arXiv subjects

Alexander A. Serga

Publications and source records attributed to Alexander A. Serga.

At least 37 records · Page 2Linked to original sources

arXiv2021

Stabilization of a nonlinear bullet coexisting with a Bose-Einstein condensate in a rapidly cooled magnonic system driven by a spin-orbit torque

We have recently shown that injection of magnons into a magnetic dielectric via the spin-orbit torque (SOT) effect in the adjacent layer of a heavy metal subjected to the action of short (0.1 $μ$s) current pulses allows for control of a magnon Bose-Einstein Condensate (BEC). Here, the BEC was formed in the process of rapid cooling (RC), when the electric current heating the sample is abruptly terminated. In the present study, we show that the application of a longer (1.0 $μ$s) electric current pulse triggers the formation of a nonlinear localized magnonic bullet below the linear magnon spectrum. After pulse termination, the magnon BEC, as before, is formed at the bottom of the linear spectrum, but the nonlinear bullet continues to exist, stabilized for additional 30 ns by the same process of RC-induced magnon condensation. Our results suggest that a stimulated condensation of excess magnons to all highly populated magnonic states occurs.

cond-mat.quant-gas↗

arXiv2021

Double accumulation and anisotropic transport of magneto-elastic bosons in yttrium iron garnet films

Interaction between quasiparticles of a different nature, such as magnons and phonons in a magnetic medium, leads to the mixing of their properties and the formation of hybrid states in the areas of intersection of individual spectral branches. We recently reported the discovery of a new phenomenon mediated by the magnon-phonon interaction: the spontaneous bottleneck accumulation of magneto-elastic bosons under electromagnetic pumping of pure magnons into a ferrimagnetic yttrium iron garnet film. Here, by studying the transport properties of the accumulated magneto-elastic bosons, we reveal that such accumulation occurs in two frequency-distant groups of quasiparticles: quasi-phonons and quasi-magnons. They propagate with different speeds in different directions relative to the magnetization field. The theoretical model we propose qualitatively describes the double accumulation effect, and the analysis of the two-dimensional spectrum of quasiparticles in the hybridization region allows us to determine the wavevectors and frequencies of each of the groups.

cond-mat.mes-hall↗

arXiv2021

Evolution of room-temperature magnon gas toward coherent Bose-Einstein condensate

The appearance of spontaneous coherence is a fundamental feature of a Bose-Einstein condensate and an essential requirement for possible applications of the condensates for data processing and quantum computing. In the case of a magnon condensate in a magnetic crystal, such computing can be performed even at room temperature. So far, the process of coherence formation in a magnon condensate was inaccessible. We study the evolution of magnon radiation spectra by direct detection of microwave radiation emitted by magnons in a parametrically driven yttrium iron garnet crystal. By using specially shaped bulk samples, we show that the parametrically overpopulated magnon gas evolves to a state, whose coherence is only limited by the natural magnon relaxation into the crystal lattice.

cond-mat.mtrl-sci↗

arXiv2020

Bose-Einstein Condensation of Nonequilibrium Magnons in Confined Systems

We study the formation of a room temperature magnon Bose-Einstein condensate (BEC) in nanoscopic systems and demonstrate that its lifetime is influenced by the spatial confinement. We predict how dipolar interactions and nonlinear magnon scattering assist in the generation of a metastable magnon BEC in energy-quantized nanoscopic devices. We verify our prediction by a full numerical simulation of the Landau-Lifshitz-Gilbert equation and demonstrate the generation of magnon BEC in confined insulating magnets of yttrium iron garnet. We directly map out the nonlinear magnon scattering processes behind this phase transition to show how fast quantized thermalization channels allow the BEC formation in confined structures. Based on our results, we discuss a new mechanism to manipulate the BEC lifetime in nanoscaled systems. Our study greatly extends the freedom to study the dynamics of magnon BEC in realistic systems and to design integrated circuits for BEC-based applications at room temperature.

cond-mat.mes-hall↗

arXiv2019

Frequency-Division Multiplexing in Magnonic Logic Networks Based on Caustic-Like Spin-Wave Beams

Wave-based data processing by spin waves and their quanta, magnons, is a promising technique to overcome the challenges which CMOS-based logic networks are facing nowadays. The advantage of these quasi-particles lies in their potential for the realization of energy efficient devices on the micro- to nanometer scale due to their charge-less propagation in magnetic materials. In this paper, the frequency dependence of the propagation direction of caustic-like spin-wave beams in microstructured ferromagnets is studied by micromagnetic simulations. Based on the observed alteration of the propagation angle, an approach to spatially combine and separate spin-wave signals of different frequencies is demonstrated. The presented magnetic structure constitutes a prototype design of a passive circuit enabling frequency-division multiplexing in magnonic logic networks. It is verified that spin-wave signals of different frequencies can be transmitted through the device simultaneously without any interaction or creation of spurious signals. Due to the wave-based approach of computing in magnonic networks, the technique of frequency-division multiplexing can be the basis for parallel data processing in single magnonic devices, enabling the multiplication of the data throughput.

physics.app-ph↗

arXiv2019

Unconventional spin currents in magnetic films

A spin current - a flow of spin angular momentum - can be carried either by spin polarised free electrons or by magnons, the quanta of a moving collective oscillation of localised electron spins - a spin wave. Traditionally, it was assumed, that a spin wave in a magnetic film with spin-sink-free surfaces can transfer energy and angular momentum only along its propagation direction. In this work, using Brillouin light scattering spectroscopy in combination with a theory of dipole-exchange spin-wave spectra, we show that in obliquely magnetized free magnetic films the in-plane propagation of spin waves is accompanied by a transverse spin current along the film normal without any corresponding transverse transport of energy.

cond-mat.mes-hall↗

arXiv2018

Tunable space-time crystal in room-temperature magnetodielectrics

We report the experimental realization of a space-time crystal with tunable periodicity in time and space in the magnon Bose-Einstein Condensate (BEC), formed in a room-temperature Yttrium Iron Garnet (YIG) film by radio-frequency space-homogeneous magnetic field. The magnon BEC is prepared to have a well defined frequency and non-zero wavevector. We demonstrate how the crystalline "density" as well as the time and space textures of the resulting crystal may be tuned by varying the experimental parameters: external static magnetic field, temperature, thickness of the YIG film and power of the radio-frequency field. The proposed space-time crystals provide a new dimension for exploring dynamical phases of matter and can serve as a model nonlinear Floquet system, that brings in touch the rich fields of classical nonlinear waves, magnonics and periodically driven systems.

cond-mat.quant-gas↗

arXiv2018

Long-distance supercurrent transport in a room-temperature Bose-Einstein magnon condensate

The term supercurrent relates to a macroscopic dissipation-free collective motion of a quantum condensate and is commonly associated with such famous low-temperature phenomena as superconductivity and superfluidity. Another type of motion of quantum condensates is second sound - a wave of the density of a condensate. Recently, we reported on an enhanced decay of a parametrically induced Bose-Einstein condensate (BEC) of magnons caused by a supercurrent outflow of the BEC phase from the locally heated area of a room temperature magnetic film. Here, we present the direct experimental observation of a long-distance spin transport in such a system. The condensed magnons being pushed out from the potential well within the heated area form a density wave, which propagates through the BEC many hundreds of micrometers in the form of a specific second sound pulse - Bogoliubov waves - and is reflected from the sample edge. The discovery of the long distance supercurrent transport in the magnon BEC further advances the frontier of the physics of quasiparticles and allows for the application of related transport phenomena for low-loss data transfer in perspective magnon spintronics devices.

cond-mat.quant-gas↗

arXiv2018

From Kinetic Instability to Bose-Einstein Condensation and Magnon Supercurrents

Evolution of an overpopulated gas of magnons to a Bose-Einstein condensate and excitation of a magnon supercurrent, propelled by a phase gradient in the condensate wave function, can be observed at room-temperature by means of the Brillouin light scattering spectroscopy in an yttrium iron garnet material. We study these phenomena in a wide range of external magnetic fields in order to understand their properties when externally pumped magnons are transferred towards the condensed state via two distinct channels: A multistage Kolmogorov-Zakharov cascade of the weak-wave turbulence or a one-step kinetic-instability process. Our main result is that opening the kinetic instability channel leads to the formation of a much denser magnon condensate and to a stronger magnon supercurrent compared to the cascade mechanism alone.

cond-mat.quant-gas↗

arXiv2018

Spin Seebeck effect and ballistic transport of quasi-acoustic magnons in room-temperature yttrium iron garnet films

We studied the transient behavior of the spin current generated by the longitudinal spin Seebeck effect (LSSE) in a set of platinum-coated yttrium iron garnet (YIG) films of different thicknesses. The LSSE was induced by means of pulsed microwave heating of the Pt layer and the spin currents were measured electrically using the inverse spin Hall effect in the same layer. We demonstrate that the time evolution of the LSSE is determined by the evolution of the thermal gradient triggering the flux of thermal magnons in the vicinity of the YIG/Pt interface. These magnons move ballistically within the YIG film with a constant group velocity, while their number decays exponentially within an effective propagation length. The ballistic flight of the magnons with energies above 20K is a result of their almost linear dispersion law, similar to that of acoustic phonons. By fitting the time-dependent LSSE signal for different film thicknesses varying by almost an order of magnitude, we found that the effective propagation length is practically independent of the YIG film thickness. We consider this fact as strong support of a ballistic transport scenario - the ballistic propagation of quasi-acoustic magnons in room temperature YIG.

cond-mat.mes-hall↗

arXiv2018

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.

cond-mat.mes-hall↗

arXiv2017

Temperature dependent relaxation of dipole-exchange magnons in yttrium iron garnet films

Low energy consumption enabled by charge-free information transport, which is free from ohmic heating, and the ability to process phase-encoded data by nanometer-sized interference devices at GHz and THz frequencies are just a few benefits of spin-wave-based technologies. Moreover, when approaching cryogenic temperatures, quantum phenomena in spin-wave systems pave the path towards quantum information processing. In view of these applications, the lifetime of magnons$-$spin-wave quanta$-$is of high relevance for the fields of magnonics, magnon spintronics and quantum computing. Here, the relaxation behavior of parametrically excited magnons having wavenumbers from zero up to $6\cdot 10^5 \mathrm{rad~cm}^{-1}$ was experimentally investigated in the temperature range from 20 K to 340 K in single crystal yttrium iron garnet (YIG) films epitaxially grown on gallium gadolinium garnet (GGG) substrates as well as in a bulk YIG crystal$-$the magnonic materials featuring the lowest magnetic damping known so far. As opposed to the bulk YIG crystal in YIG films we have found a significant increase in the magnon relaxation rate below 150 K$-$up to 10.5 times the reference value at 340 K$-$in the entire range of probed wavenumbers. This increase is associated with rare-earth impurities contaminating the YIG samples with a slight contribution caused by coupling of spin waves to the spin system of the paramagnetic GGG substrate at the lowest temperatures.

cond-mat.mtrl-sci↗

arXiv2017

Experimental investigation of the temperature-dependent magnon density and its influence on studies of spin-transfer-torque-driven systems

We present the temperature dependence of the thermal magnon density in a thin ferromagnetic layer. By employing Brillouin light scattering and varying the temperature, an increase of the magnon density accompanied by a lowering of the spin-wave frequency is observed with increasing temperature. The magnon density follows the temperature according to the Bose-Einstein distribution function which leads to an approximately linear dependency. In addition, the influence of this effect in spin-transfer-torque-driven systems is presented. In particular, the increase in the magnon density with temperature sets the limit for a suppression of magnons in charge current-driven systems. Hence, the maximum possible suppression of thermal magnons occurs at a finite current.

cond-mat.mes-hall↗

arXiv2016

Bottleneck accumulation of hybrid magneto-elastic bosons

It is known that an ensemble of magnons, quanta of spin waves, can be prepared as a Bose gas of weakly interacting quasiparticles with conservation of the particle number. Furthermore, the thermalization of the overpopulated magnon gas can lead to the formation of a Bose-Einstein condensate at the bottom of a spin-wave spectrum. However, magnon-phonon scattering processes can significantly modify this scenario and new quasiparticles are formed - magneto-elastic bosons. Our observations of a parametrically populated magnon gas in a single-crystal film of Yttrium Iron Garnet by means of wavevector-resolved Brillouin light scattering spectroscopy including magneto-elastic coupling resulted in the discovery of a novel condensation phenomenon: A spontaneous accumulation of hybrid magneto-elastic bosonic quasiparticles at the intersection of the lowest magnon mode and a transversal acoustic wave.

cond-mat.quant-gas↗

arXiv2016

On supercurrents in Bose-Einstein magnon condensates in YIG ferrimagnet

Recently E. Sonin commented [1] on our preprint "Supercurrent in a room temperature Bose-Einstein magnon condensate" [2,3], arguing that our "claim of detection of spin supercurrent is premature and has not been sufficiently supported by presented experimental results and their theoretical interpretation." We consider the appearance of this Comment as a sign of significant interest into the problem of supercurrents in Bose-Einstein magnon condensates. Here, we explicitly address E. Sonin's comments and show that our interpretation of our experimental results as a detection of a magnon supercurrent is fully supported not only by the experimental results themselves, but also by independent theoretical analysis [4].

cond-mat.other↗

arXiv2016

Damping of parametrically excited magnons in the presence of the longitudinal spin Seebeck effect

The impact of the longitudinal spin Seebeck effect (LSSE) on the magnon damping in magnetic-insulator/nonmagnetic-metal bilayers was recently discussed in several reports. However, results of those experiments can be blurred by multimode excitation within the measured linewidth. In order to avoid possible intermodal interference, we investigated the damping of a single magnon group in a platinum covered Yttrium Iron Garnet (YIG) film by measurement of the threshold of its parametric excitation. Both dipolar and exchange spin-wave branches were probed. It turned out that the LSSE-related modification of spin-wave damping in a micrometer-thick YIG film is too weak to be observed in the entire range of experimentally accessible wavevectors. At the same time, the change in the mean temperature of the YIG layer, which can appear by applying a temperature gradient, strongly modifies the damping value.

cond-mat.mes-hall↗

arXiv2016

Spin Seebeck effect at microwave frequencies

We experimentally study the transient voltage response of yttrium iron garnet/platinum bilayer samples subject to periodic heating up to gigahertz frequencies. We observe an intrinsic cutoff frequency of the induced thermopower voltage, which characteristically depends on the thickness of the yttrium iron garnet film. The cutoff frequency reaches values of up to 350 MHz in a 50 nm thick yttrium iron garnet film, but drops to below 1 MHz for several-micrometer-thick films. These data corroborate the notion that the magnon spectrum responsible for the spin current emission in the spin Seebeck effect can be shaped by tuning the thickness of the ferromagnetic layer.

cond-mat.mtrl-sci↗

arXiv2016

Low-damping transmission of spin waves through YIG/Pt-based layered structures for spin-orbit-torque applications

We show that in YIG-Pt bi-layers, which are widely used in experiments on the spin transfer torque and spin Hall effects, the spin-wave amplitude significantly decreases in comparison to a single YIG film due to the excitation of microwave eddy currents in a Pt coat. By introducing a novel excitation geometry, where the Pt layer faces the ground plane of a microstrip line structure, we suppressed the excitation of the eddy currents in the Pt layer and, thus, achieved a large increase in the transmission of the Damon-Eshbach surface spin wave. At the same time, no visible influence of an external dc current applied to the Pt layer on the spin-wave amplitude in the YIG-Pt bi-layer was observed in our experiments with YIG films of micrometer thickness.

cond-mat.mes-hall↗