arXiv subjects

Burkard Hillebrands

Publications and source records attributed to Burkard Hillebrands.

At least 19 recordsLinked to original sources

arXiv2026

Bulk-mediated reflection of chirality-protected surface spin waves

Surface spin waves of the Damon-Eshbach type exhibit intrinsically nonreciprocal transport properties with a chiral dynamical field structure that localizes counterpropagating waves at opposite film surfaces. Such chirality has been predicted to suppress direct backscattering in thin films within frequency ranges free of bulk modes. However, how chirality influences reflection in thicker three-dimensional magnetic media, where a dense spectrum of bulk excitations overlaps with surface waves, remains unclear. Here we demonstrate that, in micrometer-thick yttrium iron garnet films, reflection of the chiral Damon-Eshbach wave from the boundary of the magnetic medium is accompanied by excitation of spatially localized thickness-quantized bulk modes, whereas reciprocal backward-volume waves reflect nearly elastically. Brillouin light scattering spectroscopy, infrared thermography, and micromagnetic simulations reveal standing bulk excitations at the reflecting boundary and quantify the associated magnon energy accumulation and dissipation. These results identify bulk-mode excitations as the physical pathway enabling reversal of chirally localized surface waves in thick films, thereby defining the limits of chirality-based backscattering immunity and providing a general framework for wave transport in nonreciprocal magnetic media.

cond-mat.other↗

arXiv2025

Enhancement of magnon flux toward a Bose-Einstein condensate

We present a combined theoretical and experimental study of angle-dependent parametric pumping of magnons in Yttrium Iron Garnet films, with a focus on the mechanisms that transfer parametrically injected magnons toward the spectral minimum where Bose-Einstein condensation occurs. Using a classical Hamiltonian formalism, we analyze the threshold conditions for parametric instability as a function of the angle between the microwave pumping field and the external magnetic field, continuously tracing the transition between parallel and transverse pumping. We also describe two competing four-magnon scattering mechanisms that transfer parametric magnons toward the bottom of their frequency spectrum: The step-by-step Kolmogorov-Zakharov cascade, which is allowed for all magnetic field values, and the kinetic instability mechanisms that provide a much more efficient single-step channel in transferring magnons directly to the lowest-energy states, but occurs within specific regions of the pumping angle and the external magnetic field where the conservation laws permit it. In the experimental part, we employ microfocused Brillouin light scattering spectroscopy in combination with a vector magnet, allowing for angle-resolved mapping of the magnon population spectrum under controlled pumping angle. We observe that transverse pumping, although characterized by a higher instability threshold, yields a markedly stronger population at the spectral minimum compared to parallel pumping. These observations demonstrate that the kinetic instability channel plays a dominant role in transferring magnons to the spectral minimum under such conditions. These results reveal the crucial role of pumping geometry in shaping the magnon distribution and provide guidelines for optimizing the flux of magnons into the condensate, thereby advancing the control of magnon Bose-Einstein condensation in magnetic insulators.

cond-mat.quant-gas↗

arXiv2025

Spontaneous Emergence of Phase Coherence in a quasiparticle Bose-Einstein Condensate

Since their prediction by Einstein at the dawn of quantum mechanics, Bose-Einstein condensates (BECs), owing to their property to show quantum phenomena on macroscopic scales, are drawing increasing attention across various fields in physics. They are the subject of many fascinating observations in various physical systems, from liquid helium to diluted atomic gases. In addition to real particles like atoms and composite bosons such as Cooper pairs or excitons, this phenomenon is also observed in gases of quasiparticles such as polaritons and magnons - quanta of spin-wave excitations in magnetic media. The fundamental property of the BEC state is its coherence, which is represented by a precisely defined phase of the corresponding wave function, which arises spontaneously and encompasses all particles gathered at the bottom of their spectrum. Until now, the BEC phase was only revealed in phenomena depending on the spatial phase difference, such as interference, second order coherence, and macroscopic BEC motions - supercurrents, superfluidity and Josephson oscillations. Here, we present a method for the direct time-domain measurement of the magnon BEC coherent state phase relative to an outside reference signal. We report the emergence of spontaneous coherence from a freely evolving magnon gas, which manifests as the condensation of magnons into a uniform precession state with minimal energy and a well-defined phase. These findings confirm all postulated fundamental properties of quasiparticle condensates, provide access to a new degree of freedom in such systems, and open up the possibility of information processing using microwave-frequency magnon BECs.

cond-mat.quant-gas↗

arXiv2024

Anisotropy-assisted magnon condensation in ferromagnetic thin films

We theoretically demonstrate that adding an easy-axis magnetic anisotropy facilitates magnon condensation in thin yttrium iron garnet (YIG) films. Dipolar interactions in a quasi-equilibrium state stabilize room-temperature magnon condensation in YIG. Even though the out-of-plane easy-axis anisotropy generally competes with the dipolar interactions, we show that adding such magnetic anisotropy may even assist the generation of the magnon condensate electrically via the spin transfer torque mechanism. We use analytical calculations and micromagnetic simulations to illustrate this effect. Our results may explain the recent experiment on Bi-doped YIG and open a pathway toward applying current-driven magnon condensation in quantum spintronics.

cond-mat.mes-hall↗

arXiv2023

Towards an experimental proof of the magnonic Aharonov$-$Casher effect

Controlling the phase and amplitude of spin waves in magnetic insulators with an electric field opens the way to fast logic circuits with ultra-low power consumption. One way to achieve such control is to manipulate the magnetization of the medium via magnetoelectric effects. In experiments with magnetostatic spin waves in an yttrium iron garnet film, we have obtained the first evidence of a theoretically predicted phenomenon: The change of the spin-wave phase due to the magnonic Aharonov$-$Casher effect$-$the geometric accumulation of the magnon phase as these quasiparticles propagate through an electric field region.

cond-mat.mes-hall↗

arXiv2023

Local temperature control of magnon frequency and direction of supercurrents in a magnon Bose-Einstein condensate

The creation of temperature variations in magnetization, and hence in the frequencies of the magnon spectrum in laser-heated regions of magnetic films, is an important method for studying Bose-Einstein condensation of magnons, magnon supercurrents, Bogoliubov waves, and similar phenomena. In our study, we demonstrate analytically, numerically, and experimentally that, in addition to the magnetization variations, it is necessary to consider the connected variations of the demagnetizing field. In case of a heat induced local minimum of the saturation magnetization, the combination of these two effects results in a local increase in the minimum frequency value of the magnon dispersion at which the Bose-Einstein condensate emerges. As a result, a magnon supercurrent directed away from the hot region is formed.

cond-mat.quant-gas↗

arXiv2023

Bose-Einstein condensation in systems with flux equilibrium

We consider flux equilibrium in dissipative nonlinear wave systems subject to external energy pumping. In such systems, the elementary excitations, or quasiparticles, can create a Bose-Einstein condensate. We develop a theory on the Bose-Einstein condensation of quasiparticles for various regimes of external excitation, ranging from weak and stationary to ultra-strong pumping, enabling us to determine the number of quasiparticles near the bottom of the energy spectrum and their distribution along wave vectors. We identify physical phenomena leading to condensation in each of the regimes. For weak stationary pumping, where the distribution of quasiparticles deviates only slightly from thermodynamic equilibrium, we define a range of pumping parameters where the condensation occurs and estimate the density of the condensate and the fraction of the condensed quasiparticles. As the pumping amplitude increases, a powerful influx of injected quasiparticles is created by the Kolmogorov-Zakharov scattering cascade, leading to their Bose-Einstein condensation. With even stronger pumping, kinetic instability may occur, resulting in a direct transfer of injected quasiparticles to the bottom of the spectrum. For the case of ultra-strong parametric pumping, we have developed a stationary nonlinear theory of kinetic instability. The theory agrees qualitatively with experimental data obtained using Brillouin light scattering spectroscopy during parametric pumping of magnons in room-temperature films of yttrium-iron garnet.

cond-mat.quant-gas↗

arXiv2023

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 non-equilibrium 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 non-equilibrium magnon distribution.

cond-mat.mtrl-sci↗

arXiv2023

Spin transport and magnetic proximity effect in CoFeB/normal metal/Pt trilayers

We present a study of the damping and spin pumping properties of CoFeB/X/Pt systems with $\rm X=Al,Cr$ and $\rm Ta$. We show that the total damping of the CoFeB/Pt systems is strongly reduced when an interlayer is introduced independently of the material. Using a model that considers spin relaxation, we identify the origin of this contribution in the magnetically polarized Pt formed by the magnetic proximity effect (MPE), which is suppressed by the introduction of the interlayer. The induced ferromagnetic order in the Pt layer is confirmed by transverse magneto-optical Kerr spectroscopy at the M$_{2,3}$ and N$_7$ absorption edges as an element-sensitive probe. We discuss the impact of the MPE on parameter extraction in the spin transport model.

cond-mat.mtrl-sci↗

arXiv2023

Correlation-enhanced interaction of a Bose-Einstein condensate with parametric magnon pairs and virtual magnons

Nonlinear interactions are crucial in science and engineering. Here, we investigate wave interactions in a highly nonlinear magnetic system driven by parametric pumping leading to Bose--Einstein condensation of spin-wave quanta -- magnons. Using Brillouin light scattering spectroscopy in yttrium-iron garnet films, we found and identified a set of nonlinear processes resulting in off-resonant spin-wave excitations -- virtual magnons. In particular, we discovered a dynamically-strong, correlation-enhanced four-wave interaction process of the magnon condensate with pairs of parametric magnons having opposite wavevectors and fully correlated phases.

cond-mat.quant-gas↗

arXiv2023

Rapid-prototyping of microscopic thermal landscapes in Brillouin light scattering spectroscopy

Since temperature and its spatial and temporal variations affect a wide range of physical properties of material systems, they can be used to create reconfigurable spatial structures of various types in physical and biological objects. This paper presents an experimental optical setup for creating tunable two-dimensional temperature patterns on a micrometer scale. As an example of its practical application, we have produced temperature-induced magnetization landscapes in ferrimagnetic yttrium iron garnet films and investigated them using micro-focused Brillouin light scattering spectroscopy. It is shown that, due to the temperature dependence of the magnon spectrum, temperature changes can be visualized even for microscale thermal patterns.

cond-mat.quant-gas↗

arXiv2023

Competing signatures of intersite and interlayer spin transfer in the ultrafast magnetization dynamics

Optically driven intersite and interlayer spin transfer are individually known as the fastest processes for manipulating the spin order of magnetic materials on the sub 100 fs time scale. However, their competing influence on the ultrafast magnetization dynamics remains unexplored. In our work, we show that optically induced intersite spin transfer (also known as OISTR) dominates the ultrafast magnetization dynamics of ferromagnetic alloys such as Permalloy (Ni80Fe20) only in the absence of interlayer spin transfer into a substrate. Once interlayer spin transfer is possible, the influence of OISTR is significantly reduced and interlayer spin transfer dominates the ultrafast magnetization dynamics. This provides a new approach to control the magnetization dynamics of alloys on extremely short time scales by fine-tuning the interlayer spin transfer.

cond-mat.mtrl-sci↗

arXiv2023

Quantifying symmetric exchange in ultrathin ferromagnetic films with chirality

The symmetric (Heisenberg) exchange interaction is fundamental to magnetism and assumes critical importance in designing magnetic materials for novel emergent phenomena and device applications. However, quantifying exchange is extremely challenging for ultrathin ($\sim$ 1 nm) magnetic films, as techniques and approximations reliably used for bulk materials are largely inapplicable in the two-dimensional (2D) limit. Here we present and contrast the measurement of exchange stiffness, $A$, by several methods on a series of five Co/Pt-based ultrathin ($1-2$ nm) films. We compare results from (a) spin-wave spectroscopy by Brillouin light scattering (BLS), (b) three analytical models describing the temperature dependence of magnetization obtained by magnetometry, (c) microscopic domain periodicity measurements and simulations, and (d) ab initio density functional theory (DFT) calculations. While different methods present some qualitatively consistent trends across samples, we note, for any given sample, considerable differences (up to $5\times$) in the absolute values of $A$ across the techniques, consistent with discrepancies of $A$ reported in literature for nominally similar samples. We analyze possible sources of the discrepancies across various methods, notably including their relationship to the spin-wave dispersion, and the wave-vector ranges probed. We compare the strengths and limitations of the techniques, and outline directions for their future use in characterizing exchange interactions in ultrathin films.

cond-mat.mtrl-sci↗

arXiv2022

Confinement of Bose-Einstein magnon condensates in adjustable complex magnetization landscapes

Coherent wave states such as Bose-Einstein condensates (BECs), which spontaneously form in an overpopulated magnon gas even at room temperature, have considerable potential for wave-based computing and information processing at microwave frequencies. The ability to control the transport properties of magnon BECs plays an essential role for their practical use. Here, we demonstrate spatio-temporal control of the BEC density distribution through the excitation of magnon supercurrents in an inhomogeneously magnetized yttrium iron garnet film. The BEC is created by microwave parametric pumping and probed by Brillouin light scattering spectroscopy. The desired magnetization profile is prepared by heating the film with optical patterns projected onto its surface using a phase-based wavefront modulation technique. Specifically, we observe a pronounced spatially localized magnon accumulation caused by magnon supercurrents flowing toward each other originating in two heated regions. This accumulation effect increases the BEC lifetime due to the constant influx of condensed magnons into the confinement region. The shown approach to manipulate coherent waves provides an opportunity to extend the lifetime of freely evolving magnon BECs, create dynamic magnon textures, and study the interaction of magnon condensates formed in different regions of the sample.

cond-mat.quant-gas↗

arXiv2022

Parametric Excitation and Instabilities of Spin Waves driven by Surface Acoustic Waves

The parametric excitation of spin waves by coherent surface acoustic waves is demonstrated experimentally in metallic magnetic thin film structures. The involved magnon modes are analyzed with micro-focused Brillouin light scattering spectroscopy and complementary micromagnetic simulations combined with analytical modelling are used to determine the origin of the spin-wave instabilities. Depending on the experimental conditions, we observe spin-wave instabilities originating from different phonon-magnon and magnon-magnon scattering processes. Our results demonstrate that an efficient excitation of high amplitude, strongly nonlinear magnons in metallic ferromagnets is possible by surface acoustic waves, which opens novel ways to create micro-scaled nonlinear magnonic systems for logic and data processing that can profit from the high excitation efficiency of phonons using piezoelectricity.

cond-mat.other↗

arXiv2022

THz emission from Fe/Pt spintronic emitters with L1$_{0}$-FePt alloyed interface

Recent developments in nanomagnetism and spintronics have enabled the use of ultrafast spin physics for terahertz (THz) emission. Spintronic THz emitters, consisting of ferromagnetic FM / non-magnetic (NM) thin film heterostructures, have demonstrated impressive properties for the use in THz spectroscopy and have great potential in scientific and industrial applications. In this work, we focus on the impact of the FM/NM interface on the THz emission by investigating Fe/Pt bilayers with engineered interfaces. In particular, we intentionally modify the Fe/Pt interface by inserting an ordered L1$_{0}$-FePt alloy interlayer. Subsequently, we establish that a Fe/L1$_{0}$-FePt (2\,nm)/Pt configuration is significantly superior to a Fe/Pt bilayer structure, regarding THz emission amplitude. The latter depends on the extent of alloying on either side of the interface. The unique trilayer structure opens new perspectives in terms of material choices for the next generation of spintronic THz emitters.

cond-mat.mtrl-sci↗

arXiv2022

Accumulation of magnetoelastic bosons in yttrium iron garnet: kinetic theory and wave vector resolved Brillouin light scattering

We derive and solve quantum kinetic equations describing the accumulation of magnetoelastic bosons in an overpopulated magnon gas realized in a thin film of the magnetic insulator yttrium iron garnet. We show that in the presence of a magnon condensate, there is a non-equilibrium steady state in which incoherent magnetoelastic bosons accumulate in a narrow region in momentum space for energies slightly below the bottom of the magnon spectrum. The results of our calculations agree quite well with Brillouin light scattering measurements of the stationary non-equilibrium state of magnons and magnetoelastic bosons in yttrium iron garnet.

cond-mat.mes-hall↗

arXiv2022

Fully Resonant Magneto-elastic Spin-wave Excitation by Surface Acoustic Waves under Conservation of Energy and Linear Momentum

We report on the resonant excitation of spin waves in micro-structured magnetic thin films by surface acoustic waves (SAWs). The spin waves as well as the acoustic waves are studied by micro-focused Brillouin light scattering spectroscopy. Besides the excitation of the ferromagnetic resonance, a process which does not fulfill momentum conservation, also the excitation of finite-wavelength spin waves can be observed at low magnetic fields. Using micromagnetic simulations, we verify that during this excitation both energy and linear momentum are conserved and fully transferred from the SAW to the spin wave.

cond-mat.other↗