We study magnon hybridization in a ferrimagnetic heterostructure consisting of ultrathin gadolinium iron garnet and yttrium iron garnet layers and show the localized and extended spatial profiles of the magnon modes with different polarizations. These modes are expected to have distinct thermal excitation properties in the presence of a temperature gradient across the heterostructure. From a quantitative analysis of their consequences on longitudinal spin Seebeck effect, we predict an observable shift of the sign-changing temperature with respect to the one previously observed in gadolinium iron garnet. Moreover, the sign-changing point of spin Seebeck signal is found to be tunable by YIG thickness. Our results suggest the necessity of taking into account the temperature difference between the magnon modes in ferrimagnetic heterostructures.
We propose a new drag phenomenon, an interband magnon drag, and report on interaction effects and multiband effects in magnon transport of ferrimagnetic insulators. We study a spin-Seebeck coefficient $S_{\textrm{m}}$, a magnon conductivity $σ_{\textrm{m}}$, and a magnon thermal conductivity $κ_{\textrm{m}}$ of interacting magnons for a minimal model of ferrimagnetic insulators using a $1/S$ expansion of the Holstein-Primakoff method, the linear-response theory, and a method of Green's functions. We show that the interband magnon drag enhances $σ_{\textrm{m}}$ and reduces $κ_{\textrm{m}}$, whe…
Magnon-polarons are coherently mixed quasiparticles that originate from the strong magnetoelastic coupling of lattice vibrations and spin waves in magnetic-ordered materials. Recently, magnon-polarons have attracted a lot of attention since they provide a powerful tool to manipulate magnons, which is essential for magnon-based spintronic devices. In this work, we report the experimental observation of facet-dependent magnon-polarons in epitaxial ferrimagnetic Fe3O4 thin films via spin Seebeck effect measurement. The critical magnetic fields for the magnon-polarons in the (110)- and (100)-orien…
D-Wave Phonon Angular Momentum Texture in Altermagnets by Magnon-Phonon Hybridization
Institute of Physics, Martin Luther University Halle-Wittenberg and Halle-Berlin-Regensburg Cluster of Excellence “Center for Chiral Electronics”（马丁·路德哈雷-维滕贝格大学物理研究所）；Institute of Physics, Otto von Guericke University Magdeburg（奥托·冯·格里克马格德堡大学物理研究所）；Institute of Solid State Theory, University of Münster（明斯特大学固体理论研究所）；Institute of Physics, Johannes Gutenberg University Mainz（约翰内斯·古腾堡美因茨大学物理研究所）
We investigate magnonic band structure in thin homogeneous permalloy film decorated with periodic array of elliptically shaped permalloy dots and separated by non-magnetic Pt spacer. We demonstrated experimentally formation of the magnonic band structure for Damon-Eshbach wave propagating in permalloy film with the band gap opened at the Brillouin zone border and band splitting at smaller wavenumbers, due to the Bragg interference and interaction of propagating wave of the continuous film with a standing resonant mode of the nano-ellipses, respectively. The shape anisotropy of the permalloy na…
We present a theoretical study of excitations formed by hybridization between magnons and phonons - magnon-polarons - in antiferromagnets. We first outline a general approach to determining which magnon and phonon modes can and cannot hybridize in a system thereby addressing the qualitative questions concerning magnon-polaron formation. As a specific and experimentally relevant case, we study Nickel Oxide quantitatively and find perfect agreement with the qualitative analysis, thereby highlighting the strength of the former. We find that there are two distinct features of antiferromagnetic mag…
Sub-picosecond magnetisation manipulation via femtosecond optical pumping has attracted wide attention ever since its original discovery in 1996. However, the spatial evolution of the magnetisation is not yet well understood, in part due to the difficulty in experimentally probing such rapid dynamics. Here, we find evidence of rapid magnetic order recovery in materials with perpendicular magnetic anisotropy via nonlinear magnon processes. We identify both localisation and coalescence regimes, whereby localised magnetic textures nucleate and subsequently evolve in accordance with a power law fo…
In this paper, we introduce Berry curvature, topological Chern number and topological chiral edge mode, that emerge from a hybridization between magnon and electromagnetic wave in a ferromagnet insulator. By focusing on the energy conservation, we first reformulate the Landau-Lifshitz-Maxwell equation into a Hermitian eigenvalue equation. From the eigenvalue equation, we define the Berry curvature of the magnon-photon coupled waves. We show that the Berry curvature thus introduced shows a prominent peak around a hybridization point between magnon mode and photon mode, and a massive hybrid mode…
Recent studies show that hybrid quantum systems based on magnonics provide a new and promising platform for generating macroscopic quantum states involving a large number of spins. Here we show how to entangle two magnon modes in two massive yttrium-iron-garnet (YIG) spheres using cavity optomagnonics, where magnons couple to high-quality optical whispering gallery modes supported by the YIG sphere. The spheres can be as large as 1 mm in diameter and each sphere contains more than $10^{18}$ spins. The proposal is based on the asymmetry of the Stokes and anti-Stokes sidebands generated by the m…
Topological Magnetic Phases and Magnon-Phonon Hybridization in the Presence of Strong Dzyaloshinskii-Moriya Interaction
School of Physics and Astronomy, Shanghai Jiao Tong University（上海交通大学物理与天文学院）；Wilczek Quantum Center, School of Physics and Astronomy, Shanghai Jiao Tong University（上海交大物理与天文学院李政道量子中心）；Shanghai Research Center for Quantum Sciences（上海量子科学研究中心）；Department of Physics, The Chinese University of Hong Kong（香港中文大学物理系）；Institute for Quantum Science and Technology, Shanghai University（上海大学量子科学与技术研究院）；Department of Physics, Shanghai University（上海大学物理系）
We theoretically study the Berry curvature of the magnon induced by the hybridization with the acoustic phonons via the spin-orbit and dipolar interactions. We first discuss the magnon-phonon hybridization via the dipolar interaction, and show that the dispersions have gapless points in momentum space, some of which form a loop. Next, when both spin-orbit and dipolar interactions are considered, we show anisotropic texture of the Berry curvature and its divergence with and without gap-closing. Realistic evaluation of the consequent anomalous velocity is given for yttrium iron garnet.
Van der Waals magnets are uniquely positioned at the intersection between two-dimensional materials, antiferromagnetic spintronics, and magnonics. The interlayer exchange interaction in these materials enables antiferromagnetic resonances to be accessed at GHz frequencies. Consequently, these layered antiferromagnets are intriguing materials out of which quantum hybrid magnonic devices can be fashioned. Here, we use both a modified macrospin model and micromagnetic simulations to demonstrate a comprehensive antiferromagnetic resonance spectra in van der Waals magnets near the ultrathin (monola…
We theoretically study magnon-phonon hybrid excitations (magnon-polarons) in two-dimensional antiferromagnets on a honeycomb lattice. With an in-plane Dzyaloshinskii-Moriya interaction (DMI) allowed from mirror symmetry breaking from phonons, we find non-trivial Berry curvature around the anti-crossing rings among magnon and both optical and acoustic phonon bands, which gives rise to finite Chern numbers. We show that the Chern numbers of the magnon-polaron bands can be manipulated by changing the magnetic field direction or strength. We evaluate the thermal Hall conductivity reflecting the no…
Magnon-phonon hybrid excitations are studied theoretically in a two-dimensional antiferromagnet with an easy axis normal to the plane. We show that two magnon bands and one phonon band are intertwined by the magnetoelastic coupling through a nontrivial SU(3) topology, which can be intuitively perceived by identifying a skyrmion structure in the momentum space. We develop a continuum theory as the long-wavelength approximation to the tight-binding model, showing our results are insensitive to lattice details and generally applicable to two-dimensional antiferromagnets. The theoretical results c…
We study the diffusive transport of magnons through the compensated ferrimagnetic insulator Gd3Fe5O12 (GdIG). The magnons are injected and detected electrically in a non-local measurement configuration via two parallel Pt strips deposited on top of the ferrimagnet. GdIG exhibits a rich magnon spectrum, with several thermally populated magnon bands at room temperature. We observe a strong temperature and field dependence of the non-local voltage in the detector strip. Just below the magnetization compensation temperature we find that the increasing magnetic field causes an unexpected enhancemen…
Magnon-polaron control in a surface magnetoacoustic wave resonator
Rheinland-Pfälzische Technische Universität Kaiserslautern-Landau（莱茵兰-普法尔茨技术大学凯泽斯劳滕-兰道校区）；University of Oxford（牛津大学）；Japan Atomic Energy Agency（日本原子能机构）
Light-matter interactions lie at the heart of condensed matter physics, providing physical insight into material behaviour while enabling the design of new devices. Perhaps this is most evident in the push to develop quantum information and spintronic technologies. On the side of quantum information, engineered light-matter interactions offer a powerful means to access and control quantum states, while at the same time new insights into spin-photon manipulation will benefit the development of spintronic technologies. In this context the recent discovery of hybridization between ferromagnets an…
Level attraction is a type of mode hybridization in open systems where instead of forming a hybridization gap, the energy spectrum of two modes coalesce in a region bounded by exceptional points. We demonstrate that this phenomenon can be realized in a Floquet system with memory, which appears in describing linear excitations in a nonlinear driven system with a limit cycle. Linear response of the system in this state is different from its response near thermodynamic equilibrium. We develop a general formalism and provide an example in the context of cavity magnonics, where we show that magneti…
We theoretically study the optical generation of dc spin current (i.e., a spin-current solar cell) in ordered antiferromagnetic and ferrimagnetic insulators, motivated by a recent study on the laser-driven spinon spin current in noncentrosymmetric quantum spin chains [H. Ishizuka and M. Sato, Phys. Rev. Lett. 122, 197702 (2019)]. Using a non-linear response theory for magnons, we analyze the dc spin current generated by a linearly-polarized electromagnetic wave (typically, terahertz or gigahertz waves). Considering noncentrosymmetric two-sublattice magnets as an example, we find a finite dc sp…
We study the impact of Ga ion exposure on the local and non-local magnetotransport response in heterostructures of the ferrimagnetic insulator yttrium iron garnet and platinum. In particular, we cut the yttrium iron garnet layer in between two electrically separated wires of platinum using a Ga ion beam, and study the ensuing changes in the magnetoresistive response. We find that the non-local magnetoresistance signal vanishes when the yttrium iron garnet film between the Pt wires is fully cut, although the local spin Hall magnetoresistance signal remains finite. This observation corroborates …
Using a semiclassical approach, we study the persistent magnetization current of a mesoscopic ferrimagnetic ring in a nonuniform magnetic field. At zero temperature, there exists persistent spin current because of the quantum fluctuation of magnons, similar to the case of an antiferromagnetic spin ring. At low temperature, the current shows activation behavior because of the field-induced gap. At higher temperature, the magnitude of the spin current is proportional to temperature T, similar to the reported result of a ferromagnetic spin ring.
We observe a strong thermally-controlled magnon-mediated interlayer coupling of two ferromagnetic layers via an antiferromagnetic spacer in spin-valve type trilayers. The effect manifests itself as a field-induced coherent switching of the two ferromagnets, which can be controlled by varying temperature and the spacer thickness. We explain the observed behavior as due to a strong hybridization of the ferro- and antiferro-magnetic magnon modes in the trilayer at temperatures just below the Néel temperature of the antiferromagnetic spacer.
Spin-Hall nano-oscillators (SHNOs) are promising spintronic devices to realize current controlled GHz frequency signals in nanoscale devices for neuromorphic computing and creating Ising systems. However, traditional SHNOs -- devices based on transition metals -- have high auto-oscillation threshold currents as well as low quality factors and output powers. Here we demonstrate a new type of hybrid SHNO based on a permalloy (Py) ferromagnetic-metal nanowire and low-damping ferrimagnetic insulator, in the form of epitaxial lithium aluminum ferrite (LAFO) thin films. The superior characteristics …
Recent research on hybrid magnonics has been restricted by the long magnon wavelengths of the ferromagnetic resonance modes. We present an experiment on the hybridization of 250-nm wavelength magnons with microwave photons in a multimode magnonic system consists of a planar cavity and a magnetic bilayer. The coupling between magnon modes in the two magnetic layers, i.e., the uniform mode in Permalloy (Py) and the perpendicular standing spin waves (PSSWs) in YIG, serves as an effective means for exciting short-wavelength PSSWs, which is further hybridized with the photon mode of the microwave r…
In spintronics the propagation of spin-wave excitations in magnetically ordered materials can also be used to transport and process information. One of the most popular materials in this regard is the ferrimagnetic insulator yttrium-iron-garnet due its exceptionally small spin-wave damping parameter. While the small relaxation rate allows for large propagation length of magnetic excitations, it also leads to non-locality of the magnetic properties. By imaging spin waves their band structure is mapped. In doing so wave vector selection is shown to suppress dispersion effects to a large extent a…
Hybrid quantum devices expand the tools and techniques available for quantum sensing in various fields. Here, we experimentally demonstrate quantum sensing of the steady-state magnon population in a magnetostatic mode of a ferrimagnetic crystal. Dispersively coupling the magnetostatic mode to a superconducting qubit allows the detection of magnons using Ramsey interferometry with a sensitivity on the order of $10^{-3}$ $\text{magnons}/\sqrt{\text{Hz}}$. The protocol is based on dissipation as dephasing via fluctuations in the magnetostatic mode reduces the qubit coherence proportionally to the…
The spin resonance of electrons can be coupled to a microwave cavity mode to obtain a photon-magnon hybrid system. These quantum systems are widely studied for both fundamental physics and technological quantum applications. In this article, the behavior of a large number of ferrimagnetic spheres coupled to a single cavity is put under test. We use second-quantization modeling of harmonic oscillators to theoretically describe our experimental setup and understand the influence of several parameters. The magnon-polariton dispersion relation is used to characterize the system, with a particular …
When there is a certain amount of field inhomogeneity, the biased ferrimagnetic crystal will exhibit the higher-order magnetostatic (HMS) mode in addition to the uniform-precession Kittel mode. In cavity magnonics, we show both experimentally and theoretically the cross-Kerr-type interaction between the Kittel mode and HMS mode. When the Kittel mode is driven to generate a certain number of excitations, the HMS mode displays a corresponding frequency shift and vice versa. The cross-Kerr effect is caused by an exchange interaction between these two spin-wave modes. Utilizing the cross-Kerr effe…
The ultrastrong coupling of (quasi-)particles has gained considerable attention due to its application potential and richness of the underlying physics. Coupling phenomena arising due to electromagnetic interactions are well explored. In magnetically ordered systems, the quantum-mechanical exchange-interaction should furthermore enable a fundamentally different coupling mechanism. Here, we report the observation of ultrastrong intralayer exchange-enhanced magnon-magnon coupling in a compensated ferrimagnet. We experimentally study the spin dynamics in a gadolinium iron garnet single crystal us…
For solid-state spin systems, the collective spin motion in a single crystal embodies multiple magnetostatic modes. Recently, it was found that the cross-Kerr interaction between the higher-order magnetostatic mode and the Kittel mode introduces a new operable degree of freedom. In this work, we propose a scheme to entangle two magnon modes via the cross-Kerr nonlinearity when the bias field is inhomogeneous and the system is driven. Quantum entanglement persists at the steady state, as demonstrated by numerical results using experimentally feasible parameters. Furthermore, we also demonstrate…
Spin Inertia as a Source of Topological Magnons: Chiral Edge States from Coupled Precession and Nutation
Indian Institute of Technology (Indian School of Mines)（印度理工学院（印度矿业学院））；RWTH Aachen University（亚琛工业大学）；University of Münster（明斯特大学）；HUN-REN Wigner Research Centre for Physics（匈牙利研究网络维格纳物理研究中心）；Budapest University of Technology and Economics（布达佩斯技术与经济大学）
Magnon systems used in quantum devices require low damping if coherence is to be maintained. The ferrimagnetic electrical insulator yttrium iron garnet (YIG) has low magnon damping at room temperature and is a strong candidate to host microwave magnon excitations in future quantum devices. Monocrystalline YIG films are typically grown on gadolinium gallium garnet (GGG) substrates. In this work, comparative experiments made on YIG waveguides with and without GGG substrates indicate that the material plays a significant role in increasing the damping at low temperatures. Measurements reveal that…
Anticrossing behavior between magnons in a non-collinear chiral magnet Cu$_2$OSeO$_3$ and a two-mode X-band microwave resonator was studied in the temperature range 5-100K. In the field-induced ferrimagnetic phase, we observed a strong coupling regime between magnons and two microwave cavity modes with a cooperativity reaching 3600. In the conical phase, cavity modes are dispersively coupled to a fundamental helimagnon mode, and we demonstrate that the magnetic phase diagram of Cu$_2$OSeO$_3$ can be reconstructed from the measurements of the cavity resonance frequency. In the helical phase, a …
Magnons in ferrimagnetic insulators such as yttrium iron garnet (YIG) have recently emerged as promising candidates for coherent information processing in microwave circuits. Here we demonstrate optical whispering gallery modes of a YIG sphere interrogated by a silicon nitride photonic waveguide, with quality factors approaching $10^6$ in the telecom c-band after surface treatments. Moreover, in contrast to conventional Faraday setup, this implementation allows input photon polarized colinearly to the magnetization to be scattered to a sideband mode of orthogonal polarization. This Brillouin s…
Ultrastrong magnon-photon coupling in superconductor/antiferromagnet/superconductor heterostructures at terahertz frequencies
Moscow Institute of Physics and Technology（莫斯科物理技术学院）；School of Physics, Huazhong University of Science and Technology（华中科技大学物理学院）；National Research University Higher School of Economics（国立研究型高等经济大学）
The hybridization of magnons (spin waves) with phonons, if sufficiently strong and comprising long wavelength excitations, may offer a new playground when manipulating the magnetically ordered systems with light. Applying a magnetic field to a quasi-2D antiferromagnet, FePS3, we tune the magnon-gap excitation towards coincidence with the initially lower-in-energy phonon modes. Hybrid magnon-phonon modes, the magnon polarons are unveiled with demonstration of a pronounced avoided crossing between the otherwise bare magnon and phonon excitations. The magnon polarons in FePS3 are primary traced w…
