Coupling Of Magnons Research Articles

Unconventional magnon blockade in a dissipative photon–magnon coupling system

We investigate unconventional magnon blockade (UMB) in a dissipative photon–magnon coupling cavity, where the coherent and dissipative coupling between photon and magnon can be adjusted by the position of the yttrium iron garnet sphere in the cavity. An approximate analytical solution for the statistical property of magnon is provided, which is in good agreement with the results obtained by numerically solving the Lindblad master equation. Our results indicate that UMB can be achieved in the case of dissipative photon–magnon coupling, but not in the case of coherent photon–magnon coupling. Secondly, by adjusting the amplitude of the external laser field, dynamic control of UMB can be achieved. Finally, by solving the Lindblad master equation containing the occupation number of thermal magnon and photon, the statistical properties of magnons will transition from nonclassical to classical. It is shown that the above phenomena originate from the interference between different quantum paths. Our research may provide theoretical possibilities for quantum manipulation schemes at the single-magnon level and open up a new avenue for designing single-magnon sources in a dissipative photon–magnon coupling system.

Generation of second-order sidebands and slow–fast light in cavity magnomechanics with a parametric amplifier

In this work, we theoretically investigate the optical second-order sideband (OSS) efficiency and group delay of an output probe field in a two-coupled cavity magnomechanics (CMM) system. The setup comprises a gain (active) cavity and a passive (loss) cavity, which simultaneously includes an optical parametric amplifier (OPA) and a yttrium iron garnet sphere to facilitate magnon–photon coupling. Employing experimentally attainable parameters, we show that the presence of an OPA can significantly enhance the optical transmission rate and OSS efficiency. We show that photon–magnon strong coupling can enhance OSS efficiency substantially when compared to weak photon–magnon coupling. Our results, in particular, show that the OSS’s efficiency may be significantly improved for active–passive-coupled cavities as compared to the passive–passive cavities system. Furthermore, we demonstrate that modifying the system configurations may change the group delay, affecting the transition from rapid to slow light propagation, and vice versa. Our results provide a novel and easy approach to designing CMM devices for altering light propagation, with possible applications including optical switching, information storage, and accurate measurement of weak signals.

Manipulating the nonreciprocal microwave transmission by using a pump-induced magnon mode

We realize the electromagnetic regulation of nonreciprocal microwave transmission by introducing a pump-induced magnon mode (PIM) into a cavity magnonic device with dissipative photon–magnon coupling. As a peculiar spin wave, the PIM's dynamic properties, including its spin number and resonant frequency, can be easily tuned by the microwave pump. Hence, it facilitates the precise control of the coupling process between the PIM and the cavity magnonic device by regulating the pump signal. Along with these manipulations, the nonreciprocal microwave transmission produced by the dissipative photon–magnon coupling is regulated without reconfiguring the system. In the experiment, we achieve a pump-controlled nonreciprocal bandwidth of 16 MHz and a pump-tunable isolation range of up to 40 dB. Our work demonstrates the control of a microwave with another microwave. It has a great potential in the design of fast microwave switches and programmable isolators for information processing.

Enhanced spin pumping in heterostructures of coupled ferrimagnetic garnets

Spin pumping has significant implications for spintronics, providing a mechanism to manipulate and transport spins for information processing. Understanding and harnessing spin currents through spin pumping is critical for the development of efficient spintronic devices. The use of a magnetic insulator with low damping enhances the signal-to-noise ratio in crucial experiments such as spin-torque ferromagnetic resonance (FMR) and spin pumping. A magnetic insulator coupled with a heavy metal or quantum material offers a more straightforward model system, especially when investigating spin-charge interconversion processes to greater accuracy. This simplicity arises from the absence of unwanted effects caused by conduction electrons unlike in ferromagnetic metals. Here, we investigate the spin pumping in coupled ferrimagnetic (FiM) Y3Fe5O12 (YIG)/Tm3Fe5O12 (TmIG) bilayers combined with heavy-metal (Pt) using the inverse spin Hall effect. It is observed that magnon transmission occurs at both of the FiMs FMR positions. The enhancement of spin pumping voltage (Vsp) in the FiM garnet heterostructures is observed. The plausible reason might be the interfacial exchange coupling between FiMs. The modulation of Vsp is achieved by tuning the bilayer structure. Further, the spin mixing conductance for these coupled systems is found to be ≈1018 m−2. Our findings describe a coupled FiM system for the investigation of magnon coupling providing opportunities for magnonic devices.

Mapping the magnon–magnon hybrid state onto the Bloch sphere

We investigate magnon–magnon hybrid states using a non-Hermitian Hamiltonian and the concept of magnon hybridization. By comparing our model with micromagnetic simulations conducted on a synthetic antiferromagnet with strong magnon–magnon coupling, the two-level model reproduces not only the resonance frequencies and linewidths but also the phases and amplitudes of the magnon eigenmode. The coherent coupling between magnons results in both the anticrossing of the energy spectra and the mixing of the linewidths. Specially, it forms a two-level system and makes the eigenmode of the hybrid state as a linear combination of the pure acoustic and optic modes. After that, we map the magnon–magnon hybrid state, including the magnon state of exceptional point, onto a Bloch sphere, which enhances the ability to manipulate hybrid magnons for coherent information processing.

The role of non-uniform magnetization texture for magnon–magnon coupling in an antidot lattice

We numerically study the spin-wave dynamics in an antidot lattice based on a Co/Pd multilayer structure with reduced perpendicular magnetic anisotropy at the edges of the antidots. This structure forms a magnonic crystal with a periodic antidot pattern and a periodic magnetization configuration consisting of out-of-plane magnetized bulk and in-plane magnetized rims. Our results show a different behavior of spin waves in the bulk and in the rims under varying out-of-plane external magnetic field strength, revealing complex spin-wave spectra and hybridizations between the modes of these two subsystems. A particularly strong magnon–magnon coupling, due to exchange interactions, is found between the fundamental bulk spin-wave mode and the second-order radial rim modes. However, the dynamical coupling between the spin-wave modes at low frequencies, involving the first-order radial rim modes, is masked by the changes in the static magnetization at the bulk–rim interface with magnetic field changes. The study expands the horizons of magnonic-crystal research by combining periodic structural patterning and non-collinear magnetization texture to achieve strong magnon–magnon coupling, highlighting the significant role of exchange interactions in the hybridization.

Entanglement and quantum coherence of two YIG spheres in a hybrid Laguerre–Gaussian cavity optomechanics

We theoretically investigate continuous variable entanglement and macroscopic quantum coherence in the hybrid L–G rotational cavity optomechanical system containing two YIG spheres. In this system, a single L–G cavity mode and both magnon modes (which are due to the collective excitation of spins in two YIG spheres) are coupled through the magnetic dipole interaction whereas the L–G cavity mode can also exchange orbital angular momentum (OAM) with the rotating mirror (RM). We study in detail the effects of various physical parameters like cavity and both magnon detunings, environment temperature, optorotational and magnon coupling strengths on the bipartite entanglement and the macroscopic quantum coherence as well. We also explore parameter regimes to achieve maximum values for both of these quantum correlations. We also observed that the parameters regime for achieving maximum bipartite entanglement is completely different from macroscopic quantum coherence. So, our present study shall provide a method to control various nonclassical quantum correlations of macroscopic objects in the hybrid L–G rotational cavity optomechanical system and have potential applications in quantum sensing, quantum meteorology, and quantum information science.

Interlayer Interaction of Excitons and Magnons in Graphene/WS2/Néel‐Type Manganese Phosphorus Trichalcogenide Heterostructures

Abstract2D materials, particularly 2D semiconductors, and layered antiferromagnetic (AFM) compounds, exhibit rich light‐matter coupling phenomena in low‐dimensional systems. In this work, the occurrence of interlayer exciton–magnon coupling (EMC) is observed in Néel‐type AFM of MnPX3 (X = S or Se) and monolayer (1L) WS2 heterostructures. The energy of neutral exciton in 1L WS2 can be tuned by the adjacent AFM order, resulting in an additional energy shift of 10–14 meV. Furthermore, by filtering the photoluminescence spectrum with graphene and conducting magnetic measurements, the correlation is elucidated between the interlayer EMC and AFM transition process. In MnPS3, this coupling behavior exhibits sensitivity to the changes of magnetic structures, manifesting with a correlated length ξ of 8.34 Å at temperatures higher than Néel temperature (TN). The decoupling process in WS2/MnPS3 heterostructure below 50 K is originated from a weak out‐of‐plane ferromagnetic order, confirming the presence of an XY‐type magnetic phase transition in MnPS3 at sub‐TN temperature. This study provides a fundamental understanding of EMC and its potential applications in the integration of optical and magnetic devices.

Ultrastrong magnon-magnon coupling and chiral spin-texture control in a dipolar 3D multilayered artificial spin-vortex ice

Strongly-interacting nanomagnetic arrays are ideal systems for exploring reconfigurable magnonics. They provide huge microstate spaces and integrated solutions for storage and neuromorphic computing alongside GHz functionality. These systems may be broadly assessed by their range of reliably accessible states and the strength of magnon coupling phenomena and nonlinearities. Increasingly, nanomagnetic systems are expanding into three-dimensional architectures. This has enhanced the range of available magnetic microstates and functional behaviours, but engineering control over 3D states and dynamics remains challenging. Here, we introduce a 3D magnonic metamaterial composed from multilayered artificial spin ice nanoarrays. Comprising two magnetic layers separated by a non-magnetic spacer, each nanoisland may assume four macrospin or vortex states per magnetic layer. This creates a system with a rich 16N microstate space and intense static and dynamic dipolar magnetic coupling. The system exhibits a broad range of emergent phenomena driven by the strong inter-layer dipolar interaction, including ultrastrong magnon-magnon coupling with normalised coupling rates of Δfν=0.57\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\frac{\\Delta f}{\ u }=0.57$$\\end{document}, GHz mode shifts in zero applied field and chirality-control of magnetic vortex microstates with corresponding magnonic spectra.

Magnon-Phonon Coupling of Synthetic Antiferromagnets in a Surface Acoustic Wave Cavity Resonator.

We used a surface acoustic wave (SAW) cavity resonator to study the coupling of acoustic magnons in a synthetic antiferromagnet (SAF) and phonons carried by SAWs. The SAF is composed of a CoFeB/Ru/CoFeB trilayer, and the scattering matrix of the SAW resonator is studied to assess the coupling. We find that the spectral line width of the SAW resonator is modulated when the frequency of the excited magnons approaches the SAW resonance frequency. Such a change in the spectral linewidth can be well reproduced using macrospin-like model calculations. From the model analyses, we estimate the magnon-phonon coupling strength to be ∼9.9 MHz at a SAW resonance frequency of 1.8 GHz: the corresponding magnomechanical cooperativity is ∼0.66. As the spectral shape hardly changes in a CoFeB single-layer reference sample, these results show that SAF provides an ideal platform to study magnon-phonon coupling in an SAW cavity resonator.

Reconfigurable spin current transmission and magnon–magnon coupling in hybrid ferrimagnetic insulators

Coherent spin waves possess immense potential in wave-based information computation, storage, and transmission with high fidelity and ultra-low energy consumption. However, despite their seminal importance for magnonic devices, there is a paucity of both structural prototypes and theoretical frameworks that regulate the spin current transmission and magnon hybridization mediated by coherent spin waves. Here, we demonstrate reconfigurable coherent spin current transmission, as well as magnon–magnon coupling, in a hybrid ferrimagnetic heterostructure comprising epitaxial Gd3Fe5O12 and Y3Fe5O12 insulators. By adjusting the compensated moment in Gd3Fe5O12, magnon–magnon coupling was achieved and engineered with pronounced anticrossings between two Kittel modes, accompanied by divergent dissipative coupling approaching the magnetic compensation temperature of Gd3Fe5O12 (TM,GdIG), which were modeled by coherent spin pumping. Remarkably, we further identified, both experimentally and theoretically, a drastic variation in the coherent spin wave-mediated spin current across TM,GdIG, which manifested as a strong dependence on the relative alignment of magnetic moments. Our findings provide significant fundamental insight into the reconfiguration of coherent spin waves and offer a new route towards constructing artificial magnonic architectures.

Laser excitation of magnons in NiO via spin–phonon coupling

Antiferromagnetic materials have recently been proposed as new types of terahertz (THz) range spintronic devices owing to their ultrafast spin dynamics. Manipulating their spin dynamics expediently, however, remains a key challenge. Here, we demonstrate the laser excitation of magnons in a prototypical antiferromagnet NiO via spin–phonon coupling. The terahertz time-domain spectrum revealed the frequencies of antiferromagnetic magnons near 1 THz. Laser excitations in the visible spectrum caused a noticeable softening of the magnons. Raman spectroscopy results established the presence of optical phonons. The laser heating effect was excluded by finite-element analysis and variable-temperature measurements. The temperature- and power-dependent properties suggest an optical phonon–magnon coupling mechanism. Laser excitation raises the optical phonon temperature, linked with the magnon temperature, via the magnon–phonon interaction. Consequently, the magnon temperature rises, and the magnon mode frequency softens. Our findings shed light on spin–phonon coupling in antiferromagnetic insulators and open a route for creating rapid opto-spintronic devices that utilize antiferromagnetic materials.

Enhancement of photon–magnon coupling strength by inverted split-ring resonator at GHz

A series of inverse split-ring resonators (ISRR) loaded with an yttrium iron garnet (YIG) film were numerically and experimentally studied to provide strong photon–magnon coupling strength and cooperativity enhancement. An increase in the photon–magnon coupling strength and cooperativity has been achieved due to the simultaneous increase in the spatial overlapping between the ISRR microwave electromagnetic field magnetic component and the uniform magnon mode in the YIG, and the appearance of spatial areas with higher ISRR electromagnetic field magnetic component strength. Practically, this was reached for the ISRR by increasing the conducting gap length and changing the conducting gap shape by protrusions forming. Higher values of spin-number-normalized photon–magnon coupling strength gN and cooperativity C were reached in all cases for the ISRR with protrusion (compared with ISSR with identical conducting gap length but without protrusion). All resonators presented in this paper seem to be more prospective for the realization in quantum hybrid systems than conventional split-ring resonators and ISRR.

Fabrication of compact circular ring antenna loaded with Gd-YIG ferrites: Photon–magnon interaction

In telecommunications technology, transmitting and receiving signals is not possible without an integrated antenna. Recently, novel approaches of transmitting and receiving signals using advancement in quantum technology as “information coherent transfer” have been emphasized which overcomes the limitations of superconducting qubits. In the present investigation, a compact-sized circular-ring antenna was designed for a 12 GHz centre frequency. The antenna was topped with Gd(x)-doped YIG (x = 0.0, 0.1, 0.2, 0.3) thin disks to investigate its tunable operation over the tri-band (C to Ku) frequencies due to the photon–magnon coupling, which enables coherent qubit information transmission from an electromagnetic (EM) carrier to the magnons. The proposed prototype of the antenna is validated using HFSS simulation. To characterize the frequency tunability of the designed antenna, we measured the return loss (S11) via a vector network analyzer as a function of dc magnetic field. Different concentrations of Gd-doped YIG ferrites (Gdx-YIG) have been loaded on ring antennas to observe strong anti-crossing phenomena with the application of an external magnetic field. The designed antenna has a tunability of around 300% for a small applied magnetic field up to 3.7 kOe. The maximum gain of dual resonance antenna is 1.4 for higher magnetic field of 3.7 kOe. The highest efficiency and lowest bandwidth of 30% and 14% respectively, for Gd0.1-YIG disk are observed. The measured normal mode splitting spectrum as a function of bias magnetic field that gives the maximum coupling strength is 153 MHz for a Gd concentration of 0.1. For practical applications, such antennas are suitable for use in tunable tri-band or multi-band applications (C to Ku).

Terahertz Cavity Magnon Polaritons

AbstractHybrid light–matter coupled states, or polaritons, in magnetic materials have attracted significant attention due to their potential for enabling novel applications in spintronics and quantum information processing. However, most magnon‐polariton studies in the strong coupling regime to date have been carried out for ferromagnetic materials with magnon excitations at gigahertz frequencies. Here, strong resonant photon–magnon coupling at frequencies above 1 terahertz is investigated for the first time in a prototypical room‐temperature antiferromagnetic insulator, NiO, inside a Fabry–Pérot cavity. The cavity is formed by the crystal itself with a thickness adjusted to an optimal value. Terahertz time‐domain spectroscopy measurements in magnetic fields up to 25 T reveal the evolution of the magnon frequency through Fabry–Pérot cavity modes with photon–magnon anticrossing behavior, demonstrating clear vacuum Rabi splittings exceeding the polariton linewidths. These results show that NiO is a promising platform for exploring antiferromagnetic spintronics and cavity magnonics in the terahertz frequency range.

Large enhancement of spin–orbit torques under a MHz modulation due to phonon–magnon coupling

The discovery of spin–orbit torques (SOTs) generated through the spin Hall or Rashba effects provides an alternative write approach for magnetic random-access memory (MRAM), igniting the development of spin orbitronics in recent years. Quantitative characterization of SOTs highly relies on the SOT-driven ferromagnetic resonance (ST-FMR), where a modulated microwave current is used to generate ac SOTs and the modulation frequency is usually less than 100 kHz (the limit of conventional lock-in amplifiers). Here, we have investigated the SOT of typical SOT material/ferromagnet bilayers in an extended modulation-frequency range, up to MHz, by developing the ST-FMR measurement. Remarkably, we found that the measured SOTs are enhanced about three times in the MHz range, which cannot be explained according to the present SOT theory. We attribute the enhancement of SOT to additional magnon excitations due to phonon–magnon coupling, which is also reflected in the slight changes of resonant field and linewidth in the acquired ST-FMR spectra, corresponding to the modifications of effective magnetization and damping constant, respectively. Our results indicate that the write current of SOT-MRAM may be reduced with the assistance of phonon–magnon coupling.

Floquet engineering of selective magnon–magnon coupling in synthetic antiferromagnets

Floquet engineering is a widely applied method for temporally periodic driving in various quantum systems, capable of inducing emergent phenomena, such as Floquet states, Floquet topological insulators, and temporal crystals. In this work, we study the selective magnon–magnon coupling between sidebands by introducing Floquet engineering into magnonic system: coupling between Floquet sidebands occurs only when there is an odd order difference. In addition to the coherent coupling between the optical and acoustic modes in synthetic antiferromagnet, a Floquet coupling is observed as the secondary splitting of the two hybridized magnon modes. The splitting width of Floquet coupling exhibits different dependence with driving amplitude, i.e., linearly increasing for weak driving regime and nonlinearly varying for strong driving regime. Specifically, the nonlinear dependence arises from the coupling between hybridized sidebands that exhibit odd order differences in their components. Our findings could promote the further development of Floquet magnonics and its application for quantum information.

Long-lived spin waves in a metallic antiferromagnet

Collective spin excitations in magnetically ordered crystals, called magnons or spin waves, can serve as carriers in novel spintronic devices with ultralow energy consumption. The generation of well-detectable spin flows requires long lifetimes of high-frequency magnons. In general, the lifetime of spin waves in a metal is substantially reduced due to a strong coupling of magnons to the Stoner continuum. This makes metals unattractive for use as components for magnonic devices. Here, we present the metallic antiferromagnet CeCo2P2, which exhibits long-living magnons even in the terahertz (THz) regime. For CeCo2P2, our first-principle calculations predict a suppression of low-energy spin-flip Stoner excitations, which is verified by resonant inelastic X-ray scattering measurements. By comparison to the isostructural compound LaCo2P2, we show how small structural changes can dramatically alter the electronic structure around the Fermi level leading to the classical picture of the strongly damped magnons intrinsic to metallic systems. Our results not only demonstrate that long-lived magnons in the THz regime can exist in bulk metallic systems, but they also open a path for an efficient search for metallic magnetic systems in which undamped THz magnons can be excited.

Phonon–magnon conversion using longitudinal leaky surface acoustic waves through magnetoelastic coupling

Surface acoustic waves (SAWs) can transmit magnetization oscillation inside magnetoelastic films in the form of magnetoacoustic waves. If the frequency and wavenumber of SAWs match those of spin waves, magnon excitation can be observed. In this work, we studied the phonon–magnon conversion excited by longitudinal leaky SAWs, which possess the same dominant strain component like Rayleigh-type SAWs but a much higher phase velocity. The measured transmission power absorptions of both SAW modes due to spin wave resonance follow a linear frequency dependence and exhibit a fourfold symmetry. We demonstrate that longitudinal leaky SAWs can serve as a very effective means to excite stronger phonon–magnon coupling than Rayleigh SAWs at a lower wave number.

Acousto‐Plasmo‐Magnonics: Coupling Spin Waves with Hybridized Phonon‐Plasmon Waves in a 2D Artificial Magnonic Crystal Deposited on a Plasmonic Material

AbstractCoupling between spin waves (SWs) and other waves in nanostructured media has emerged as an important topic of research because of the rich physics and the potential for disruptive technologies. Herein, a new phenomenon is reported in this family involving coupling between SWs and hybridized phonon‐plasmon waves in a 2D periodic array of magnetostrictive nanomagnets deposited on a silicon substrate with an intervening thin film of aluminium that acts as a source of surface plasmons. Hybridized phonon‐plasmon waves naturally form in this composite material when exposed to ultrashort laser pulses and they non‐linearly couple with SWs to produce a new breed of waves – acousto‐plasmo‐spin waves that can exhibit a “frequency comb” spanning more than one octave. This phenomenon, that we call acousto‐plasmo‐magnonics resulting from tripartite coupling of magnons, phonons and plasmons, is studied with time‐resolved magneto‐optical‐Kerr‐effect microscopy. The findings also reveal the presence of parametric amplification in this system; energy is transferred from the hybridized phonon‐plasmon modes to the acousto‐plasmo‐spin wave modes to amplify the latter. This opens a path to design novel active metamaterials with tailored and enhanced response. It may enable high‐efficiency magneto‐mechanical‐plasmonic frequency mixing in the GHz−THz frequency regime and provide a unique avenue to study non‐linear coupling, parametric amplification, and frequency comb physics.