Magnon Modes Research Articles

Rectification of the spin Seebeck current in noncollinear antiferromagnets

In the absence of an external magnetic field and a spin-polarized charge current, an antiferromagnetic system supports two degenerate magnon modes. An applied thermal bias activates the magnetic dynamics, leading to a magnon flow from the hot to the cold edge (magnonic spin Seebeck current). Both degenerate bands contribute to the magnon current but the orientations of the magnetic moments underlying the magnons are opposite in different bands. Therefore, while the magnon current is nonzero, the net spin current is zero. To obtain a nonzero net spin current, one needs to apply either a magnetic field or a spin-polarized charge current that lifts the bands' degeneracy. Here, attaching a thermal contact to one edge of a helical nanowire, we study three different magnonic spin currents: (i) the exchange, and (ii) Dzyaloshinskii--Moriya spin currents flowing along the helical nanowire, and (iii) magnonic spin current pumped into the adjacent normal-metal layer. We find that the combination of Dzyaloshinskii--Moriya interaction and external magnetic field substantially enhances the spin current compared to the current generated solely through a magnetic field. Due to nonreciprocal magnons and magnon dichroism effect, the Dzyaloshinskii--Moriya and exchange spin currents show left-right propagation asymmetry, with $20%$ of current rectification. The spin pumping current shows a slight asymmetry only in the case of a strong Dzyaloshinskii--Moriya interaction. The observed effects are explained in terms of the magnon dispersion relations and the magnon Doppler effect.

Magnon squeezing enhanced ground-state cooling in cavity magnomechanics

Cavity magnomechanics has recently become a new platform for studying macroscopic quantum phenomena. The magnetostriction induced vibration mode of a large-size ferromagnet or ferrimagnet reaching its ground state represents a genuine macroscopic quantum state. Here we study the ground-state cooling of the mechanical vibration mode in a cavity magnomechanical system, and focus on the role of magnon squeezing in improving the cooling efficiency. The magnon squeezing is obtained by exploiting the magnon self-Kerr nonlinearity. We find that the magnon squeezing can significantly and even completely suppress the magnomechanical Stokes scattering. It thus becomes particularly useful in realizing ground-state cooling in the unresolved-sideband regime, where the conventional sideband cooling protocols become inefficient. We also find that the coupling to the microwave cavity plays only an adverse effect in mechanical cooling. This makes essentially the two-mode magnomechanical system (without involving the microwave cavity) a preferred system for cooling the mechanical motion, in which the magnon mode is established by a uniform bias magnetic field and a microwave drive field.

Steady-entangled-state generation via the cross-Kerr effect in a ferrimagnetic crystal

For solid-state spin systems, the collective spin motion in a single crystal embodies multiple magnetostatic modes. Recently, it was found that the cross-Kerr interaction between the higher-order magnetostatic mode and the Kittel mode introduces a new operable degree of freedom. In this work, we propose a scheme to entangle two magnon modes via the cross-Kerr nonlinearity when the bias field is inhomogeneous and the system is driven. Quantum entanglement persists at the steady state, as demonstrated by numerical results using experimentally feasible parameters. Furthermore, we also demonstrate that entangled states can survive better in the system where self-Kerr and cross-Kerr nonlinearities coexist. Our work provides insights and guidance for designing experiments to observe entanglement between different degrees of freedom within a single ferrimagnetic crystal. Additionally, it may stimulate potential applications in quantum information processing using spintronic devices.

Anisotropic magnon damping by zero-temperature quantum fluctuations in ferromagnetic CrGeTe3

Spin and lattice are two fundamental degrees of freedom in a solid, and their fluctuations about the equilibrium values in a magnetic ordered crystalline lattice form quasiparticles termed magnons (spin waves) and phonons (lattice waves), respectively. In most materials with strong spin-lattice coupling (SLC), the interaction of spin and lattice induces energy gaps in the spin wave dispersion at the nominal intersections of magnon and phonon modes. Here we use neutron scattering to show that in the two-dimensional (2D) van der Waals honeycomb lattice ferromagnetic CrGeTe3, spin waves propagating within the 2D plane exhibit an anomalous dispersion, damping, and breakdown of quasiparticle conservation, while magnons along the c axis behave as expected for a local moment ferromagnet. These results indicate the presence of dynamical SLC arising from the zero-temperature quantum fluctuations in CrGeTe3, suggesting that the observed in-plane spin waves are mixed spin and lattice quasiparticles fundamentally different from pure magnons and phonons.

Hybrid magnon-atom entanglement and magnon blockade via quantum interference

In this paper, we propose a quantum interference mediated control of switching between magnon-atom entanglement and magnon blockade. A three-level $\mathrm{\ensuremath{\Lambda}}$ configuration atomic ensemble and a yttrium iron garnet sphere are simultaneously coupled to a microwave cavity mode through magnetic dipole interaction. By applying two strong fields to drive the atoms, the resonant coupling between a dressed-state atomic transition and the magnon mode, without direct interaction, is established by adiabatically eliminating the cavity field. Interestingly, the anti-Jaynes-Cummings (JC) interaction and the JC interaction are alternatively formed when the atoms are trapped into different superposition states induced by quantum interference, causing the appearance of hybrid magnon-atom entanglement and magnon quantum blockade, respectively. The coherent-controlled hybrid magnon-atom entanglement and magnon blockade may find potential applications in quantum information processing.

Experimental observation of interlayer perpendicular standing spin wave mode with low damping in skyrmion-hosting [Pt/Co/Ta]10 multilayer

An interlayer perpendicular standing spin wave mode is observed in the skyrmion-hosting [Pt/Co/Ta]10 multilayer by measuring the time-resolved magneto-optical Kerr effect. The observed interlayer mode depends on the interlayer spin-pumping and spin transfer torque among the neighboring Co layers. This mode shows monotonically increasing frequency-field dependence which is similar to the ferromagnetic resonance mode, but within higher frequency range. Besides, the damping of the interlayer mode is found to be a relatively low constant value of 0.027 which is independent of the external field. This work expounds the potential application of the [heavy-metal/ferromagnetic-metal] n multilayers to skyrmion-based magnonic devices which can provide multiple magnon modes, relatively low damping, and skyrmion states, simultaneously.

Controlling quantum coherence and entanglement in cavity magnomechanical systems

We investigate theoretically macroscopic quantum coherence and tripartite entanglement in a three-mode magnomechanical system with a yttrium iron garnet sphere, which is simultaneously driven by a magnetic pump and a microwave field pump to establish an effective magnomechanical coupling. We discuss mainly the dependence of quantum properties on the driving power, the cavity field detuning and decay, the magnomechanical coupling strength, the bias magnetic field, and the temperature of an environment. It is found that the quantum coherence of the system can be attained easily in a large range of parameters. In contrast, tripartite photon-magnon-phonon entanglement can only be realized in a relatively small range of optical and magnetic parameters. In particular, the quantum coherence of the mechanical mode can be larger than that of the photon and magnon modes, which means that the environmental incoherence of the mechanical mode in the cavity magnomechanical system can be suppressed significantly. These results may help manipulate simultaneously the macroscopic quantum coherence and the tripartite entanglement of massive objects in coupled magnomechanical system, which may have potential applications for realizing highly tunable information processing through the flexible control of quantum properties.

Low-energy excitations and magnetic anisotropy of the layered van der Waals antiferromagnet Ni2P2S6

The quasi-two-dimensional antiferromagnet Ni$_2$P$_2$S$_6$ belongs to the family of magnetic van der Waals compounds that provides a rich material base for the realization of fundamental models of quantum magnetism in low dimensions. Here, we report high-frequency/high-magnetic field electron spin resonance measurements on single crystals of Ni$_2$P$_2$S$_6$. The results enable to reliably determine the positive, 'easy'-plane type of the single ion anisotropy of the Ni$^{2+}$ ions with the upper limit of its magnitude $\lesssim 0.7$ meV. The resonance response reveals strongly anisotropic spin fluctuations setting in shortly above the N\'eel temperature $T_{\rm N} = 158$ K and extending in the antiferromagnetically ordered state down to low temperatures. There, a new low-energy magnon excitation gapped from the ground state by only $\sim 1$ meV was found. This magnon mode may explain unusual low temperature relaxation processes observed in a latest nuclear magnetic resonance experiments and together with the estimate of the single ion anisotropy they should enable setting up a realistic spin model for an accurate description of magnetic properties of Ni$_2$P$_2$S$_6$.

Cavity-mediated level attraction and repulsion between magnons

We characterize some of the distinctive hallmarks of magnon-magnon interaction mediated by the intracavity field of a microwave cavity, along with their testable ramifications. In general, we foreground two widely dissimilar parameter domains that bring forth the contrasting possibilities of level splitting and level crossing. The former is observed in the regime of strong magnon-photon couplings, particularly when the three modes bear comparable relaxation rates. This character is marked by the appearance of three distinguishable and nonconverging polariton branches in the spectral response to a cavity drive. However, when the bare modes are resonant and the couplings are perfectly symmetrical, one of the spectral peaks gets wiped out. This anomalous extinction of polaritonic response can be traced down to the existence of a conspicuous dark mode alongside two frequency-shifted bright modes. In an alternate parameter regime, where the magnon modes are weakly coupled to the cavity, features of level attraction unfold, subject to a large relaxation rate for the cavity mode. Concurrently, for antisymmetric detunings to the magnon modes, a transmission window springs into existence, exhibiting transparency in the limit of negligible dissipation from the magnons. The emergence of level attraction can be reconciled with a theoretical model that embodies the dynamics of the magnon-magnon subsystem when the cavity field decays rapidly into its steady state. In this limit, we identify a purely dissipative coupling between the magnon modes.

All-magnonic Stern–Gerlach effect in antiferromagnets

The Stern–Gerlach (SG) effect is well known as the spin-dependent splitting of a beam of atoms carrying magnetic moments by a magnetic-field gradient, leading to the concept of electron spin. Antiferromagnets can accommodate two magnon modes with opposite spin polarizations, which is equivalent to the spin property of electrons. Here, we propose an all-magnonic SG effect in an antiferromagnetic magnonic system, where a linearly polarized spin-wave beam is deflected by a straight Dzyaloshinskii–Moriya interaction (DMI) interface into two opposite polarized spin-wave beams propagating in two discrete directions. Moreover, we observe bi-focusing of antiferromagnetic spin waves induced by a curved DMI interface, which can also spatially separate thermal magnons with opposite polarizations. Our findings provide a unique perspective to understand the rich phenomena associated with antiferromagnetic magnon spin and would be helpful for polarization-dependent application of antiferromagnetic spintronic devices.

Enhanced entanglement and quantum steering of directly and indirectly coupled modes in a magnomechanical system

This study is based on a theoretical proposal for quantum steering, non-locality, and entanglement in a two microwave cavities based magnomechanical system. The system is composed of two microwave (MW) cavity mode photons, phonons associated with mechanical mode and a magnon mode. We have employed a standard damping approach and found that steerability and entanglements among different bipartite subsystems strongly depend upon the detuning of indirectly coupled cavity mode and effective magnon detuning. One/two-way quantum Steering and non-local correlation have been found in the system for different choices of bipartitions. In addition, entanglement generated in the system enhances and has shown to be more robust against the thermal effects, as compared with the earlier studies. Interestingly, we have shown that the magnomechanical entanglements among different bipartitions have a complementary relation due to the transfer entanglement. This shows an additional insight into the system dynamics which shows that there exists a ‘tie-up’ relationship among these bipartitions. The present magnomechanical system will open new perspectives in quantum tasks that demand the entanglement of microwave cavity fields.

Magnon drag in a metal–insulating antiferromagnet bilayer

We study a bilayer structure consisting of an antiferromagnetic insulator and a normal metal. An electron current is driven in the normal metal with direction parallel to the interface between the materials. Due to interfacial exchange coupling between the localized spins in the antiferromagnet and the itinerant electrons in the normal metal, a magnon current can then be induced in the antiferromagnet. Using an uncompensated antiferromagnetic interface, creating an asymmetry in the interfacial coupling to the two degenerate magnon modes, we find that it is possible to generate a magnon spin current. The magnon spin current can be enhanced by increasing the temperature or by spin-splitting the magnon modes.

Accelerated adiabatic passage in cavity magnomechanics

Cavity magnomechanics provides a readily controllable hybrid system, consisting of a cavity mode, magnon mode, and phonon mode, for quantum state manipulation and transfer. To implement a fast-and-robust state transfer between the hybrid photon-magnon mode and the phonon mode, we apply two accelerated-adiabatic-passage protocols based on the counterdiabatic Hamiltonian for transitionless quantum driving and the Levis-Riesenfeld invariant for inverse engineering. We construct the counterdiabatic Hamiltonian in a state-resolved way and express it through the creation and annihilation operators rather than the system eigenstates and their time derivatives. We can optimize the invariant-based inverse-engineering protocol with respect to the stability against the systematic errors of the coupling strength and frequency detuning. Both protocols apply for continuous-variable systems with arbitrary target states. We also discuss the decoherence effects from the thermal environment and the counterrotating interactions on both protocols. Our work contributes to the quantum memory for photonic and magnonic quantum information.

PT -symmetric magnon laser in cavity optomagnonics

Nonlinear magnonics based on the coupling of magnons and optical photons has become a crucial topic in the quantum magnonic field. Here, we demonstrate the magnon laser, the analog of the optical laser in magnonics, in a parity-time ($\mathcal{PT}$) symmetric cavity optomagnonical system involving active and passive optical whispering gallery modes and the magnon mode. We show that the stimulated emitted magnon number can be coherently amplified above a threshold driving power in the $\mathcal{PT}$-symmetric regime where the splitting of optical supermodes is resonant with the magnon frequency, indicating the occurrence of the magnon laser. And this magnon laser action can work with low threshold when the system approaches the optical gain-loss balance and when increasing the optomagnonic coupling. Furthermore, the frequency of the magnon laser can be continuously tuned by adjusting the applied magnetic field strength, which is a distinct feature compared to the photon or phonon amplification by stimulated emission. The study of the $\mathcal{PT}$-symmetric magnon laser may promote the intersection and merging of various disciplines like non-Hermitian physics and quantum magnonics, and provide the theoretical basis and reference for the research and development of new frequency-tunable laser devices.

Analog spacetimes from nonrelativistic Goldstone modes in spinor condensates

It is well established that linear dispersive modes in a flowing quantum fluid behave as though they are coupled to an Einstein-Hilbert metric and exhibit a host of phenomena coming from quantum field theory in curved space, including Hawking radiation. We extend this analogy to any nonrelativistic Goldstone mode in a flowing spinor Bose-Einstein condensate. In addition to showing the linear dispersive result for all such modes, we show that the quadratically dispersive modes couple to a special nonrelativistic spacetime called a Newton-Cartan geometry. The kind of spacetime (Einstein-Hilbert or Newton-Cartan) is intimately linked to the mean-field phase of the condensate. To illustrate the general result, we further provide the specific theory in the context of a pseudo-spin-1/2 condensate where we can tune between relativistic and nonrelativistic geometries. We uncover the fate of Hawking radiation upon such a transition: it vanishes and remains absent in the Newton-Cartan geometry despite the fact that any fluid flow creates a horizon for certain wave numbers. Finally, we use the coupling to different spacetimes to compute and relate various energy and momentum currents in these analog systems. While this result is general, present day experiments can realize these different spacetimes including the magnon modes for spin-1 condensates such as $^{87}$Rb, $^{7}$Li, $^{41}$K (Newton-Cartan), and $^{23}$Na (Einstein-Hilbert).

Theory for the spin caloritronic nano-oscillator

Auto-oscillations of the magnetization in ferromagnetic hybrid nanostructures driven by spin currents produced by a variety of processes have attracted attention for their challenging physics and possible applications, such as microwave assisted magnetic recording, neuromorphic computing, and chip to chip wireless communications. Of particular interest are applications in which the spin current is produced by a thermal gradient in the configuration of the spin Seebeck effect, because it makes it possible to harvest the thermal energy generated in nanodevices. A few years ago, it was demonstrated experimentally that in a simple bilayer made of a thin film of the insulating ferrimagnet yttrium iron garnet in contact with a platinum layer, the application of a temperature difference across the bilayer produced a coherent microwave auto-oscillation. This device was called a spin caloritronic nano-oscillator. Here we show that these experiments are explained quantitatively by a theory based on a mechanism in which one magnon in the spin current splits into two magnons, one of them being the magnon mode resonating at the nanostructure. The theoretical value of the critical temperature gradient necessary to overcome the magnetic damping to produce auto-oscillations is in good agreement with the one employed in the experiments.

Anisotropic spin-wave excitations in multiferroic BiFeO3

Polarized inelastic neutron-scattering experiments have been performed to elucidate the anisotropic behavior of the low-energy spin-wave excitations in a multiferroic ${\mathrm{BiFeO}}_{3}$, which shows a cycloidal spin structure below 640 K. Using neutron polarization analysis for single magnetic domain crystals, magnetic excitation modes in and out of the cycloidal plane below 6 meV were separated successfully. The magnetic excitation spectra were analyzed using linear spin-wave theory. The low-energy magnon density of states consist of several magnon modes, including the two anisotropic modes, $\mathrm{\ensuremath{\Phi}}$ and $\mathrm{\ensuremath{\Psi}}$ modes, distributed in and out of the cycloidal plane, respectively, which were previously observed using optical spectroscopies. Furthermore, there are other magnon modes that are not active in optical measurements. A model spin Hamiltonian, which reproduces the spin-wave frequencies observed using optical spectroscopies, explains the overall spectra reasonably well.

Spin stiffness, spectral weight, and Landau damping of magnons in metallic spiral magnets

We analyze the properties of magnons in metallic electron systems with spiral magnetic order. Our analysis is based on the random phase approximation for the susceptibilities of tight binding electrons with a local Hubbard interaction in two or three dimensions. We identify three magnon branches from poles in the susceptibilities, one associated with in-plane, the other two associated with out-of-plane fluctuations of the spiral order parameter. We derive general expressions for the spin stiffnesses and the spectral weights of the magnon modes, from which also the magnon velocities can be obtained. Moreover, we determine the size of the decay rates of the magnons due to Landau damping. While the decay rate of the in-plane mode is of the order of its excitation energy, the decay rate of the out-of-plane mode is smaller so that these modes are asymptotically stable excitations even in the presence of Landau damping.

Frustration model and spin excitations in the helimagnet FeP

The metallic compound FeP belongs to the class of materials that feature a complex noncollinear spin order driven by magnetic frustration. While its double-helix magnetic structure with a period $\lambda_{\text{s}} \approx 5c$, where $c$ is the lattice constant, was previously well determined, the relevant spin-spin interactions that lead to that ground state remain unknown. By performing extensive inelastic neutron scattering measurements, we obtained the spin-excitation spectra in a large part of the momentum-energy space. The spectra show that the magnons are gapped with a gap energy of $\sim$5 meV. Despite the 3D crystal structure, the magnon modes display strongly anisotropic dispersions, revealing a quasi-one-dimensional character of the magnetic interactions in FeP. The physics of the material, however, is not determined by the dominating exchange, which is ferromagnetic. Instead, the weaker two-dimensional antiferromagnetic interactions between the rigid ferromagnetic spin chains drive the magnetic frustration. Using linear spin-wave theory, we were able to construct an effective Heisenberg Hamiltonian with an anisotropy term capable of reproducing the observed spectra. This enabled us to quantify the exchange interactions in FeP and determine the mechanism of its magnetic frustration.

Hybrid Magnonics for Short-Wavelength Spin Waves Facilitated by a Magnetic Heterostructure

Recent research on hybrid magnonics has been restricted by the long magnon wavelengths of the ferromagnetic resonance modes. We present an experiment on the hybridization of 250-nm-wavelength magnons with microwave photons in a multimode magnonic system consisting of a planar cavity and a magnetic bilayer. The coupling between magnon modes in the two magnetic layers, i.e., the uniform mode in Permalloy and the perpendicular standing spin waves (PSSWs) in yttrium iron garnet, serves as an effective means for exciting short-wavelength PSSWs, which is further hybridized with the photon mode of the microwave resonator. The demonstrated magnon-photon coupling approaches the superstrong coupling regime, and can even be achieved near zero bias field.