arXiv subjects

Korbinian Baumgaertl

Publications and source records attributed to Korbinian Baumgaertl.

9 recordsLinked to original sources

arXiv2026

Multiband condensate of magnons in two dimensions

Wavelike bosonic particles can accumulate in a single mode characterized by a particular wavelength, and such condensates are at the heart of phenomena such as superconductivity and superfluidity where usually a single complex number describes their state. If there are several flavors of excitations or particles, a vector containing several complex numbers can characterize multicomponent condensation, thus opening new possibilities for textures, dynamics, and devices. Thus far, multicomponent condensates have long represented a rewarding subfield of cold atom physics, as well as a theme for research on exotic superconductors where the constituent bosons are not atoms but electron pairs. Here we consider the case where the bosons are magnons (collective spin excitations), injected by microwaves into high-quality yttrium iron garnet (YIG) crystals. Recent advances in fabrication yield thin (128 nm) films of sufficiently high quality to display multiple magnon bands quantized along the film normal. Microwave pumping can populate these bands, providing a new two-dimensional multiband condensate optimized in a narrow range of powers and frequencies due to a four-magnon scattering resonance. We establish a phase diagram for this magnonic system, reminiscent of that for exotic superconductors, revealing both single and multiband condensation.

cond-mat.quant-gas↗

arXiv2026

Soft-X-ray momentum microscopy of nonlinear magnon interactions below 100-nm wavelength

Magnons are quantised collective excitations of long-range ordered spins. At nanometre wavelengths, exchange interactions increasingly govern their dynamics, giving rise to a largely unexplored regime of couplings between magnons and other quasiparticles. Yet, detecting such short-wavelength spin waves has remained a key experimental challenge. Here, we introduce Magnon Momentum Microscopy (MMM) -- a quasi-elastic, resonant magnetic soft-X-ray scattering technique that directly images magnon populations across two-dimensional momentum space. Owing to its remarkable sensitivity, MMM can capture nonlinear magnon-magnon interactions over large regions of the dispersion plane. Applying MMM to the prototypical magnonic material yttrium iron garnet (YIG), we uncover a rich variety of previously unobserved nonlinear magnon interactions. With its element specificity, bulk sensitivity, as well as intrinsic access to nanometre-scale wavelengths without frequency limitation, soft-X-ray MMM establishes a powerful and versatile platform for exploring short-wavelength and nonlinear magnonics.

cond-mat.mes-hall↗

arXiv2023

Magnon-assisted magnetization reversal of Ni81Fe19 nanostripes on Y3Fe5O12 with different interfaces

Magnetic bit writing by short-wave magnons without conversion to the electrical domain is expected to be a game-changer for in-memory computing architectures. Recently, the reversal of nanomagnets by propagating magnons was demonstrated. However, experiments have not yet explored different wavelengths and the nonlinear excitation regime of magnons required for computational tasks. We report on the magnetization reversal of individual 20-nm-thick Ni81Fe19 (Py) nanostripes integrated onto 113-nm-thick yttrium iron garnet (YIG). We suppress direct interlayer exchange coupling by an intermediate layer such as Cu and SiO2. Exciting magnons in YIG with wavelengths λ down to 148 nm we observe the reversal of the integrated ferromagnets in a small opposing field of 14 mT. Magnons with a small wavelength of λ = 195 nm, i.e., twice the width of the Py nanostripes, induced the reversal at an unprecedentedly small spin precessional power of about 1 nW after propagating over 15 μm in YIG. Considerations based on dynamic dipolar coupling explain the observed wavelength dependence of magnon-induced reversal efficiency. For an increased power the stripes reversed in an opposing field of only about 1 mT. Our findings are important for the practical implementation of nonvolatile storage of broadband magnon signals in YIG by means of bistable nanomagnets without the need of an appreciable global magnetic field.

cond-mat.mes-hall↗

arXiv2023

Magnetization Reversal of 50-nm-wide Ni81Fe19 Nanostripes by Ultrashort Magnons in Yttrium Iron Garnet for Memory-Enhanced Magnonic Circuits

Spin waves (magnons) can enable wave-based neuromorphic computing by which one aims at overcoming limitations inherent to conventional electronics and the von Neumann architecture. In this study, we explore the storage of magnon signals and the magnetization switching of periodic and aperiodic arrays of Ni81Fe19 (Py) nanostripes with widths (w) between 50 nm and 200 nm. Spin waves excited with low microwave power in yttrium iron garnet induce the reversal of the nanostripes of different w in a small opposing field. Exploiting microwave-to-magnon transducers for magnon modes with ultrashort wavelengths, we demonstrate the reversal of 50-nm-wide Py nanostripes by magnons with wavelength ~ 100 nm after they have propagated over 25 micrometer in YIG. The findings are important for designing a magnon-based in-memory computing device.

cond-mat.mes-hall↗

arXiv2023

Nonreciprocal Spin Waves in Nanoscale Domain Walls Detected by Scanning X-ray Microscopy in Perpendicular Magnetic Anisotropic Fe/Gd Multilayers

Spin wave nonreciprocity in domain walls (DWs) allows for unidirectional signal processing in reconfigurable magnonic circuits. Using scanning transmission x-ray microscopy (STXM), we examined coherently-excited magnons propagating in Bloch-like DWs in amorphous Fe/Gd multilayers with perpendicular magnetic anisotropy (PMA). Near 1 GHz we detected magnons with short wavelengths down to $λ= 281$ nm in DWs whose minimum width amounted to $δ_{\rm DW} = 52$ nm. Consistent with micromagnetic simulations, the STXM data reveal their nonreciprocal magnon band structures. We identified Bloch points which disrupted the phase evolution of magnons and induced different $λ$ adjacent to the topological defects. Our observations provide direct evidence of nonreciprocal spin waves within Bloch-like DWs, serving as programmable waveguides in magnonic devices with directed information flow.

cond-mat.mes-hall↗

arXiv2022

Reversal of nanomagnets by propagating magnons in ferrimagnetic yttrium iron garnet enabling nonvolatile magnon memory

Despite the unprecedented downscaling of CMOS integrated circuits, memory-intensive machine learning and artificial intelligence applications are limited by data conversion between memory and processor. There is a challenging quest for novel approaches to overcome this so-called von Neumann bottleneck. Magnons are the quanta of spin waves and transport angular momenta through magnets. They enable power-efficient computation without charge flow and would solve the conversion problem if spin wave amplitudes could be stored directly in a magnetic memory cell. Here, we report the reversal of ferromagnetic nanostripes by spin waves which propagate through an underlying spin-wave bus made from yttrium iron garnet. Thereby, the charge-free angular momentum flow is stored after transmission over a macroscopic distance. We show that spin waves can reverse large arrays of ferromagnetic stripes at a strikingly small power level of nW. Combined with the already existing wave logic, our discovery is path-breaking for the new era of magnonics-based in-memory computation and beyond von Neumann computer architectures.

cond-mat.mes-hall↗

arXiv2022

Spin Dynamics, Loop Formation and Cooperative Reversal in Artificial Quasicrystals with Tailored Exchange Coupling

Aperiodicity and un-conventional rotational symmetries allow quasicrystalline structures to exhibit unprecedented physical and functional properties. In magnetism, artificial ferromagnetic quasicrystals exhibited knee anomalies suggesting reprogrammable magnetic properties via nonstochastic switching. However, the decisive roles of short-range exchange and long-range dipolar interactions have not yet been clarified for optimized reconfigurable functionality. We report broadband spin-wave spectroscopy and X-ray photoemission electron microscopy on different quasicrystal lattices consisting of ferromagnetic Ni81Fe19 nanobars arranged on aperiodic Penrose and Ammann tilings with different exchange and dipolar interactions. We imaged the magnetic states of partially reversed quasicrystals and analyzed their configurations in terms of the charge model, geometrical frustration and the formation of flux-closure loops. Only the exchange-coupled lattices are found to show aperiodicity-specific collective phenomena and non-stochastic switching. Both, exchange and dipolarly coupled quasicrystals show magnonic excitations with narrow linewidths in minor loop measurements. Thereby reconfigurable functionalities in spintronics and magnonics become realistic.

cond-mat.mes-hall↗

arXiv2021

Bistable nanomagnet as programmable phase inverter for spin waves

To realize spin wave logic gates programmable phase inverters are essential. We image with phase-resolved Brillouin light scattering microscopy propagating spin waves in a one-dimensional magnonic crystal consisting of dipolarly coupled magnetic nanostripes. We demonstrate phase shifts upon a single nanostripe of opposed magnetization. Using micromagnetic simulations we model our experimental finding in a wide parameter space of bias fields and wave vectors. We find that low-loss phase inversion is achieved, when the internal field of the oppositely magnetized nanostripe is tuned such that the latter supports a resonant standing spin wave mode with odd quantization number at the given frequency. Our results are key for the realization of phase inverters with optimized signal transmission.

cond-mat.mes-hall↗

arXiv2020

Nonreciprocal surface acoustic wave propagation via magneto-rotation coupling

One of the most fundamental forms of magnon-phonon interaction is an intrinsic property of magnetic materials, the "magnetoelastic coupling". This particular form of interaction has been the basis for describing magnetic materials and their strain related applications, where strain induces changes of internal magnetic fields. Different from the magnetoelastic coupling, more than 40 years ago, it was proposed that surface acoustic waves may induce surface magnons via rotational motion of the lattice in anisotropic magnets. However, a signature of this magnon-phonon coupling mechanism, termed magneto-rotation coupling, has been elusive. Here, we report the first observation and theoretical framework of the magneto-rotation coupling in a perpendicularly anisotropic ultra-thin film Ta/CoFeB(1.6 nm)/MgO, which consequently induces nonreciprocal acoustic wave attenuation with a unprecedented ratio up to 100$\%$ rectification at the theoretically predicted optimized condition. Our work not only experimentally demonstrates a fundamentally new path for investigating magnon-phonon coupling, but also justify the feasibility of the magneto-rotation coupling based application.

cond-mat.mes-hall↗