Phonons and magnons are engineered by periodic potential landscapes in phononic and magnonic crystals, and their combined studies may enable valley phonon transport tunable by the magnetic field. Through nonreciprocal surface acoustic wave transmission, we demonstrate valley-selective phonon-magnon scattering in magnetoelastic superlattices. The lattice symmetry and the out-of-plane magnetization component control the sign of nonreciprocity. The phonons in the valleys play a crucial role in generating nonreciprocal transmission by inducing circularly polarized strains that couple with the magnons. The transmission spectra show a nonreciprocity peak near a transmission gap, matching the phononic band structure. Our results open the way for manipulating valley phonon transport through periodically varying magnon-phonon coupling.
We study the topological property of the magnetoelastic excitation in non-collinear antiferromagnets. As a toy model, we consider the magnon-phonon coupling in a triangular antiferromagnet with a $120^\circ$ Nèel order. We find that in the presence of out-of-plane external magnetic field, the magnon-polaron bands, which arise from hybridization of magnons and phonons, can carry Chern number, even though the individual magnon and phonon bands are topologically trivial. Large Berry curvature is induced from the anti-crossing regions between the magnon and phonon bands, which renormalizes the the…
We study the formation of magnon-polaron excitations and the consequences of different time scales between the magnon and lattice dynamics. The spin-spin interactions along the 1D lattice are ruled by a Heisenberg Hamiltonian in the anisotropic form XXZ, in which each spin exhibits a vibrational degree of freedom around its equilibrium position. By considering a magnetoelastic coupling as a linear function of the relative displacement between nearest-neighbor spins, results provide an original framework for achieving a hybridized state of magnon-polaron. Such state is characterized by high coo…
Development of energy efficient techniques for generation of spin waves (magnons) is important for implementation of low-dissipation spin-wave-based logic circuits and memory elements. A promising approach to achieve this goal is based on the injection of short strain pulses into ferromagnetic films with a strong magnetoelastic coupling between spins and strains. Here we report micromagnetoelastic simulations of the magnetization and strain dynamics excited in Fe$_{81}$Ga$_{19}$ films by picosecond and nanosecond acoustic pulses created in a GaAs substrate by a transducer subjected to an optic…
We theoretically investigate the magnetoelastic coupling (MEC) and its effect on magnon transport in two-dimensional antiferromagnets with a honeycomb lattice. MEC coeffcient along with magnetic exchange parameters and spring constants are computed for monolayers of transition metal trichalcogenides with Néel order ($\text{MnPS}_3$ and $\text{VPS}_3$) and zigzag order ($\text{CrSiTe}_3$, $\text{NiPS}_3$ and $\text{NiPSe}_3$) by $ab$ $initio$ calculations. Using these parameters, we predict that the spin-Nernst coefficient is significantly enhanced due to magnetoelastic coupling. Our study show…
We derive and solve quantum kinetic equations describing the accumulation of magnetoelastic bosons in an overpopulated magnon gas realized in a thin film of the magnetic insulator yttrium iron garnet. We show that in the presence of a magnon condensate, there is a non-equilibrium steady state in which incoherent magnetoelastic bosons accumulate in a narrow region in momentum space for energies slightly below the bottom of the magnon spectrum. The results of our calculations agree quite well with Brillouin light scattering measurements of the stationary non-equilibrium state of magnons and magn…
We show how to measure a steady-state magnon population in a magnetostatic mode of a ferromagnet or ferrimagnet, such as yttrium iron garnet. We adopt an optomechanical approach and utilize the magnetoelasticity of the ferromagnet. The magnetostrictive force dispersively couples magnons to the deformation displacement of the ferromagnet, which is proportional to the magnon population. By further coupling the mechanical displacement to an optical cavity that is resonantly driven by a weak laser, the magnetostrictively induced displacement can be sensed by measuring the phase quadrature of the o…
Giant spin Nernst effect in a two-dimensional antiferromagnet due to magnetoelastic coupling-induced gaps and interband transitions between magnon-like bands
Optical probing of magnons and phonons in Ni80Fe20 nanodot arrays
S. N. Bose National Centre for Basic Sciences（S.N. 玻色基础科学国家中心）；Polish Academy of Sciences（波兰科学院）；University of Gothenburg（哥德堡大学）；Adam Mickiewicz University（亚当·密茨凯维奇大学）；Shiv Nadar Institution of Eminence (Delhi NCR)（希夫纳达尔卓越机构（德里NCR））
Motivated by recent experiments, we investigate the nonreciprocal magnetoelastic interaction between the surface acoustic phonons of dielectric non-magnetic substrates and magnons of proximity nanomagnets. The magnetization dynamics exerts rotating forces at the edges of the nanomagnet that causes the nonreciprocal interaction with surface phonons due to its rotation-momentum locking. This coupling induces the nonreciprocity of the surface phonon transmission and a nearly complete phonon diode effect by several (tens of) magnetic nanowires of high (ordinary) magnetic quality. Phase-sensitive m…
Magnetoelastic coupling in stripe-domain states of yttrium iron garnet
University College London（伦敦大学学院）；Imperial College London（帝国理工学院）；Japan Atomic Energy Agency（日本原子能机构）；The University of Tokyo（东京大学）；RIKEN Center for Emergent Matter Science (CEMS)（理化学研究所新兴物质科学中心）；WPI-Advanced Institute for Materials Research, Tohoku University（东北大学材料研究高等研究院）
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 theoretically study the effects of strong magnetoelastic coupling on the transport properties of magnetic insulators. We develop a Boltzmann transport theory for the mixed magnon-phonon modes (magnon polarons) and determine transport coefficients and spin diffusion length. Magnon-polaron formation causes anomalous features in the magnetic field and temperature dependence of the spin Seebeck effect when the disorder scattering in the magnetic and elastic subsystems is sufficiently different. Experimental data by Kikkawa et al. [PRL 117, 207203 (2016)] on yttrium iron garnet films can be expl…
In this work, we report the theoretical possibility of generating magnon polaron excitations through a space-varying magnetic field. The spatial dependence of the magnetic field in the Zeeman interaction gives rise to a magnon-phonon coupling when a magnetic field gradient is applied, and such a coupling depends directly on the strength of the gradient. It is also predicted that the direction of the magnetic field gradient allows control over which phonon polarization couples to the magnons in the material. Here we develop the calculations of the magnon-phonon coupling for an arbitrary (anti)f…
We theoretically study the intrinsic thermal Hall and spin Nernst effect in collinear ferrimagnets on a honeycomb lattice with broken inversion symmetry. The broken inversion symmetry allows in-plane Dzyaloshinskii-Moriya interaction between the nearest neighbors, which does not affect the magnon spectrum in the linear spin wave theory. However, the Dzyaloshinskii-Moriya interaction can induce large Berry curvature in the magnetoelastic excitation spectrum through the magnon-phonon interaction to produce thermal Hall current. Furthermore, we find that the magnetoelastic excitations transport s…
The spin Seebeck effect (SSE) is observed in magnetic insulator|heavy metal bilayers as an inverse spin Hall effect voltage under a temperature gradient. The SSE can be detected nonlocally as well, viz. in terms of the voltage in a second metallic contact (detector) on the magnetic film, spatially separated from the first contact that is used to apply the temperature bias (injector). Magnon-polarons are hybridized lattice and spin waves in magnetic materials, generated by the magnetoelastic interaction. Kikkawa et al. [Phys. Rev. Lett. \textbf{117}, 207203 (2016)] interpreted a resonant enhanc…
One of the most fundamental forms of magnon-phonon interaction is an intrinsic property of magnetic materials, the "magnetoelastic coupling". This particular form of interaction has been the basis for describing magnetic materials and their strain related applications, where strain induces changes of internal magnetic fields. Different from the magnetoelastic coupling, more than 40 years ago, it was proposed that surface acoustic waves may induce surface magnons via rotational motion of the lattice in anisotropic magnets. However, a signature of this magnon-phonon coupling mechanism, termed ma…
Phase-resolved imaging of coherent phonon-magnon coupling
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 Groningen（格罗宁根大学）；University of Augsburg（奥格斯堡大学）
We theoretically investigate topological spin transport of the magnon-polarons in bilayer magnet with two-dimensional square lattices. Our theory is motivated by recent reports on the van der Waals magnets which show the reversible electrical switching of the interlayer magnetic order between antiferromagnetic and ferromagnetic orders. The magnetoelastic interaction opens band gaps and allows the interband transition between different excitation states. In the layered antiferromagnet, due to the interband transition between the magnon-polaron states, the spin Berry curvature which allows the t…
Harnessing the causal relationships between mechanical and magnetic properties of van der Waals materials presents a wealth of untapped opportunity for scientific and technological advancement, from precision sensing to novel memories. This can, however, only be exploited if the means exist to efficiently interface with the magnetoelastic interaction. Here, we demonstrate acoustically-driven spin-wave resonance in a crystalline antiferromagnet, chromium trichloride, via surface acoustic wave irradiation. The resulting magnon-phonon coupling is found to depend strongly on sample temperature and…
We investigate phonon spin transport in an insulating ferromagnet-nonmagnet-ferromagnet heterostructure. We show that the magnetoelastic interaction between the spins and the phonons leads to nonlocal spin transfer between the magnets. This transfer is mediated by a local phonon spin current and accompanied by a phonon spin accumulation. The spin conductance depends nontrivially on the system size, and decays over millimeter length scales for realistic material parameters, far exceeding the decay lengths of magnonic spin currents.
We report ferromagnetic resonance in the normal configuration of an electrically insulating magnetic bilayer consisting of two yttrium iron garnet (YIG) films epitaxially grown on both sides of a 0.5-mm-thick nonmagnetic gadolinium gallium garnet (GGG) slab. An interference pattern is observed and it is explained as the strong coupling of the magnetization dynamics of the two YIG layers either in phase or out of phase by the standing transverse sound waves, which are excited through a magnetoelastic interaction. This coherent mediation of angular momentum by circularly polarized phonons throug…
We investigate a hybrid quantum system where an ensemble of nitrogen-vacancy (NV) centers in diamond is interfaced with a piezomagnetic superlattice that supports surface phonon polaritons (SPhPs). We show that the strong magnetic coupling between the collective spin waves in the NV spin ensemble and the quantized SPhPs can be realized, thanks to the subwavelength nature of the SPhPs and relatively long spin coherence times. The magnon-polariton coupling allows different modes of the SPhPs to be mapped and orthogonally stored in different spatial modes of excitation in the solid medium. Becaus…
Recently [1], we proposed a new way to engineer a flexible acoustomechanical coupling between the center-of-mass motion of an isolated micromagnet and one of its internal acoustic phonons by using a magnon as a passive mediator. In our approach, the coupling is enabled by the strong magnetoelastic interaction between magnons and acoustic phonons which originates from the small particle size. Here, we substantially extend our previous work. First, we provide the full theory of the quantum acoustomagnonic interaction in small micromagnets and analytically calculate the magnon-phonon coupling rat…
Magnomechanical Coupling in Suspended 2D van der Waals Ferromagnets
Kavli Institute of Nanoscience, Delft University of Technology（代尔夫特理工大学卡弗里纳米科学研究所）；Department of Precision and Microsystems Engineering, Delft University of Technology（代尔夫特理工大学精密与微系统工程系）
