mode is driven to generate a certain number of excitations, the temperature of the whole YIG sample rises and the HMS mode can display an induced frequency shift, and vice versa. This cross effect provides a new method to study the magnetization dynamics and paves a way for novel cavity magnonic devices by including the heating effect as an operational degree of freedom.

Theoretical Study of Magnon Spin Currents in Chromium Trihalide Hetero-bilayers: Implications for Magnonic and Spintronic Devices

The intriguing magnetic behavior of two-dimensional chromium trihalide materials holds great promises for spintronics and magnonics. Here, we study the stacking-dependent magnetism in (CrBr3/CrI3) hetero-bilayers by using first-principles calculations. We find that different stackings yield ferromagnetic ground states or favor an antiferromagnetic (AFM) interlayer coupling. Starting from ab initio results, we study the magnonic spectra and magnon spin textures in the hetero-bilayer. Based on our findings, we propose an experimental setup for the optical generation and electric detection of pure magnon spin currents in layered AFM (LAFM) (CrBr3/CrI3) hetero-bilayers. Furthermore, we propose a device to simultaneously realize magnon spin currents and topological domain walls in the hetero-bilayers. The advantages of (CrBr3/CrI3) hetero-bilayers over the (CrI3)2 homo-bilayers to realize magnon spin currents are highlighted. The interesting features of magnons in LAFM hetero-bilayers and their potential for spin-charge conversion could pave the way for their implementation in spin-based transistors and logic devices.

Inverse design of magnonic filter

Monochromatic spin wave (SW) is the substrates for the realizing the practical function of magnonic devices, however the generating perfect monochromatic SW is still a unsolved but fundamental problems. In our latest work (Xing et al., 2022), the resonant tunneling effect was found in a magnonic crystals (MCs), which demonstrates a similar effect of Fabry–Pérot interferometer, and shows a promising application of band-pass magnonic filter (MF) for generating perfect monochromatic SW. The optimization of such MF is a complex problem, due to that the natural nonlinearity of spin-wave scattering behavior enhances sensitivity to device parameters. In this work, the inverse design (ID) approach is developed for designing the magnonic filters (MFs). To determine the optimizable variables, the fundamental properties of antiferromagnetically coupled heterojunction based MCs were theoretically investigated. The particle swarm algorithms (PSA) is used for automatically finding MFs which meet the inputting requirements. Here, the input of algorithms is a vector of SW frequency and output is a vector of parameters of MFs. Besides, the PSA is further optimized to be self-adapting to required accuracies, showing a higher computing efficiency than the convectional one. Finally, the output data of predicted MFs with various filtering characteristics are briefly summarized in a table as an alternative proposal for the future experimental design. These encouraging results show high potential of using ID approach in magnetism and boosting optimization of magnetic multilayer heterojunction devices, such as magnon transistor and magnon space–time crystals.

Magnonic frequency combs based on the resonantly enhanced magnetostrictive effect

A magnonic counterpart to optical frequency combs is vital for high-precision magnonic frequency metrology and spectroscopy. Here, we present an efficient mechanism for the generation of robust magnonic frequency combs in a yttrium iron garnet (YIG) sphere via magnetostrictive effects. We show that magnonic and vibrational dynamics in the ferrimagnetic sphere can be substantively modified in the presence of magnetostrictive effects, which results in degenerate and non-degenerate magnonic four-wave mixing and frequency conversion. Particularly, resonantly enhanced magnetostrictive effects can induce phonon laser action above a threshold, which leads to significant magnonic nonlinearity and enables a potentially practical scheme for the generation of robust magnonic frequency combs. Numerical calculations of both magnonic and phononic dynamics show excellent agreement with this theory. These results deepen our understanding of magnetostrictive interaction, open a novel and efficient pathway to realize magnonic frequency conversion and mixing in a magnonic device, and provide a sensitive tool for precision measurement.

Efficient Gating of Magnons by Proximity Superconductors.

Electrostatic gating confines and controls the transport of electrons in integrated circuits. Magnons, the quanta of spin waves of the magnetic order, are promising alternative information carriers, but difficult to gate. Here we report that superconducting strips on top of thin magnetic films can totally reflect magnons by their diamagnetic response to the magnon stray fields. The induced large frequency shifts unidirectionally block the magnons propagating normal to the magnetization. Two superconducting gates parallel to the magnetization create a magnonic cavity. The option to gate coherent magnons adds functionalities to magnonic devices, such as reprogrammable logical devices and increased couplings to other degrees of freedom.

Simulation on spin wave transmission and domain wall dynamics in a permalloy nanostrip

Spin waves (SWs), the magnons, can potentially be used for fast and energy efficient information processing and the interaction of SWs with a magnetic domain wall (DW), which is quite complex, can play a key role in the development of magnonic devices. Manipulation of SW amplitude and phase is the key step in realization of spin wave devices. The SW transmission through DW is strongly dependent on the magnetization angle inside the DW and spatial movement of DW is coupled to this interaction. Magnonic spin transfer torque and linear momentum transfer of SW, collectively, control the motion of the DW and the rotation of magnetization inside the DW. In the present work, micromagnetic simulations were performed considering a ferromagnetic permalloy nanostrip with perpendicular magnetic anisotropy (PMA) using mumax3 platform to study SW transmission through DW. The correlation between rotation in magnetization tilt angle (δϕ) inside an initially Néel-type DW and SW-transmission ratio (TD) was investigated at different SW-frequencies ranging from 18 GHz to 28 GHz. Applied SW amplitude at each frequency was sufficiently large for transverse DW to show oscillatory motion along the length of the strip. The results showed that the dependence of TD on rotation in magnetization angle (δϕ) decreases as the frequency of SW is increased and it approaches a nearly isotropic behavior at higher frequencies.

Reconfigurable Magnonic Crystals Based on Imprinted Magnetization Textures in Hard and Soft Dipolar-Coupled Bilayers.

Reconfigurable magnetization textures offer control of spin waves with promising properties for future low-power beyond-CMOS systems. However, materials with perpendicular magnetic anisotropy (PMA) suitable for stable magnetization-texture formation are characterized by high damping, which limits their applicability in magnonic devices. Here, we propose to overcome this limitation by using hybrid structures, i.e., a PMA layer magnetostatically coupled to a low-damping soft ferromagnetic film. We experimentally show that a periodic stripe-domain texture from a PMA layer is imprinted upon the soft layer and induces a nonreciprocal dispersion relation of the spin waves confined to the low-damping film. Moreover, an asymmetric bandgap features the spin-wave band diagram, which is a clear demonstration of collective spin-wave dynamics, a property characteristic for magnonic crystals with broken time-reversal symmetry. The composite character of the hybrid structure allows for stabilization of two magnetic states at remanence, with parallel and antiparallel orientation of net magnetization in hard and soft layers. The states can be switched using a low external magnetic field; therefore, the proposed system obtains an additional functionality of state reconfigurability. This study offers a link between reconfigurable magnetization textures and low-damping spin-wave dynamics, providing an opportunity to create miniaturized, programmable, and energy-efficient signal processing devices operating at high frequencies.

Magnon Dynamics in Parity-Time-Symmetric Dipolarly Coupled Waveguides and Magnonic Crystals

We consider the magnonic properties of two dipolarly coupled magnetic stripes, both deposited on a normal conductive substrate with strong spin-orbit coupling. A charge current in the substrate acts on the adjacent magnets with spin-orbit torques, which result in magnonic damping or antidamping of the spin waves, and hence a gain-loss coupling of the two magnetic stripes. The whole setup is demonstrated to exhibit features typical for parity-time- ($PT$) symmetric systems. Phenomena are demonstrated that can be functionalized in magnonic devices, including reconfigurable magnonic diodes and logic devices. Alternative stripe designs and $PT$-symmetric, periodic, coupled magnonic textures are studied. Analytical and full numerical analysis identify the conditions for the appearance of exceptional points (EPs), where magnonic gain and loss are balanced and evidence nonreciprocal magnon propagation and enhanced magnon excitation around EPs. Furthermore, the dipolar coupling is shown to bring in a wave-vector-dependent $PT$-symmetric behavior. Proposing and simulating a $PT$-symmetric magnonic crystal, we show how EPs and hence associated phenomena can be steered to a particular wave vector in a gaped spectrum via material design. The phenomena offer additional tools for magnonic based communication and computational devices.

Magnonic band-pass and band-stop filters with structurally modulated waveguides

Magnonics is a fascinating and emerging field, which mainly studies processing information with spin waves. Magnonic devices with in-plane magnetization have recently been realized. Because of the isotropic propagation, magnonic devices based on perpendicular magnetization are attracting extensive interest. Here, we numerically demonstrate two magnonic filters with out-of-plane magnetization using micromagnetic simulations. The band-pass and the band-stop functions have been realized in two structurally modulated waveguides, respectively. The intensity of spin waves is manipulated when they arrive at the uniformly/non-uniformly magnetized modulators, which results in the variation of transmission coefficients. It is found that the proposed filters can work at multiple frequencies, which can be further adjusted by the external magnetic field. Our designed magnonic devices with Néel-type skyrmion could promote the development of spin wave computing using spin textures.

Spin-wave interconversion via thermoelectric point-contact control

The magnonic functionalities of magnonic devices, such as logic gates, majority gates, and multiplexers, critically depend on anisotropic spin-wave dispersions. Further, each magnonic function utilizes the advantages of distinct spin-wave modes. However, thus far, no method of directly combining magnonic functions to construct an integrated magnonic circuit has been proposed. Therefore, this paper presents a method for the interconversion of surface spin-waves and backward volume spin-waves using a thermoelectrically controlled magnetization gradient. The results of time- and frequency-domain spectroscopies, heat transfer analyses, and micromagnetic simulations demonstrate the effectiveness of the proposed method, through which spin-wave interconversion via thermoelectric control may be employed to develop integrated magnonic circuits.

Spin-Wave Channeling in Magnetization-Graded Nanostrips.

Magnetization-graded ferromagnetic nanostrips are proposed as potential prospects to channel spin waves. Here, a controlled reduction of the saturation magnetization enables the localization of the propagating magnetic excitations in the same way that light is controlled in an optical fiber with a varying refraction index. The theoretical approach is based on the dynamic matrix method, where the magnetic nanostrip is divided into small sub-strips. The dipolar and exchange interactions between sub-strips have been considered to reproduce the spin-wave dynamics of the magnonic fiber. The transition from one strip to an infinite thin film is presented for the Damon-Eshbach geometry, where the nature of the spin-wave modes is discussed. An in-depth analysis of the spin-wave transport as a function of the saturation magnetization profile is provided. It is predicted that it is feasible to induce a remarkable channeling of the spin waves along the zones with a reduced saturation magnetization, even when such a reduction is tiny. The results are compared with micromagnetic simulations, where a good agreement is observed between both methods. The findings have relevance for envisioned future spin-wave-based magnonic devices operating at the nanometer scale.

Excitation and detection of coherent sub-terahertz magnons in ferromagnetic and antiferromagnetic heterostructures

Excitation of coherent high-frequency magnons (quanta of spin waves) is critical to the development of high-speed magnonic devices. Here we computationally demonstrate the excitation of coherent sub-terahertz (THz) magnons in ferromagnetic (FM) and antiferromagnetic (AFM) thin films by a photoinduced picosecond acoustic pulse. Analytical calculations are also performed to reveal the magnon excitation mechanism. Through spin pumping and spin-charge conversion, these magnons can inject sub-THz charge current into an adjacent heavy-metal film which in turn emits electromagnetic (EM) waves. Using a dynamical phase-field model that considers the coupled dynamics of acoustic waves, spin waves, and EM waves, we show that the emitted EM wave retains the spectral information of all the sub-THz magnon modes and has a sufficiently large amplitude for near-field detection. These predictions indicate that the excitation and detection of sub-THz magnons can be realized in rationally designed FM or AFM thin-film heterostructures via ultrafast optical-pump THz-emission-probe spectroscopy.