Explore the vast range of titles available.

Reservoir Computing Wataru Namiki, Daiki Nishioka, Takashi Tsuchiya, Kazuya Terabe, and co‐workers experimentally demonstrate high‐performance reservoir computing on the basis of chaotic interference of spin waves in an yttrium iron garnet single crystal (see article number 2300228). The subject computing system achieves excellent performance when used for hand‐written digit recognition, second‐order nonlinear dynamical tasks, and nonlinear autoregressive moving average (NARMA).

We investigate the effect of magnon–magnon interactions on the dispersion and polarization of magnon modes in collinear antiferromagnetic (AF) insulators at finite temperatures. In two-sublattice AF systems with uniaxial easy-axis and biaxial easy-plane magneto-crystalline anisotropies, we implement a self-consistent Hartree–Fock mean-field approximation to explore the nonlinear thermal interactions. The resulting nonlinear magnon interactions separate into two-magnon intra- and interband scattering processes. Furthermore, we compute the temperature dependence of the magnon bandgap and AF resonance modes due to nonlinear magnon interactions for square and hexagonal lattices. In addition, we study the effect of magnon interactions on the polarization of magnon modes. We find that although the noninteracting eigenmodes in the uniaxial easy-axis case are circularly polarized, but in the presence of nonlinear thermal interactions the U(1) symmetry of the magnon Hamiltonian is broken. The attractive nonlinear interactions squeeze the low energy magnon modes and make them elliptical. In the biaxial easy-plane case, on the other hand, the bare eigenmodes of low energy magnons are elliptically polarized but thermal nonlinear interactions squeeze them further. Direct measurements of the predicted temperature-dependent AF resonance modes and their polarization can be used as a tool to probe the nonlinear interactions. Our findings establish a framework for exploring the effect of thermal magnon interactions in technologically important magnetic systems, such as magnetic stability of recently discovered two-dimensional magnetic materials, coherent transport of magnons, Bose–Einstein condensation of magnons, and magnonic topological insulators.

Rigidity of an ordered phase in condensed matter results in collective excitation modes spatially extending to macroscopic dimensions. A magnon is a quantum of such collective excitation modes in ordered spin systems. Here, we demonstrate the coherent coupling between a single-magnon excitation in a millimeter-sized ferromagnetic sphere and a superconducting qubit, with the interaction mediated by the virtual photon excitation in a microwave cavity. We obtain the coupling strength far exceeding the damping rates, thus bringing the hybrid system into the strong coupling regime. Furthermore, we use a parametric drive to realize a tunable magnon-qubit coupling scheme. Our approach provides a versatile tool for quantum control and measurement of the magnon excitations and may lead to advances in quantum information processing.

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 electron-phonon interaction. Unlike higher order magnon-only scattering, we find that the coupling with phonons induce a scattering which is very small for low-energy (or subthermal) magnons but increases sharply above a certain energy-rendering magnons above this energy poor spin-current transporters.

Ultrafast laser excitation of ferromagnetic metals gives rise to correlated, highly non‐equilibrium dynamics of electrons, spins and lattice, which are, however, poorly described by the widely‐used three‐temperature model. Here, a fully ab initio parameterized out‐of‐equilibrium theory based on a quantum kinetic approach – termed (N+2) temperature model – is developed to describe magnon occupation dynamics due to electron‐magnon scattering. This model is applied to perform quantitative simulations on the ultrafast, laser‐induced generation of magnons in iron and demonstrates that on these timescales the magnon distribution is non‐thermal: predominantly high‐energy magnons are created, while the magnon occupation close to the center of the Brillouin zone even decreases, due to a repopulation toward higher energy states via a so‐far‐overlooked scattering term. It is demonstrated that the simple relation between magnetization and temperature computed at equilibrium does not hold in the ultrafast regime. The ensuing Gilbert damping, furthermore, becomes strongly magnon wavevector dependent and requires a description beyond the conventional Landau‐Lifshitz‐Gilbert spin dynamics. The ab initio parameterized calculations show that ultrafast generation of non‐thermal magnons provides a sizable demagnetization within 200 fs in excellent comparison with experimentally observed laser‐induced demagnetizations. This investigation thus emphasizes the importance of non‐thermal magnon excitations for the ultrafast demagnetization process. A fully ab initio parametrized model is developed to describe out‐of‐equilibrium magnon dynamics in ferromagnetic metals. Applying this model to ultrafast laser‐induced demagnetization, it is revealed that the generation of non‐thermal magnons provides a sizable demagnetization within 200 fs in excellent comparison with experiments, which establishes the importance of non‐thermal magnon excitations for the ultrafast demagnetization process.

Spin wave has attracted significant attention in various fields because of its rich physics and potential applications in the development of spintronics devices in the post-Moore era. However, the analog of a subluminal-like propagation in the field of spin waves has not been well discussed. Here, we theoretically demonstrate the ultra-slow spin waves propagation in a nanoscale two-dimensional ferromagnetic film in the presence of magnon-skyrmion interaction. The minimum spin waves propagation velocity was estimated to be as low as 1.8 m s −1 by adjusting the system parameters properly, and the spin waves group delay and advance are dynamically tunable via the intensity or detuning of the control field, which allows the possibility of observing superluminal- and subluminal-like spin waves propagation in a single experimental setup. These results deepen our understanding of the spin wave–skyrmion interactions, open a novel and efficient pathway to realize ultra-slow spin waves propagation, and are expected to be applied to magnetic information storage and quantum operations of magnons.

The tunability of magnons enables their interaction with various other quantum excitations, including photons, paving the route for novel hybrid quantum systems. Here, we study magnon-photon coupling using a high-quality factor split-ring resonator and single-crystal yttrium iron garnet (YIG) sphere at room temperature. We investigate the dependence of the coupling strength on the size of the sphere and find that the coupling is stronger for spheres with a larger diameter as predicted by theory. Furthermore, we demonstrate strong magnon-photon coupling by varying the position of the YIG sphere within the resonator. Our experimental results reveal the expected correlation between the coupling strength and the rf magnetic field. These findings demonstrate the control of coherent magnon-photon coupling through the theoretically predicted square-root dependence on the spin density in the ferromagnetic medium and the magnetic dipolar interaction in a planar resonator.

The rich and unconventional physics in layered 2D magnets can open new avenues for topological magnonics and magnon valleytronics. In particular, two-dimensional (2D) bilayer quantum magnets are gaining increasing attention due to their intriguing stacking-dependent magnetism, controllable ground states, and topological excitations induced by magnetic spin–orbit couplings (SOCs). Despite the substantial research on these materials, their topological features remain widely unexplored to date. The present study comprehensively investigates the magnon topology and magnon valley-polarization in honeycomb bilayers with collinear magnetic order. We elucidate the separate and combined effects of the SOC, magnetic ground-states, stacking order, and inversion symmetry breaking on the topological phases, magnon valley transport, and the Hall and Nernst effects. The comprehensive analysis suggests clues to determine the SOC’s nature and predicts unconventional Hall and Nernst conductivities in topologically trivial phases. We further report on novel bandgap closures in layered antiferromagnets and detail their topological implications. We believe the present study provides important insights into the fundamental physics and technological potentials of topological 2D magnons.

Since longitudinal spin–spin interaction is ubiquitous in magnetic materials, it is very interesting to explore the interplay between topology and longitudinal spin–spin interaction. Here, we examine the role of longitudinal spin–spin interaction on topological magnon excitations. Remarkably, even for single-magnon excitations, we discover topological edge states and defect edge states of magnon excitations in a dimerized Heisenberg XXZ chain and their topological properties can be distinguished via adiabatic quantum transport. We uncover topological phase transitions induced by longitudinal spin–spin interactions whose boundary is analytically obtained via the transfer matrix method. For multi-magnon excitations, even-magnon bound states are found to be always topologically trivial, but odd-magnon bound states may be topologically nontrivial due to the interplay between the transverse dimerization and the longitudinal spin–spin interaction. For two-dimensional spin systems, the longitudinal spin–spin interaction contributes to the coexistence of defect corner states, second-order topological corner states and first-order topological edge states. We propose an experimental scheme to realize and measure the magnon boundary states in superconducting qubits. Our work opens an avenue for exploring topological magnon excitations and has potential applications in topological magnon devices.

Recently, the photon-magnon coherent interaction based on the collective spins excitation in ferromagnetic materials has been achieved experimentally. Under the prospect, the magnons are proposed to store and process quantum information. Meanwhile, cavity-optomagnonics which describes the interaction between photons and magnons has been developing rapidly as an interesting topic of the cavity quantum electrodynamics. Here in this short review, we mainly introduce the recent theoretical and experimental progress in the field of optomagnetic coupling and optical manipulation based on cavity-optomagnonics. According to the frequency range of the electromagnetic field, cavity optomagnonics can be divided into microwave cavity optomagnonics and optical cavity optomagnonics, due to the different dynamics of the photon-magnon interaction. As the interaction between the electromagnetic field and the magnetic materials is enhanced in the cavity-optomagnonic system, it provides great significance to explore the nonlinear characteristics and quantum properties for different magnetic systems. More importantly, the electromagnetic response of optomagnonics covers the frequency range from gigahertz to terahertz which provides a broad frequency platform for the multi-mode controlling in quantum systems.