We theoretically investigate the energy relaxation rate of magnons and phonons near the resonance points to clarify the underlying mechanism of heat transport in ferromagnetic materials. We find that the simple two-temperature model is valid for the one-phonon/one-magnon process, as the rate of energy exchange between magnons and phonons is proportional to the temperature difference between them, and it is independent of temperature in the high temperature limit. We found that the magnon-phonon relaxation time due to the one-phonon/one-magnon interaction could be reduced to 1.48 $μs$ at the resonance point by applying an external magnetic field. It means that the resonance effect plays a significant role in enhancing the total magnon-phonon energy exchange rate, apart from the higher order interaction processes.
Nonlinear relaxation between spin waves (magnons) and the crystal lattice (phonons) in an insulating ferromagnet is investigated theoretically. Magnons and phonons are described by the equilibrium Bose-Einstein distributions with different temperatures. The nonlinear heat current from magnons to phonons is calculated microscopically in terms of the Cherenkov radiation of phonons by magnons. The results are discussed in comparison with the well-known theoretical results on the nonlinear electron-phonon relaxation in metals [Kaganov, Lifshitz, Tanatarov, J. Exp. Theor. Phys. 31, 232 (1956)]. The…
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 propose a magnon realization of 3D topological insulator in the AIII (chiral symmetry) topological class. The topological magnon gap opens due to the presence of Dzyaloshinskii-Moriya interactions. The existence of the topological invariant is established by calculating the bulk winding number of the system. Within our model, the surface magnon Dirac cone is protected by the sublattice chiral symmetry. By analyzing the magnon surface modes, we confirm that the backscattering is prohibited. By weakly breaking the chiral symmetry, we observe the magnon Hall response on the surface due to open…
We study transport phenomena cross-correlated among the heat and electric currents of magnons and Dirac electrons on the surface of ferromagnetic topological insulators. For a perpendicular magnetization, we calculate magnon- (electron-) drag anomalous Nernst/Seebeck (anomalous Ettingshausen/Peltier) effects and magnon-/electron-drag thermal Hall effects. The magnon-drag thermoelectric effects are interpreted to be caused by magnon-induced electromotive force. When the magnetization has in-plane components, there arise thermal/thermoelectric analogs of anisotropic magnetoresistance (AMR). In t…
Tripartite Phonon-Magnon-Plasmon Coupling, Parametric Amplification, and Formation of a Phonon-Magnon-Plasmon Polariton in a Two-Dimensional Periodic Array of Magnetostrictive/Plasmonic Bilayered Nanodots
The plethora of recent discoveries in the field of topological electronic insulators has inspired a search for boson systems with similar properties. There are predictions that ferromagnets on a two-dimensional honeycomb lattice may host chiral edge magnons. In such systems, we theoretically study how magnons and phonons couple. We find topological magneto-polarons around the avoided crossings between phonons and topological magnons. Exploiting this feature along with our finding of Rayleigh edge phonons in armchair ribbons, we demonstrate the existence of chiral edge modes with a phononic cha…
We investigate the spin transport properties of a topological magnon insulator, a magnetic insulator characterized by topologically nontrivial bulk magnon bands and protected magnon edge modes located in the bulk band gaps. Employing the Landau-Lifshitz-Gilbert phenomenology, we calculate the spin current driven through a normal metal$|$topological magnon insulator$|$normal metal heterostructure by a spin accumulation imbalance between the metals, with and without random lattice defects. We show that bulk and edge transport are characterized by different length scales. This results in a charac…
Bilayer Heisenberg ferromagnetic insulators hold degenerate terahertz Dirac magnon modes associated with two opposite valleys of the hexagonal Brillouin zone. We show that this energy degeneracy can be removed by breaking of the inversion symmetry (I), leading to a topological magnon valley current. We show furthermore that this current leads to valley Seebeck effect for magnons and is thereby detectable. We perform calculations in the specific example of bilayer CrBr3, where I can be broken by electrostatic doping.
We consider a magnetic insulator in contact with a normal metal. We derive a self-consistent Keldysh effective action for the magnon gas that contains the effects of magnon-magnon interactions and contact with the metal to lowest order. Self-consistent expressions for the dispersion relation, temperature and chemical potential for magnons are derived. Based on this effective action, we study instabilities of the magnon gas that arise due to spin-current flowing across the interface between the normal metal and the magnetic insulator. We find that the stability phase diagram is modified by an i…
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.
Magnon-mediated spin transport across nonmagnetic metal (NM) and ferromagnetic insulator (FI) interface depends critically on electron-magnon coupling. We propose a novel route to enhance electron-magnon coupling dynamically from transport viewpoint. Using non-equilibrium Green's function a theoretical formalism for magnon-mediated spin current is developed. In the language of transport, the effective electron-magnon coupling at NM/FI interface is determined by self-energy of FI lead, which is proportional to density of states (DOS) at NM/FI interface due to nonlinear process of electron-magno…
We develop a linear-response transport theory of diffusive spin and heat transport by magnons in magnetic insulators with metallic contacts. The magnons are described by a position dependent temperature and chemical potential that are governed by diffusion equations with characteristic relaxation lengths. Proceeding from a linearized Boltzmann equation, we derive expressions for length scales and transport coefficients. For yttrium iron garnet (YIG) at room temperature we find that long-range transport is dominated by the magnon chemical potential. We compare the model's results with recent ex…
Angular momentum transport is one of the cornerstones of spintronics. Spin angular momentum is not only transported by mobile charge carriers, but also by the quantized excitations of the magnetic lattice in magnetically ordered systems. In this regard, magnetically ordered insulators provide a platform for magnon spin transport experiments without additional contributions from spin currents carried by mobile electrons. In combination with charge-to-spin current conversion processes in conductors with finite spin-orbit coupling it is possible to realize all-electrical magnon transport schemes …
We propose a new mechanism for the thermal Hall effect in exchange spin-wave systems, which is induced by the magnon-phonon interaction. Using symmetry arguments, we first show that this effect is quite general, and exists whenever the mirror symmetry in the direction of the magnetization is broken. We then demonstrate our result in a collinear ferromagnet on a square lattice, with perpendicular easy-axis anisotropy and Dzyaloshinskii-Moriya interaction from mirror symmetry breaking. We show that the thermal Hall conductivity is controlled by the resonant contribution from the anti-crossing po…
The Doppler shift of the quasiparticle dispersion by charge currents is responsible for the critical supercurrents in superconductors and instabilities of the magnetic ground state of metallic ferromagnets. Here we predict an analogous effect in thin films of magnetic insulators in which microwaves emitted by a proximity stripline generate coherent chiral spin currents that cause a Doppler shift in the magnon dispersion. The spin-wave instability is suppressed by magnon-magnon interactions that limit spin currents to values close to but below the threshold for the instability. The spin current…
In this work, we theoretically study the magnon spin transport in easy-plane antiferromagnetic insulators in the presence of an in-plane magnetic field. By exactly calculating the magnon spectrum, we find the band splitting due to the magnetic anisotropy can be fully compensated by the external field at a particular strength, which makes its dynamics nearly equivalent to an easy-axis antiferromagnet. As a result, the intrinsic magnon spin Hall effect due to the dipole-dipole interaction, previously predicted in easy-axis antiferromagnets is activated in easy-plane antiferromagnets. The compens…
Magnon transport through a magnetic insulator can be controlled by current-biased heavy-metal gates that modulate the magnon conductivity via the magnon density. Here, we report nonlinear modulation effects in 10$\,$nm thick yttrium iron garnet (YIG) films. The modulation efficiency is larger than 40\%/mA. The spin transport signal at high DC current density (2.2$\times 10^{11}\,$A/m$^{2}$) saturates for a 400$\,$nm wide Pt gate, which indicates that even at high current levels a magnetic instability cannot be reached in spite of the high magnetic quality of the films.
We generalize the two-temperature model [Sanders and Walton, Phys. Rev. B, 15, 1489 (1977)] for coupled phonon-magnon diffusion to include the effect of the concurrent magnetization flow. Working within the framework of Boltzmann transport equation, we derive the constitutive equations for coupled phonon-magnon transport driven by gradients of both temperature and external magnetic fields, and the corresponding conservation laws. Our equations reduce to the original Sanders-Walton two-temperature model under a uniform external field, but predict a new magnon cooling effect driven by a non-unif…
Recent experimental evidence points to low-energy magnons as the primary contributors to the spin Seebeck effect. This spectral dependence is puzzling since it is not observed on other thermocurrents in the same material. Here, we argue that the physical origin of this behavior is the magnon-magnon scattering mediated by phonons, in a process which conserves the number of magnons. To assess the importance and features of this kind of scattering, we derive the effective magnon-phonon interaction from a microscopic model, including band energy, a screened electron-electron interaction and the el…
In this Letter we present a comprehensive study of topological magnon bands and thermal Hall effect in non-collinear antiferromagnetic systems on the honeycomb lattice with an intrinsic Dzyaloshinskii-Moriya interaction. We theoretically show that the system possesses topological magnon bands with Chern number protected edge modes accompanied by a nonzero thermal magnon Hall effect. These features result from non-collinearity of the magnetic moments due to an applied out-of-plane magnetic field. Our results provide an experimental clue towards the realization of topological magnon transports i…
Electrical resistivities can be different for charge currents travelling parallel or perpendicular to the magnetization in magnetically ordered conductors or semiconductors, resulting in the well-known planar Hall effect and anisotropic magnetoresistance. Here, we study the analogous anisotropic magnetotransport behavior for magnons in a magnetic insulator Y$_{3}$Fe$_{5}$O$_{12}$. Electrical and thermal magnon injection, and electrical detection methods are used at room temperature with transverse and longitudinal geometries to measure the magnon planar Hall effect and anisotropic magnetoresis…
Electronic spin current is convertible to magnonic spin current via the creation or annihilation of thermal magnons at the interface of a magnetic insulator and a metal with a strong spin-orbital coupling. So far this phenomenon was evidenced in the linear regime. Based on analytical and fulledged numerical results for the non-linear regime we demonstrate that the generated thermal magnons or magnonic spin current in the insulator is asymmetric with respect to the charge current direction in the metal and exhibits a nonlinear dependence on the charge current density, which is explained by the …
We investigated the effect of an external magnetic field on the diffusive spin transport by magnons in the magnetic insulator yttrium iron garnet (YIG), using a non-local magnon transport measurement geometry. We observed a decrease in magnon spin diffusion length $λ_m$ for increasing field strengths, where $λ_m$ is reduced from 9.6$\pm1.2$ $μ$m at 10 mT to 4.2$\pm0.6$ $μ$m at 3.5 T at room temperature. In addition, we find that there must be at least one additional transport parameter that depends on the external magnetic field. Our results do not allow us to unambiguously determine whether t…
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…
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…
We calculate the phonon, electron and magnon temperature profiles in yttrium iron garnet/platinum bilayers by diffusive theory with appropriate boundary conditions, in particular taking into account interfacial thermal resistances. Our calculations show that in thin film hybrids, the interface magnetic heat conductance qualitatively affects the magnon temperature. Based on published material parameters we assess the degree of non-equilibrium at the yttrium iron garnet/platinum interface. The magnitude of the spin Seebeck effect derived from this approach compares well with experimental results…
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 examine the combined effects of a Kekule coupling texture (KC) and a Dzyaloshinskii-Moriya interaction (DMI) in a two-dimensional ferromagnetic honeycomb lattice. By analyzing the gap closing conditions and the inversions of the bulk bands, we identify the parameter range in which the system behaves as a trivial or a nontrivial topological magnon insulator. We find four topological phases in terms of the KC parameter and the DMI strength. We present the bulk-edge correspondence for the magnons in a honeycomb lattice with an armchair or a zigzag boundary. Furthermore, we find Tamm-like edge …
The key physics of the spin valve involves spin-polarized conduction electrons propagating between two magnetic layers such that the device conductance is controlled by the relative magnetization orientation of two magnetic layers. Here, we report the effect of a magnon valve which is made of two ferromagnetic insulators (YIG) separated by a nonmagnetic spacer layer (Au). When a thermal gradient is applied perpendicular to the layers, the inverse spin Hall voltage output detected by a Pt bar placed on top of the magnon valve depends on the relative orientation of the magnetization of two YIG l…
Our joint theoretical and computer experimental study of heat-to-spin conversion reveals that noncollinear antiferromagnetic insulators are promising materials for generating magnon spin currents upon application of a temperature gradient: they exhibit spin Seebeck and spin Nernst effects. Using Kubo theory and spin dynamics simulations, we explicitly evaluate these effects in a single kagome sheet of potassium iron jarosite, KFe$_3$(OH)$_6$(SO$_4$)$_2$, and predict a spin Seebeck conversion factor of $0.2 μ\mathrm{V}/\mathrm{K}$ at a temperature of $20 \mathrm{K}$.
We demonstrate a spin to charge current conversion via magnon-phonon coupling and inverse Edelstein effect on the hybrid device Ni/Cu(Ag)/Bi$_{2}$O$_{3}$. The generation of spin current ($J_{s}\approx 10^{8}A/m^{2}$) due to magnon - phonon coupling reveals the viability of acoustic spin pumping as mechanism for the development of spintronic devices. A full in-plane magnetic field angle dependence of the power absorption and a combination of longitudinal and transverse voltage detection reveals the symmetric and asymmetric components of the inverse Edelstein effect voltage induced by Rayleigh t…
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…
Conversion of thermal energy into magnonic spin currents and/or effective electric polarization promises new device functionalities. A versatile approach is presented here for generating and controlling open circuit magnonic spin currents and an effective multiferroicity at a uniform temperature with the aid of spatially inhomogeneous, external, static electric fields. This field applied to a ferromagnetic insulator with a Dzyaloshinskii-Moriya type coupling changes locally the magnon dispersion and modifies the density of thermally excited magnons in a region of the scale of the field inhomog…
The transport of spin information has been studied in various materials, such as metals, semiconductors and graphene. In these materials, spin is transported by diffusion of conduction electrons. Here we study the diffusion and relaxation of spin in a magnetic insulator, where the large bandgap prohibits the motion of electrons. Spin can still be transported, however, through the diffusion of non-equilibrium magnons, the quanta of spin wave excitations in magnetically ordered materials. Here we show experimentally that these magnons can be excited and detected fully electrically in linear resp…
We theoretically study the electromagnetic wave radiation of magnons driven by acoustic phonons in systems with strong magnon-phonon interaction. We evaluate the field dependence of radiation intensity spectra which exhibits the avoided crossing, a characteristic of strongly coupled systems. At the crossover where the magnon and phonon eigenstates are hybridized, we demonstrate the existence of two resonant radiation frequencies with circular polarization and the enhancement of antenna radiation efficiency by over 100 times. Our results open up possibilities of developing ultra-compact antenna…
We address the theory of magnon-phonon interactions and compute the corresponding quasi-particle and transport lifetimes in magnetic insulators with focus on yttrium iron garnet at intermediate temperatures from anisotropy- and exchange-mediated magnon-phonon interactions, the latter being derived from the volume dependence of the Curie temperature. We find in general weak effects of phonon scattering on magnon transport and the Gilbert damping of the macrospin Kittel mode. The magnon transport lifetime differs from the quasi-particle lifetime at shorter wavelengths.
Strong coupling between two quanta of different excitations leads to the formation of a hybridized state which paves a way for exploiting new degrees of freedom to control phenomena with high efficiency and precision. A magnon polaron is the hybridized state of a phonon and a magnon, the elementary quanta of lattice vibrations and spin waves in a magnetically-ordered material. A magnon polaron can be formed at the intersection of the magnon and phonon dispersions, where their frequencies coincide. The observation of magnon polarons in the time domain has remained extremely challenging because …
Karlsruhe Institute of Technology（卡尔斯鲁厄理工学院）；Jiangsu University（江苏大学）；Forschungszentrum Jülich GmbH（于利希研究中心有限公司）；Johannes Gutenberg-University Mainz（美因茨约翰内斯·古腾堡大学）
A temperature-dependent approach involving Green-Kubo equilibrium atomic and spin dynamics (GKEASD) is reported to assess phonon and magnon thermal transport processes accounting for phonon-magnon interactions. Using body-center cubic (BCC) iron as a case study, GKEASD successfully reproduces its characteristic temperature-dependent spiral and lattice thermal conductivities. The non-electronic thermal conductivity, i.e., the sum of phonon and magnon thermal conductivities, calculated using GKEASD for BCC Fe agrees well with experimental measurements. Spectral energy analysis reveals that high-…
The magnon pumping effect in ferromagnetic insulators under an external time-dependent transverse magnetic field is theoretically studied. Generation of a magnon current is discussed by calculating the magnon source term in the spin continuity equation. This term represents the non-conservation of magnons arising from an applied transverse magnetic field. The magnon source term has a resonance structure as a function of the angular frequency of the transverse field, and this fact is useful to enhance the pumping effect.
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…
