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Electric field modulations of the perpendicular magnetic anisotropy (PMA) and magnetization dynamics of MgO/Co/Pt trilayers were measured by time-resolved magneto-optical Kerr effect (TRMOKE). The anisotropy field estimated from the anomalous Hall effect applying a field along in-plane was confirmed to be modified by the electric field, which results in the PMA variation with electric field of –32 fJ/(V∙m). The PMA variation was also confirmed from TRMOKE measurements, which was estimated to be –47 fJ/(V∙m). From TRMOKE measurements, we also observed the reduction of the inverse of relaxation time 1/τ of the magnetization precession with increasing the electric field. The 1/τ was analyzed by assuming the contributions of effective Gilbert damping αeff, anisotropy distribution, anisotropy angle distribution, and two magnon scattering, and we found αeff decreased with increasing the electric field whose modulation was –0.0046 nm/V, corresponding to –13% per 1 V/nm. These results are similar to the electric field effect in (001)-orientated MgO/CoFeB/Ta trilayers (A. Okada et al., Appl. Phys. Lett. 105, 052415 (2014)), however our finding indicates similar PMA and α modulations are found in MgO/Co/Pt trilayers having MgO(111)/fcc-Co interface.

Spontaneous symmetry breaking in crystalline solid often produces exotic nonreciprocal phenomena. As one such example, the unconventional optical rotation with nonreciprocity, which is termed gyrotropic birefringence, is expected to emerge from the magnetoelectric coupling. However, the fundamental nature of gyrotropic birefringence remains to be examined. Here w`e demonstrate the gyrotropic birefringence enhanced by the dynamical magnetoelectric coupling on the electrically active magnon resonance, i.e. electromagnon, in a multiferroic helimagnet. The helical spin order having both polarity and chirality is found to cause the giant gyrotropic birefringence in addition to the conventional gyrotropy, i.e. natural optical activity. It is demonstrated that the optical rotation of gyrotropic birefringence can be viewed as the nonreciprocal rotation of the optical principal axes, while the crystallographic and magnetic anisotropies are intact. The independent control of the nonreciprocal linear (gyrotropic birefringence) and circular (natural optical activity) birefringence/dichroism paves a way for the optically active devices., Gyrotropic birefringence is expected to emerge from the magnetoelectric coupling. Here, the authors find polarity and chirality arising from the helical spin order exhibit enhanced optical activities, gyrotropic birefringence and natural optical activity, on the electromagnon resonance.

The quanta of magnetic excitations - magnons - are known for their unique ability to undergo Bose-Einstein condensation at room temperature. This fascinating phenomenon reveals itself as a spontaneous formation of a macroscopic coherent state under the influence of incoherent stimuli. Spin currents have been predicted to offer electronic control of magnon Bose-Einstein condensates, but this phenomenon has not been experimentally evidenced up to now. Here we experimentally show that current-driven Bose-Einstein condensation can be achieved in nanometer-thick films of magnetic insulators with tailored dynamic magnetic nonlinearities and minimized magnon-magnon interactions. We demonstrate that, above a certain threshold, magnons injected by the spin current overpopulate the lowest-energy level forming a highly coherent spatially extended state. By accessing magnons with essentially different energies, we quantify the chemical potential of the driven magnon gas and show that, at the critical current, it reaches the energy of the lowest magnon level. Our results pave the way for implementation of integrated microscopic quantum magnonic and spintronic devices., Comment: This is a preprint of an article published in Nature Communications. The final authenticated version is available online at: https://doi.org/10.1038/s41467-021-26790-y

Magnon-mediated angular-momentum flow in antiferromagnets may become a design element for energy-efficient, low-dissipation and high-speed spintronic devices1,2. Owing to their low energy dissipation, antiferromagnetic magnons can propagate over micrometre distances3. However, direct observation of their high-speed propagation has been elusive due to the lack of sufficiently fast probes2. Here we measure the antiferromagnetic magnon propagation in the time domain at the nanoscale (≤50 nm) with optical-driven terahertz emission. In non-magnetic-Bi2Te3/antiferromagnetic-insulator-NiO/ferromagnetic-Co trilayers, we observe a magnon velocity of ~650 km s–1 in the NiO layer. This velocity far exceeds previous estimations of the maximum magnon group velocity of ~40 km s–1, which were based on the magnon dispersion measurements of NiO using inelastic neutron scattering4,5. Our theory suggests that for magnon propagation at the nanoscale, a finite damping makes the dispersion anomalous for small magnon wavenumbers and yields a superluminal-like magnon velocity. Given the generality of finite dissipation in materials, our results strengthen the prospects of ultrafast nanodevices using antiferromagnetic magnons. Magnon-mediated angular-momentum flow in antiferromagnets may become a design element for energy-efficient, low-dissipation and high-speed spintronic devices. Here, terahertz emission measurements in magnetic multilayers unveil a superluminal-like magnon velocity of ~650 km s–1 in the antiferromagnetic insulator NiO at nanoscale distances.

The article is devoted to the study of quantization of magnons in yttrium iron garnet film in the magnetic field gradient. Detected energy levels correspond to the quantization of particles in triangular potential. These energy levels are not equidistant. Therefore, they can be used to create a magnon quantum qubits.

This review presents the results about spin caloritronics obtained at the Department of Low Temperature Physics of the Kharkiv National University from 2017 to 2019 years. Several new directions in magneto-electronics: spintronics, spin caloritronics and magnonics, which emerged with the aim to reduce the energy dissipation in devices of usual semiconductor microelectronics, are discussed. Spintronic devices hold the promise of faster switching speeds, less total energy consumption, and higher density of circuit elements, lowering the heat production per switching element. Then the main results of the four papers published in Physical Review B are discussed: nonlinear relaxation between magnons and phonons in insulating ferromagnets; role of magnons and the size effect in heat transport through an insulating ferromagnet-insulator interface; spin Seebeck effect and phonon energy transfer in heterostructures containing layers of normal metal and ferroinsulator; temperature dependence of the magnon-phonon energy relaxation time in a ferromagnet insulator.

Magnonics is a research field complementary to spintronics, in which the quanta of spin waves (magnons) replace electrons as information carriers, promising less energy dissipation. The development of ultrafast nanoscale magnonic logic circuits calls for new tools and materials to generate coherent spin waves with frequencies as high, and wavelengths as short, as possible. Antiferromagnets can host spin waves at THz frequencies and are therefore seen as a future platform for the fastest and the least dissipative transfer of information. However, the generation of short-wavelength coherent propagating magnons in antiferromagnets has so far remained elusive. Here we report the efficient emission and detection of a nanometer-scale wavepacket of coherent propagating magnons in antiferromagnetic DyFeO3 using ultrashort pulses of light. The subwavelength nanoscale confinement of the laser field due to large absorption creates a strongly non-uniform spin excitation profile, thereby enabling the propagation of a broadband continuum of coherent THz spin waves. The wavepacket features magnons with detected wavelengths down to 125 nm and supersonic velocities up to 13 km/s that propagate over macroscopic distances. The long-sought source of coherent short-wavelength spin carriers demonstrated here opens up new prospects for THz antiferromagnetic magnonics and coherence mediated logic devices at THz frequencies., Comment: 33 pages, 13 figures

Recent years have witnessed a much broader use of Brillouin inelastic light-scattering spectroscopy for the investigation of phonons and magnons in novel materials, nanostructures and devices. Driven by the developments in instrumentation and the strong need for accurate knowledge on the energies of elemental excitations, Brillouin–Mandelstam spectroscopy is rapidly becoming an essential technique that is complementary to Raman inelastic light-scattering spectroscopy. We provide an overview of recent progress in the Brillouin light-scattering technique, focusing on the use of this photonic method for the investigation of confined acoustic phonons, phononic metamaterials and magnon propagation and scattering. This Review emphasizes the emerging applications of Brillouin–Mandelstam spectroscopy for phonon-engineered structures and spintronic devices, and concludes with a perspective on future directions. Nearly 100 years after the prediction of Brillouin light-scattering spectroscopy, or Brillouin–Mandelstam light-scattering spectroscopy, the effect has proved itself a powerful tool for decades. Now its application to probing confined acoustic phonons, phononic metamaterials and magnons is reviewed.

Nanomagnets are widely used to store information in non-volatile spintronic devices. Spin waves can transfer information with low-power consumption as their propagations are independent of charge transport. However, to dynamically couple two distant nanomagnets via spin waves remains a major challenge for magnonics. Here we experimentally demonstrate coherent coupling of two distant Co nanowires by fast propagating spin waves in an yttrium iron garnet thin film with sub-50 nm wavelengths. Magnons in two nanomagnets are unidirectionally phase-locked with phase shifts controlled by magnon spin torque and spin-wave propagation. The coupled system is finally formulated by an analytical theory in terms of an effective non-Hermitian Hamiltonian. Our results are attractive for analog neuromorphic computing that requires unidirectional information transmission. [Figure not available: see fulltext.]

Spin-current and spin-wave-based devices have been considered as promising candidates for next-generation information transport and processing and wave-based computing technologies with low-power consumption. Spin pumping has attracted tremendous attention and has led to interesting phenomena, including the line width broadening, which indicates damping enhancement due to energy dissipation. Recently, chiral spin pumping of spin waves has been experimentally realized and theoretically studied in magnetic nanostructures. Here, we experimentally observe by Brillouin light scattering (BLS) microscopy the line width broadening sensitive to magnetization configuration in a hybrid metal-insulator nanostructure consisting of a Co nanowire grating dipolarly coupled to a planar continuous YIG film, consistent with the results of the measured hysteresis loop. Tunable line width broadening has been confirmed independently by propagating spin-wave spectroscopy, where unidirectional spin waves are detected. Position-dependent BLS measurement unravels an oscillating-like behavior of magnon populations in Co nanowire grating, which might result from the magnon trap effect. These results are thus attractive for reconfigurable nanomagnonics devices.

Phonon and magnon excitations can be modulated by magnetoelastic and magneto-optical coupling effects, respectively. The photons not only mediate the interaction between two distant magnetic films inserted into a cavity, but also affect the phonon excitations in each film by the magnetoelastic coupling. In this work, an indirect interaction mechanism is presented for the phonons in two distant magnetic films inserted into a cavity. Although the phonons in no way pass through a very long distance from one film to the other in a vacuum environment, they can establish a correlation between two films through the magneto-optical and magnetoelastic couplings in each film. This really corresponds to an indirect interaction. In this work, the relationship between the phonon interaction and the laser intensity is studied using a quantum theory. It is found that when the laser frequency is equal to magnon frequency and the laser intensity is in a certain range, this phonon interaction energy takes very large negative values so as to satisfy the application requirement. According to a generating functional integral approach, a formula for the phonon interaction energy is derived, and an experimental approach to detect this effect is suggested. This indirect interaction effect of phonons not only adds to our understanding of the essence of matter interaction, but also reveals some potential applications to the heat management of electronic devices, long-range transfer of heat energy, and ultrasonic signal transmission in a vacuum environment.

The ability to control and tune magnetic dissipation is a key concept of emergent spintronic technologies. Magnon scattering processes constitute a major dissipation channel in nanomagnets, redefine their response to spin torque, and hold the promise for manipulating magnetic states on the quantum level. Controlling these processes in nanomagnets, while being imperative for spintronic applications, has remained difficult to achieve. Here, we propose an approach for controlling magnon scattering by a switch that generates nonuniform magnetic field at nanoscale. We provide an experimental demonstration in magnetic tunnel junction nanodevices, consisting of a free layer and a synthetic antiferromagnet. By triggering the spin-flop transition in the synthetic antiferromagnet and utilizing its stray field, magnon interaction in the free layer is toggled. The results open up avenues for tuning nonlinearities in magnetic neuromorphic applications and for engineering coherent magnon coupling in hybrid quantum information technologies.

Since the discovery of intrinsic ferromagnetism in atomically thin Cr2Gr2Te6 and CrI3 in 2017, research on two-dimensional (2D) magnetic materials has become a highlighted topic. Based on 2D magnetic materials and their heterostructures, exotic physical phenomena at the atomically thin limit have been discovered, such as the quantum anomalous Hall effect, magneto-electric multiferroics, and magnon valleytronics. Furthermore, magnetism in these ultrathin magnets can be effectively controlled by external perturbations, such as electric field, strain, doping, chemical functionalization, and stacking engineering. These attributes make 2D magnets ideal platforms for fundamental research and promising candidates for various spintronic applications. This review aims at providing an overview of the structures, properties, and external controls of 2D magnets, as well as the challenges and potential opportunities in this field.

Using time-resolved magneto-optical Kerr effect (TR-MOKE) microscopy, we demonstrate surface-acoustic-wave (SAW) induced resonant amplification of intrinsic spin-wave (SW) modes, as well as generation of new extrinsic or driven modes at the SAW frequency, in a densely packed two-dimensional array of elliptical Co nanomagnets fabricated on a piezoelectric LiNbO3 substrate. This system can efficiently serve as a magnonic crystal (MC), where the intrinsic shape anisotropy and the strong inter-element magnetostatic interaction trigger the incoherent precession of the nanomagnets' magnetization in the absence of any bias magnetic field, giving rise to the ‘intrinsic’ SW modes. The magnetoelastic coupling leads to a rich variety of SW phenomena when the SAW is launched along the major axis of the nanomagnets, such as 4–7 times amplification of intrinsic modes (at 3, 4, 7 and 10 GHz) when the applied SAW frequencies are resonant with these frequencies, and the generation of new extrinsic modes at non-resonant SAW frequencies. However, when the SAW is launched along the minor axis, a dominant driven mode appears at the applied SAW frequency. This reveals that the magnetoelastic coupling between SW and SAW is anisotropic in nature. Micromagnetic simulation results are in qualitative agreement with the experimental observations and elucidate the underlying dynamics. Our findings lay the groundwork for bias-field free magnonics, where the SW behavior is efficiently tuned by SAWs. It has important applications in the design of energy efficient on-chip microwave devices, SW logic, and extreme sub-wavelength ultra-miniaturized microwave antennas for embedded applications.

The interaction of a magnon Bose—Einstein condensate (mBEC) in a perpendicularly magnetized yttrium iron garnet film with high-order acoustic modes appearing at the sizes of an yttrium iron garnet substrate has been studied experimentally. Narrow energy absorption lines of the mBEC have been observed at the coincidence of frequencies. The minimum relaxation of the mBEC has been detected under acoustic antiresonance conditions when the precession frequency is exactly in the middle between acoustic resonance modes. This discovery will allow the optimization of coherence times when fabricating quantum magnonic devices.

The interaction between different types of wave excitation in hybrid systems is usually anisotropic. Magnetoelastic coupling between surface acoustic waves and spin waves strongly depends on the direction of the external magnetic field. However, in the present study we observe that even if the orientation of the field is supportive for the coupling, the magnetoelastic interaction can be significantly reduced for surface acoustic waves with a particular profile in the direction normal to the surface at distances much smaller than the wavelength. We use Brillouin light scattering for the investigation of thermally excited phonons and magnons in a magnetostrictive CoFeB/Au multilayer deposited on a Si substrate. The experimental data are interpreted on the basis of a linearized model of interaction between surface acoustic waves and spin waves.

Magnons, the quanta of spin waves, could be used to encode information in beyond-Moore computing applications, and magnonic device components, including logic gates, transistors, and units for non-Boolean computing, have already been developed. Magnonic directional couplers, which can function as circuit building blocks, have also been explored, but have been impractical because of their millimetre dimensions and multi-mode spectra. Here, we report a magnonic directional coupler based on yttrium iron garnet single-mode waveguides of 350 nm width. We use the amplitude of a spin-wave to encode information and to guide it to one of the two outputs of the coupler depending on the signal magnitude, frequency, and the applied magnetic field. Using micromagnetic simulations, we also propose an integrated magnonic half-adder that consists of two directional couplers and processes all information within the magnon domain with aJ energy consumption., 26 pages, 10 figures. arXiv admin note: text overlap with arXiv:1902.02855

State-of-the-art studies of dielectric magnonics and magnon spintronics are reviewed. Theoretical and experimental approaches to exploring physical processes in and calculations of the parameters of magnonic micro- and nanostructures are described. We discuss the basic concepts of magnon spintronics, the underlying physical phenomena, and the prospects for applying magnon spintronics for data processing, transmission, and reception. Special attention is paid to the feasibility of boosting the operating frequencies of magnonic devices from the gigahertz to terahertz frequency range. We also discuss specific implementations of the component base of magnonics and ways to further develop it.

At the resonant passage of a plane elastic shear wave through an acoustically continuous heterostructure consisting of magnetic and nonmagnetic layers in a symmetric nonmagnetic environment, the excitation of propagating exchangeless and exchange spin waves can lead to the formation of elastodipole analog of the Fano resonance and accompanying dynamic effects, including the collapse of the Fano resonance and the appearance of coupled magnon states in the continuum of transverse phonons.

The formation of a Bose—Einstein condensate of magnons in a perpendicularly magnetized iron yttrium garnet film under radio-frequency pumping in a stripline has been studied experimentally. The characteristics of the nonlinear magnetic resonance and spatial distribution of the Bose—Einstein condensate of magnons in the gradient of a magnetic field have been analyzed. In these experiments, the system of bosonic magnons behaves in much the same way as the Bose—Einstein condensate of magnons in antiferromagnetic superfluid 3He-B, which was previously studied in detail. The Bose—Einstein condensate of magnons forms a coherently precessing state having the properties of magnon superfluidity. Its stability is determined by the repulsion potential between excited magnons, which compensates the inhomogeneity of the magnetic field.

On-chip signal processing at microwave frequencies is key for modern mobile communication. When one aims at small footprints, low power consumption, reprogrammable filters, and delay lines, magnons in low-damping ferrimagnets offer great promise. Ferromagnetic grating couplers have been reported to be specifically useful as microwave-to-magnon transducers. However, their interconversion efficiency is unknown and real-space measurements of the emitted magnon wavelengths have not yet been accomplished. Here, we image with subwavelength spatial resolution the magnon emission process into ferrimagnetic yttrium iron garnet (YIG) at frequencies up to 8 GHz. We evidence propagating magnons of a wavelength of 98.7 nm underneath the gratings, which enter the YIG without a phase jump. Counterintuitively, the magnons exhibit an even increased amplitude in YIG, which is unexpected and due to a further wavelength conversion process. Our results are of key importance for magnonic components, which efficiently control microwave signals on the nanoscale.

Magnons are collective spin excitations in crystals with long-range magnetic order. The emergent van der Waals magnets1–3 provide a highly tunable platform to explore magnetic excitations in the two-dimensional limit with intriguing properties, manifesting from their honeycomb lattice structure and switchable magnetic configurations. Here, we report the direct observation of two-dimensional magnons through magneto-Raman spectroscopy with optical selection rules determined by the interplay between crystal symmetry, layer number and magnetic states in atomically thin CrI3. In monolayers, we observe an acoustic magnon mode at ~0.3 meV. It has strict cross-circularly polarized selection rules locked to the magnetization direction that originates from the conservation of angular momentum of photons and magnons dictated by three-fold rotational symmetry4. Additionally, we reveal optical magnon modes at ~17 meV. This mode is Raman silent in monolayers, but optically active in bilayers and bulk due to a relaxation of the parity criterion resulting from the layer index. In the layered antiferromagnetic states, we directly resolve two degenerate optical magnon modes with opposite angular momentum and conjugate optical selection rules. From these measurements, we quantitatively extract the spin-wave gap, magnetic anisotropy and intralayer and interlayer exchange constants, and establish two-dimensional magnets as a new platform for exploring magnon physics. Magnons are collective excitations that dictate many of a magnet’s low-temperature properties. By means of Raman scattering, the magnon spectra of CrI3 are measured in the monolayer limit.

A mechanism is proposed for transferring magnon energy from one ferromagnet to another by tunneling magnons through a bridging ferromagnetic wire that connects ferromagnets. In the framework of the model of Bose gas for the magnons, expressions are found for the power of the transferred magnon energy from a ferromagnet having a higher temperature to a colder ferromagnet. It is shown how the efficiency of tunneling transport depends on the parameters of exchange interactions in the “ferromagnet-wire-ferromagnet” system, the single-ion anisotropy of the structural units of the system, and the magnetic field. As an example, the role of single-ion anisotropy in the formation of resonant and non-resonant transmission of magnons between ferromagnets and adjacent wire terminal units is shown, which accordingly significantly enhances or weakens the tunneling flow of magnons from one ferromagnet to another.

In recent years, the microwave processes in artificial multiferroic media such as electromagnonic crystals have attracted increased research interest due to their potential applications for voltage-controlled spintronic devices. In contrast to the conventional magnonic crystals, the artificially created periodic structures are characterised by electrically and magnetically tunable band gaps in the wave spectrum where the propagation of the electromagnons is forbidden. In this study, an experimental realisation of an electromagnonic crystal based on a ferrite–ferroelectric–ferrite multilayer has been proposed. The authors have demonstrated for the first time a band-gap splitting, which arises from an interaction of three fundamental modes of two ferrite films separated by a ferroelectric layer. This splitting manifested itself as an additional stop-band appearance in the frequency response of the electromagnonic crystal. The obtained band structures are confirmed by numerical modelling using the coupled-mode approach and the transfer-matrix method. The authors expect that their results allow exploiting the electromagnonic crystal for enhanced logic control as well as for tunable microwave devices.

The experimental detection of the Bose-Einstein condensate of magnons in coupled nuclear-electron spin precession in antiferromagnets brings the prospect of its use for magnonics and computer calculations. In particular, an attractive feature of such systems is a relatively large spin coherence time compared to traditional iron yttrium garnet samples. However, the observed Bose-Einstein condensation of magnons contradicts the Suhl-Nakamura model and the Bloch equations, which are usually used for these systems. The results of a direct experiment in antiferromagnetic MnCO3 performed in this work indicate that the Suhl-Nakamura model and the Bloch equations cannot adequately describe the coupled nuclear-electron spin motion at large levels of magnon excitation.

Recent advances in the studies of magnon gases have opened new horizons for the investigation of room-temperature macroscopic coherent states and discovery of Bose–Einstein condensation of magnons. Although the phenomenon has been discovered almost 15 years ago, a lot of important issues connected with magnon Bose–Einstein condensation remain unclear. Here I review the recent experimental achievements in investigations of this phenomenon. I show that the magnon condensate possesses high degrees of temporal and spatial coherency, the latter leading to observation of interference of two condensate. Discovery of second sound in magnon condensate is also discussed. Finally, I demonstrate a practical way to realize magnon laser, which create a freely propagating cloud of magnons.

Magnetoresistance effects are used in a variety of devices including hard disk drives and magnetic random access memories. In particular, giant magnetoresistance and tunnelling magnetoresistance can be used to create spin valves and tunnel junctions in which the resistance depends on the relative magnetization orientations of two ferromagnetic conducting layers. Here, we report a magnetoresistance effect that occurs in a platinum layer deposited on a magnon junction consisting of two insulating magnetic yttrium iron garnet (YIG) layers separated by an antiferromagnetic nickel oxide spacer layer. The resistance of the platinum layer is found to depend on the magnetization of the YIG layer in direct contact with it (an effect known as spin Hall magnetoresistance), but also the magnetization of the adjacent YIG layer in the junction. The resistance of the platinum layer is higher when the two YIG layers are aligned antiparallel than when parallel. We assign this behaviour to a magnonic nonlocal spin Hall magnetoresistance in which spin-carrying magnon propagation across the junction affects spin accumulation at the metal interface and hence modulates the spin Hall magnetoresistance. The effect could be used to develop spintronic and magnonic devices that have spin transport properties controlled by an all-insulating magnon junction and are thus free from Joule heating. A magnetoresistance effect that occurs in a platinum layer deposited on a magnon junction consisting of two insulating magnetic yttrium iron garnet layers separated by an antiferromagnetic nickel oxide spacer layer could be used to create spintronic and magnonic devices that are free from Joule heating.

The particle size-dependent surface optical (SO) phonon and magnon modes of NiO nanoparticles were extensively studied using room-temperature confocal Raman spectroscopy. The required nanoparticles...

For the last few years, the research interest in magnetoelectric (ME) effect, which is the cross-coupling between ferroelectric and magnetic ordering in multiferroic materials, has experienced a significant revival. The extensive recent studies are not only conducted towards the design of sensors, actuators, transducers, and memory devices by taking advantage of the cross-control of polarization (or magnetization) by magnetic (or electric) fields, but also aim to create a clearer picture in understanding the sources of ME responses and the novel effects associated with them. Here we derive analytical models allowing to understand the striking and novel dynamics of ME effects in multiferroics and further confirm it with atomistic simulations. Specifically, the role of strain is revealed to lead to the existence of electroacoustic magnons, a new quasiparticle that mixes acoustic and optical phonons with magnons, which results in resonances and thus a dramatic enhancement of magnetoelectric responses. Moreover, a unique aspect of the dynamical quadratic ME response under a magnetic field with varying frequencies, which is the second harmonic generation (SHG), has not been discussed prior to the present work. These SHGs put emphasis on the fact that nonlinearities should be considered while dealing with such systems.

Nowadays, the interaction between phonon and magnon subsystems of a magnetic medium is a hot topic of research. The complexity of phonon and magnon spectra, the existence of both bulk and surface modes, the quantization effects, and the dependence of magnon properties on applied magnetic field, make this field very complex and intriguing. Moreover, the recent advances in the fields of spin caloritronics and magnon spintronics as well as the observation of the spin Seebeck effect in magnetic insulators points on the crucial role of magnons in spin-caloric transport processes. In this review, we collect the variety of different studies in which magnon-phonon interaction play important role. The scope of the paper covers the wide range of phenomena starting from the interaction of the coherent magnons with surface acoustic wave and finishing with the formation of magnon supercurrents in the thermal gradients.

In this work, we performed detailed first-principles calculation and theoretical analysis to investigate the effect of molecular adsorption on the spin-wave spectrum and magnon relaxation in a Cr2Ge2Te6 (CGT) monolayer. It is found that NH3, NO, and NO2 adsorption can enhance the exchange constant of CGT, which can result in a blue-shift in the spin-wave spectrum. At 30 K, by means of a thorough investigation of many possible lattice configurations excited by thermal fluctuation, we identify the magnon scattering rate from the intrinsic lattice vibrational modes, and find that the relaxation of optical and acoustic magnons exhibits a completely different wave vector dependence. Moreover, although the adsorption of NO2 and NH3 molecules has a negligible influence on the magnon–phonon interaction, the adsorption of NO molecules results in a significant increase in magnon scattering strength. In the long-wavelength limit, the interlayer vibrational modes induced by NO adsorption increase the magnon–phonon scattering strength by ∼12.7%. The remarkable interlayer magnon–phonon interaction is ascribed to the strong CGT–NO coupling and large molecular vibration amplitude. Considering the importance of magnon relaxation time in the application of spin devices, we suggest that both the impacts on the exchange interaction and scattering rate must be considered when manipulating two-dimensional magnets by surface functionalization.

Spin configuration inside a ferromagnetic metal influences its magnetoresistive behavior. The local spin curvature induces excess resistivity from the homogeneous ferromagnetic state. In this work, we characterize the spin curvature induced resistivity in epitaxial half-metallic CrO2 nanowires with 100% spin polarization. We control the magnitude of the spin curvature by introducing different geometric notches along the edge of the wire and applying an external magnetic field. Through magnetoresistance measurements and micromagnetic simulations, we uncover an empirical relationship between the spin curvature and the induced resistivity in this archetypal half-metallic solid. This relationship provides a quantitative method to calculate the resistance of magnetic domain walls or other spin textured states. We also study the influence of the thermal effect on the spin curvature induced resistivity across temperatures ranging from 10 K to 250 K. Thermal magnons worsen spin asymmetry considerably and suppress spin curvature induced resistivity at temperatures much lower than the ferromagnetic ordering temperature Tc.

The results of the experiments on an yttrium iron garnet film have been reported, demonstrating two types of long-lived free induction signals. The first type corresponds well to signals appearing because of the superfluid transfer of magnons, which was previously detected in antiferromagnetic superfluid 3He-B. The second, ultra-long-lived, signal also has a number of properties of coherent precession. However, it is fundamentally different from the ultra-long-lived signal in 3He-B. The mechanism of formation of the ultra-long-lived signal in yttrium iron garnet has not yet been theoretically explained.

The extension of magnon electron drag (MED) to the paramagnetic domain has recently shown that it can create a thermopower more significant than the classical diffusion thermopower resulting in a thermoelectric figure-of-merit greater than unity. Due to their distinct nature, ferromagnetic (FM) and antiferromagnetic (AFM) magnons interact differently with the carriers and generate different amounts of drag-thermopower. The question arises of whether the MED is stronger in FM or in AFM semiconductors. Two material systems, namely MnSb and CrSb, which are similar in many aspects except that the former is FM and the latter AFM, were studied in detail, and their MED properties were compared. Three features of AFMs compared to FMs, namely double degeneracy of the magnon modes, higher magnon group velocity, and longer magnon relaxation time, can lead to enhanced first-order MED thermopower. One effect, magnon–electron relaxation, leads to a higher second-order effect in AFMs that reduces the MED thermopower. However, it is generally expected that the first-order effect dominates and leads to a higher drag thermopower in AFMs, as seen in this case study.

The concept of quasi-particles forms the theoretical basis of our microscopic understanding of emergent phenomena associated with quantum mechanical many-body interactions. However, quasi-particle theory in disordered materials has proven difficult, resulting in the predominance of mean-field solutions. Here we report first-principles phonon calculations and inelastic x-ray and neutron scattering measurements on equiatomic alloys (NiCo, NiFe, AgPd, and NiFeCo) with force constant dominant disorder - confronting a key 50-year-old assumption in the Hamiltonian of all mean-field quasi-particle solutions for off-diagonal disorder. Our results have revealed the presence of a large, and heretofore unrecognized, impact of local chemical environments on the distribution of the species-pair-resolved force constant disorder that can dominate phonon scattering. This discovery not only identifies a critical analysis issue that has broad implications for other elementary excitations such as magnons and skyrmions in magnetic alloys, but also provides an important tool for the design of materials with ultra-low thermal conductivity., Comment: 25 pages, 6 figures

Using a simplified microscopic model of coupled spin and lattice excitations in a ferromagnetic insulator we evaluate the magnetic-field dependence of the spin Seebeck effect at low temperatures. The model includes Heisenberg exchange coupling, a harmonic lattice potential, and a pseudo-dipolar exchange interaction. Our approach goes beyond previous work [Phys. Rev. B 98, 134421 (2018)] in that it does not rely on the a priori assumption of a fast equilibration of the magnon and phonon distributions. Our theory shows that singular features in the magnetic-field dependence of the spin Seebeck effect at low temperatures observed by Kikkawa et al. [Phys. Rev. Lett. 117, 207203 (2016)] are independent of the relative strength of magnon-impurity and phonon-impurity scattering., 34 pages, 14 figures

Magnons are the elementary excitations of the magnetic order that carry spin, momentum, and energy. Here we compare the magnon with the ferron, i.e. the elementary excitation of the electric dipolar order that transports polarization and heat in ferroelectrics., Comment: Submitted for publication in the Virtual Special Issue (VSI) on Magnonics of the Journal of Magnetism and Magnetic Materials, in honor of Prof. Servio Rezende's 80th birthday

Magnetic resonance techniques at nuclei and electrons are applied to characterize the electronic structure and collective magnetic excitations throughout the magnetic phase diagrams of the lacunar spinels GaV4S8 and GaV4Se8 showing cycloidal, Néel-type skyrmion lattice and ferromagnetically polarized phases. 71Ga nuclear magnetic resonance (NMR) provides a local probe of the rhombohedral distortion and the resulting uniaxial magnetic anisotropy via the detection of electric field gradients (EFGs) and hyperfine coupling at the gallium sites of these lacunar spinels. Broadband electron spin resonance (ESR) allows identifying clockwise, counterclockwise, and breathing modes of the skyrmion-lattice phase supported by theoretical simulations.

The collective excitation of ordered spins, known as spin waves or magnons, can in principle radiate by emitting travelling photons to an open system when decaying to the ground state. However, in contrast to the electric dipoles, magnetic dipoles contributed by magnons are more isolated from electromagnetic environment with negligible radiation in the vacuum, limiting their application in coherent communication by photons. Recently, strong interaction between cavity standing-wave photons and magnons has been reported, indicating the possible manipulation of magnon radiation via tailoring photon states. Here, with loading an yttrium iron garnet sphere in a one-dimensional circular waveguide cavity in the presence of both travelling and standing photon modes, we demonstrate an efficient photon emissions from magnon and a significant magnon radiative damping with radiation rate found to be proportional to the local density of states (LDOS) of photon. By modulating the LDOS including its magnitude and/or polarization, we can flexibly tune the photon emission and magnon radiative damping on demand. Our findings provide a general way in manipulating photon emission from magnon radiation for harnessing energy and angular momentum generation, transfer and storage modulated by magnon in the cavity and waveguide electrodynamics., Comment: 9+7 pages, 4+2 figures

We analytically derive the dependence of the current of the spin-down minority electrons on the magnon at zero temperature for a tunnel junction composed of ferromagnetic and normal metals using th...

In this work, we introduce a computer model and theoretical approach based on the matching technique to investigate the spin precession and the magnetic properties of an ordered magnetic interface joining two ferromagnetic multilayers of type AB, made of 10 spin slabs, obtained by alternative two spin layers A and B. We simulate, particularly, the coherent magnon transmission through spins’ interface, in multilayered thin films, obtained by shearing a part of the film from the other at an angle of 30∘. The individual and total transmittance of bulk magnons of the thin film, scattering coherently at the shearing interface zone and the localized magnonic spin states, are calculated and analyzed. The transmission and reflection spin modes are derived as elements of a Landauer–Büttiker type scattering matrix. The results highlight the localized spin states on the interface shear domain and their interactions with incident magnons. The evolutions of the magnonic spectra can be presented for arbitrary directions of the incident magnons on the boundary zone, for all accessible frequencies in the propagating bands as well as for the magnetic exchange coupling between each spin A(B) and its adjacent sites and their spin intensity. The results demonstrate the dependence of the magnonic spectra for the perfect multilayered films and at the inhomogeneous domain of the interface shear. The analysis of the spectra illustrates the fluctuations, related to Fano resonances, due to the coupling between travelling magnons and the localized modes in the shear interface domain. The calculated spectra could yield useful information concerning the magnetic parameters of such interface slabs in multilayered films.

We theoretically investigate the magnetoelastic coupling (MEC) and its effect on magnon transport in two-dimensional antiferromagnets with a honeycomb lattice. MEC coefficients along with magnetic exchange parameters and spring constants are computed for monolayers of transition-metal trichalcogenides with N\'eel magnetic 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 shows that although Dzyaloshinskii-Moriya interaction can produce spin-Nernst effect in these materials, other mechanisms such as magnon-phonon coupling should be taken into account. We also demonstrate that the magnetic anisotropy is an important factor for control of magnon-phonon hybridization and enhancement of the Berry curvature and thus the spin-Nernst coefficient. Our results pave the way toward gate tunable spin current generation in two-dimensional magnets by spin-Nernst effect via electric field modulation of MEC and anisotropy.

Van der Waals magnets have emerged as a fertile ground for the exploration of highly tunable spin physics and spin-related technology. Two-dimensional (2D) magnons in van der Waals magnets are collective excitation of spins under strong confinement. Although considerable progress has been made in understanding 2D magnons, a crucial magnon device called the van der Waals magnon valve, in which the magnon signal can be completely and repeatedly turned on and off electrically, has yet to be realized. Here we demonstrate such magnon valves based on van der Waals antiferromagnetic insulator MnPS3. By applying DC electric current through the gate electrode, we show that the second harmonic thermal magnon (SHM) signal can be tuned from positive to negative. The guaranteed zero crossing during this tuning demonstrates a complete blocking of SHM transmission, arising from the nonlinear gate dependence of the non-equilibrium magnon density in the 2D spin channel. Using the switchable magnon valves we demonstrate a magnon-based inverter. These results illustrate the potential of van der Waals anti-ferromagnets for studying highly tunable spin-wave physics and for application in magnon-base circuitry in future information technology., 18 pages, 4 figures

In this work, we propose and study a one-dimensional model of the effects of optical phonon/magnon coupling. The indirect interactions between magnetic ions are conducted by the sandwiched nonmagnetic ions. The vibrations of nonmagnetic ions invoke effective magnetic fields to spins and hence couple with the magnon. The quantum solutions of the model show energy gaps in the spin-wave dispersion induced by the magnon-phonon coupling. Classical simulations reveal the existence of hybrid magnon-phonon quasiparticles and the coherent phonon (magnon) induced by magnon (phonon).

A nonreciprocal magnon laser is proposed in a compound cavity optomagnonical system consisting of an yttrium iron garnet sphere coupled to a spinning resonator. On the basis of the magnon-induced Brillouin scattering process making it possible to achieve a magnon lasing action, the Fizeau light-dragging effect caused by the spinning of the resonator further results in significant modifications in the magnon gain and the threshold power of magnon lasing for different driving directions, and then a nonreciprocal magnon laser is realized. Especially, this nonreciprocal magnon laser is highly tunable by the spinning speed and the driving direction. Our work provides an experimentally feasible pathway for manipulating spin-wave excitations and may find intriguing phenomena at the crossroad between spintronics of the magnet and nonreciprocal optics.

Description, Applying heat to a magnetoelectric antiferromagnet enables an electric field control of magnon spin currents., Pure spin currents can be generated via thermal excitations of magnons. These magnon spin currents serve as carriers of information in insulating materials, and controlling them using electrical means may enable energy efficient information processing. Here, we demonstrate electric field control of magnon spin currents in the antiferromagnetic insulator Cr2O3. We show that the thermally driven magnon spin currents reveal a spin-flop transition in thin-film Cr2O3. Crucially, this spin-flop can be turned on or off by applying an electric field across the thickness of the film. Using this tunability, we demonstrate electric field–induced switching of the polarization of magnon spin currents by varying only a gate voltage while at a fixed magnetic field. We propose a model considering an electric field–dependent spin-flop transition, arising from a change in sublattice magnetizations via a magnetoelectric coupling. These results provide a different approach toward controlling magnon spin current in antiferromagnets.

The explosive development of quantum magnonics is explained by its potential of use in quantum computers, processing information and the formation of hybrid quantum systems. The processes of spatial correlation of quantum systems are fundamental and lead to such phenomena as the Josephson effect and superconductivity. In particular, they determine the phenomenon of the magnon superfluidity and magnon Bose condensation which were first discovered in antiferromagnetic superfluid 3He. In this article, we consider the features of magnon Bose condensation in yttrium iron garnet film. We simulate the processes of magnon BEC coherency and magnon tunneling through the gap.

We report on optically induced, ultrafast magnetization dynamics in the Heusler alloy ${\mathrm{Co}}_{2}\mathrm{FeAl}$, probed by time-resolved magneto-optical Kerr effect. Experimental results are compared to results from electronic structure theory and atomistic spin-dynamics simulations. Experimentally, we find that the demagnetization time (${\ensuremath{\tau}}_{M}$) in films of ${\mathrm{Co}}_{2}\mathrm{FeAl}$ is almost independent of varying structural order, and that it is similar to that in elemental 3$d$ ferromagnets. In contrast, the slower process of magnetization recovery, specified by ${\ensuremath{\tau}}_{R}$, is found to occur on picosecond time scales, and is demonstrated to correlate strongly with the Gilbert damping parameter ($\ensuremath{\alpha}$). Based on these results we argue that for ${\mathrm{Co}}_{2}\mathrm{FeAl}$ the remagnetization process is dominated by magnon dynamics, something which might have general applicability.

Magnon transfer torque (MTT) is regarded as capable of manipulating spin by magnons or spin waves only without spatial movement of electrons, which is a key to enrich the toolbox for magnonics. Here using a magnon current through an antiferromagnetic NiO spacer, which can be deep into a magnetic insulator ${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ (YIG), we demonstrated the perpendicular magnetization of the YIG in a YIG/NiO/Pt heterostructure could be switched. When the thickness of the magnon channel NiO was 1.5 nm, the magnon current could still maintain about $84%$ of the spin current for the YIG/Pt control device, indicating high efficiency of the NiO spacer in transferring magnon torque. Switching the perpendicular magnetization in the YIG/NiO/Pt heterostructures unambiguously verified the effect of magnon transfer torque (MTT), which may advance the development of magnonic memories and logics.

A quantum network consisting of magnonic and mechanical nodes connected by light is proposed. Recent years have witnessed a significant development in cavity magnonics based on collective spin excitations in ferrimagnetic crystals, such as yttrium iron garnet (YIG). Magnonic systems are considered to be a promising building block for a future quantum network. However, a major limitation of the system is that the coherence time of the magnon excitations is limited by their intrinsic loss (typically in the order of 1 $\mu$s for YIG). Here, we show that by coupling the magnonic system to a mechanical system using optical pulses, an arbitrary magnonic state (either classical or quantum) can be transferred to and stored in a distant long-lived mechanical resonator. The fidelity depends on the pulse parameters and the transmission loss. We further show that the magnonic and mechanical nodes can be prepared in a macroscopic entangled state. These demonstrate the quantum state transfer and entanglement distribution in such a novel quantum network of magnonic and mechanical nodes. Our work shows the possibility to connect two separate fields of optomagnonics and optomechanics, and to build a long-distance quantum network based on magnonic and mechanical systems., Comment: PRX Quantum 2, 040344 (2021)