Amplification Of Spin Waves Research Articles

Antiferromagnetic Spin Wave Amplification by Scattering in the Presence of Non-Uniform Dzyaloshinskii-Moriya Interaction.

In this study, we suggest a method to amplify spin waves (SWs) in antiferromagnets (AFMs). By introducing a non-uniform Dzyaloshinskii-Moriya (DM) interaction, the potential barrier forms a resonant cavity. SWs with an opposite chirality undergo scattering and are resonantly amplified at a phase-matching condition. The calculation is performed in the insulating AFMs where the electric-field-induced DM interaction and pseudo-dipole anisotropy broaden the parabolic-like SW band for multiple resonant modes. Using a transfer matrix method, we also show numerically that scattering between SWs contributes significantly to the SW amplification. Since the electric field selectively amplifies the SWs with resonant frequencies, the proposed device works as an SW transistor and rectifier. This finding will contribute to insulating AFM-based magnon devices where Joule heating is, in principle, avoided.

True amplification of spin waves in magnonic nano-waveguides

Magnonic nano-devices exploit magnons - quanta of spin waves - to transmit and process information within a single integrated platform that has the potential to outperform traditional semiconductor-based electronics. The main missing cornerstone of this information nanotechnology is an efficient scheme for the amplification of propagating spin waves. The recent discovery of spin-orbit torque provided an elegant mechanism for propagation losses compensation. While partial compensation of the spin-wave losses has been achieved, true amplification – the exponential increase in the spin-wave intensity during propagation – has so far remained elusive. Here we evidence the operating conditions to achieve unambiguous amplification using clocked nanoseconds-long spin-orbit torque pulses in magnonic nano-waveguides, where the effective magnetization has been engineered to be close to zero to suppress the detrimental magnon scattering. We achieve an exponential increase in the intensity of propagating spin waves up to 500% at a propagation distance of several micrometers.

Electric chiral magnonic resonators utilizing spin–orbit torques

The recently proposed concept of electric chiral magnonic resonator (ECMR) has been extended to include usage of spin–orbit torques (SOT). Unlike the original version of ECMR which was based on voltage controlled magnetic anisotropy (VCMA), the spin wave amplification power by this new version of ECMR (pumped by SOT) no longer depends on the phase of the incident wave, which is highly desirable from an application point of view. The performance of the SOT pumped ECMR has been compared with the case of amplification by applying SOT pumping directly to a waveguide (without any ECMR involved). It is argued that at the expense of narrowing the bandwidth (i.e., slower amplifier response), the advantage of the former configuration (amplification by a SOT pumped ECMR) over the latter (amplification by direct SOT pumping the waveguide) is to offer gain, while at the same time, maintaining system stability (avoidance of auto-oscillations). Non-linear behavior of the SOT pumped ECMR has been analyzed. It is demonstrated that by cascading a SOT ECMR operating in an off-resonance mode together with a VCMA biased passive ECMR, it is possible to produce a magnonic neuron with a transmitted signal magnitude larger than the input in the firing state.

Stimulated Amplification of Propagating Spin Waves.

Spin-wave amplification techniques are key to the realization of magnon-based computing concepts. We introduce a novel mechanism to amplify spin waves in magnonic nanostructures. Using the technique of rapid cooling, we create a nonequilibrium state in excess of high-energy magnons and demonstrate the stimulated amplification of an externally seeded, propagating spin wave. Using an extended kinetic model, we qualitatively show that the amplification is mediated by an effective energy flux of high energy magnons into the low energy propagating mode, driven by a nonequilibrium magnon distribution.

Nonresonant amplification of spin waves through interface magnetoelectric effect and spin-transfer torque

We present a new mechanism for manipulation of the spin-wave amplitude through the use of the dynamic charge-mediated magnetoelectric effect in ultrathin multilayers composed of dielectric thin-film capacitors separated by a ferromagnetic bilayer. Propagating spin waves can be amplified and attenuated with rising and decreasing slopes of the oscillating voltage, respectively, locally applied to the sample. The way the spin accumulation is generated makes the interaction of the spin-transfer torque with the magnetization dynamics mode-selective and restricted to some range of spin-wave frequencies, which is contrary to known types of the spin-transfer torque effects. The interfacial nature of spin-dependent screening allows to reduce the thickness of the fixed magnetization layer to a few nanometers, thus the proposed effect significantly contributes toward realization of the magnonic devices and also miniaturization of the spintronic devices.

Spin–orbit-torque magnonics

The field of magnonics, which utilizes propagating spin waves for nanoscale transmission and processing of information, has been significantly advanced by the advent of the spin–orbit torque. The latter phenomenon allows one to overcome two main drawbacks of magnonic devices—low energy efficiency of the conversion of electrical signals into spin-wave signals and fast spatial decay of spin waves in thin-film waveguiding structures. At first glance, the excitation and amplification of spin waves by spin–orbit torques seem to be straightforward. Recent research indicates, however, that the lack of the mode selectivity in the interaction of spin currents with dynamic magnetic modes and the onset of dynamic nonlinear phenomena represent significant obstacles. Here, we discuss the possible route to overcoming these limitations, based on the suppression of nonlinear spin-wave interactions in magnetic systems with perpendicular magnetic anisotropy. We show that this approach enables efficient excitation of coherent magnetization dynamics and propagating spin waves in extended spatial regions and is expected to enable practical implementation of complete compensation of spin-wave propagation losses.

Spin-Wave Amplification and Lasing Driven by Inhomogeneous Spin-Transfer Torques.

We show that an inhomogeneity in the spin-transfer torques in a metallic ferromagnet under suitable conditions strongly amplifies incoming spin waves. Moreover, at nonzero temperatures the incoming thermally occupied spin waves will be amplified such that the region with inhomogeneous spin-transfer torques emits spin waves spontaneously, thus constituting a spin-wave laser. We determine the spin-wave scattering amplitudes for a simplified model and setup, and show under which conditions the amplification and lasing occurs. Our results are interpreted in terms of a so-called black-hole laser, and could facilitate the field of magnonics, which aims to utilize spin waves in logic and data-processing devices.

Excitation and Amplification of Spin Waves by Spin-Orbit Torque.

The emerging field of nanomagnonics utilizes high-frequency waves of magnetization-spin waves-for the transmission and processing of information on the nanoscale. The advent of spin-transfer torque has spurred significant advances in nanomagnonics, by enabling highly efficient local spin wave generation in magnonic nanodevices. Furthermore, the recent emergence of spin-orbitronics, which utilizes spin-orbit interaction as the source of spin torque, has provided a unique ability to exert spin torque over spatially extended areas of magnonic structures, enabling enhanced spin wave transmission. Here, it is experimentally demonstrated that these advances can be efficiently combined. The same spin-orbit torque mechanism is utilized for the generation of propagating spin waves, and for the long-range enhancement of their propagation, in a single integrated nanomagnonic device. The demonstrated system exhibits a controllable directional asymmetry of spin wave emission, which is highly beneficial for applications in nonreciprocal magnonic logic and neuromorphic computing.

Amplification and stabilization of large-amplitude propagating spin waves by parametric pumping

The interaction of a localized parametric pumping with spin waves of different amplitudes, propagating in a ferromagnetic nanowire, is studied analytically and by micromagnetic simulations. It is shown that parametric amplification of spin waves by localized pumping becomes less efficient with an increase in the spin wave amplitude due to the influence of nonlinear 4-magnon processes. In a certain range of spin wave amplitudes, the parametric amplifier acts as a stabilizer of the spin wave amplitude, as its action significantly reduces the spread of the spin wave amplitude in the vicinity of a certain mean value. The stabilization effect becomes more pronounced for higher pumping strength and larger relative lengths of the pumping localization region, compared to the spin wave mean free path. In contrast, the use of relatively short pumping localization regions allows one to efficiently amplify large-amplitude nonlinear spin waves.

Realization of a Spin-Wave Switch Based on the Spin-Transfer-Torque Effect

We investigate the amplification of externally excited spin waves via the Spin-Transfer-Torque (STT) effect in combination with the Spin-Hall-Effect (SHE) employing short current pulses. The results reveal that, in the case of an overcompensation of the spin wave damping, a strong nonlinear shift of the spin wave frequency spectrum occurs. In particular, this shift affects the spin wave amplification using the SHE-STT effect. In contrast, this effect allows for the realization of a spin wave switch. By determining the corresponding working point, an efficient spin wave excitation is only possible in the presence of the SHE-STT effect yielding an increased spin wave intensity of a factor of 20 compared to the absence of the SHE-STT effect.

High-efficiency control of spin-wave propagation in ultra-thin yttrium iron garnet by the spin-orbit torque

We study experimentally with submicrometer spatial resolution the propagation of spin waves in microscopic waveguides based on the nanometer-thick yttrium iron garnet and Pt layers. We demonstrate that by using the spin-orbit torque, the propagation length of the spin waves in such systems can be increased by nearly a factor of 10, which corresponds to the increase in the spin-wave intensity at the output of a 10 μm long transmission line by three orders of magnitude. We also show that, in the regime, where the magnetic damping is completely compensated by the spin-orbit torque, the spin-wave amplification is suppressed by the nonlinear scattering of the coherent spin waves from current-induced excitations.

Parametric Amplification of Spin Waves Using Acoustic Waves

Spin waves are a promising prospect for the development of microwave information processing devices. A major obstacle in the study of spin waves is the large intrinsic spin-wave damping in most known magnetic materials. Viable spin-wave amplification techniques are thus required to make further progress in the development of spin-wave-based logic devices. In this paper, we investigate a novel technique of parametric modulation of spin waves using bulk acoustic waves. Although it is not a conclusive proof of parametric amplification, we show that bulk acoustic waves at twice the frequency of the propagating spin waves on coupling into an yttrium iron garnet film can modulate the amplitude of the spin wave. This paper provides insight into magnon-phonon interactions, and suggests a new mechanism to compensate for inherent losses in spin-wave propagation in spin-wave-based logic devices.

Influence of interfacial Dzyaloshinskii-Moriya interaction on the parametric amplification of spin waves

The influence of the interfacial Dzyaloshinskii-Moriya interaction (IDMI) on the parametric amplification of spin waves propagating in ultrathin ferromagnetic film is considered theoretically. It is shown that the IDMI changes the relation between the group velocities of the signal and idler spin waves in a parametric amplifier, which may result in the complete vanishing of the reversed idler wave. In the optimized case, the idler spin wave does not propagate from the pumping region at all, which increases the efficiency of the amplification of the signal wave and suppresses the spurious impact of the idler waves on neighboring spin-wave processing devices.

Non-volatile Clocked Spin Wave Interconnect for Beyond-CMOS Nanomagnet Pipelines.

The possibility of using spin waves for information transmission and processing has been an area of active research due to the unique ability to manipulate the amplitude and phase of the spin waves for building complex logic circuits with less physical resources and low power consumption. Previous proposals on spin wave logic circuits have suggested the idea of utilizing the magneto-electric effect for spin wave amplification and amplitude- or phase-dependent switching of magneto-electric cells. Here, we propose a comprehensive scheme for building a clocked non-volatile spin wave device by introducing a charge-to-spin converter that translates information from electrical domain to spin domain, magneto-electric spin wave repeaters that operate in three different regimes - spin wave transmitter, non-volatile memory and spin wave detector, and a novel clocking scheme that ensures sequential transmission of information and non-reciprocity. The proposed device satisfies the five essential requirements for logic application: nonlinearity, amplification, concatenability, feedback prevention, and complete set of Boolean operations.

Parallel parametric amplification of coherently excited propagating spin waves in a microscopic Ni81Fe19 waveguide

We present parallel parametric amplification of coherently excited, propagating spin waves in a microstructured magnonic Ni81Fe19 waveguide. Amplification is achieved by the pumping field generated by a microwave current flowing through a Cu micro-stripline underneath the waveguide. By employing microfocussed Brillouin light scattering spectroscopy, we investigate the spatial decay of the propagating spin waves and their amplification by means of parallel pumping. We analyze the dependence of the intensity of the amplified spin waves on the spin-wave excitation power, pumping power, and pumping duration, revealing the most efficient working point for a noise-free amplification. This paves the way for a frequency selective amplification of spin waves in microstructured magnonic circuits.

Amplification of spin waves by the spin Seebeck effect

We observe amplification of spin-wave packets propagating along a film of single-crystal yttrium iron garnet (YIG) subject to a transverse temperature gradient. The spin waves are excited and detected with standard techniques used to study volume or surface magnetostatic waves in the 1–2 GHz frequency range. Amplification gains larger than 20 are observed in a YIG film heated by a current of 20 mA in a Pt layer in a simple YIG/Pt bilayer. The amplification is attributed to the action of a spin-transfer thermal torque acting on the magnetization that opposes the relaxation and which is created by spin currents generated through the spin Seebeck effect. The experimental data are interpreted with a spin-wave model.

Amplification of spin waves in yttrium iron garnet films through the spin Hall effect

We demonstrate that spin waves propagating in a film of yttrium iron garnet (YIG) can be amplified by a dc current in an adjacent Pt layer by means of the spin Hall effect. The experiments are done at room temperature using pulsed currents to avoid sample heating. Amplification occurs only for surface like modes propagating in a direction perpendicular to the applied in-plane field. The results are interpreted with a model for spin-wave propagation in a YIG film with magnetic losses and subject to a spin-transfer torque due to spin currents created by the spin Hall effect in the Pt layer.

Amplification of Spin Waves by Thermal Spin-Transfer Torque

We observe amplification of spin-wave packets propagating along a film of single-crystal yttrium iron garnet subject to a transverse temperature gradient. The spin waves are excited and detected with standard techniques used in magnetostatic microwave delay lines in the 1-2 GHz frequency range. The amplification is attributed to the action of a thermal spin-transfer torque acting on the magnetization that opposes the relaxation and which is created by spin currents generated through the spin-Seebeck effect. The experimental data are interpreted with a spin-wave model that gives an amplification gain in very good agreement with the data.

Current-Induced Control of Spin-Wave Attenuation

The current-induced modification of the attenuation of a propagating spin wave in a magnetic nanowire is studied theoretically and numerically. The attenuation length of spin wave can increase when the spin waves and electrons move in the same direction. It is directly affected by the nonadiabaticity of the spin-transfer torque and thus can be used to estimate the nonadiabaticity. When the nonadiabatic spin torque is sufficiently large, the attenuation length becomes negative, resulting in the amplification of spin waves.

Effect of spin-polarized electric current on spin-wave radiation by spin-valve nanocontacts

Using a spatially resolved microfocus Brillouin light scattering technique with spatial resolution of 250nm, the authors have studied radiation of spin waves by a spin-valve nanocontact into a continuous underlying magnetic film. They have found that a spin-polarized electric current flowing through the contact affects the intensity of radiated spin waves. They show that for one of the directions of the current, the spin-wave intensity is noticeably increased, whereas for the other direction, spin waves are suppressed. They associate the findings with an amplification of spin waves due to spin-torque-transfer phenomena.