arXiv subjects

Daniel Schick

Publications and source records attributed to Daniel Schick.

At least 19 recordsLinked to original sources

arXiv2026

Dynamical diffraction formalism for imaging time-dependent diffuse scattering from coherent phonons with Dark-Field X-ray Microscopy

Coherent acoustic phonons, whose damping sets the upper bound of quality factors in acoustic resonators, play a critical role in advanced telecommunication and quantum information technologies. Yet, probing their decay in the GHz regime remains challenging using conventional surface-based techniques. Dark-field X-ray microscopy (DFXM) offers a solution by enabling through-depth, non-destructive and full-field imaging of strain fields and dislocations inside bulk materials with high spatial and angular resolution. We previously used kinematic diffraction theory to describe DFXM signals based on how the Bragg peak shifts due to the strain wave, allowing us to reconstruct the frequency spectrum of coherent phonons as a function of depth through the sample. The approach of tracking the Bragg peak shifts to study phonon dynamics, however, places an upper-bound to the highest phonon frequency that can be studied, determined by the spatial resolution of the measurement. In this work, we discuss how coherent phonon dynamics can be studied with DFXM from time-dependent intensity oscillation sidebands. This approach simultaneously allows studying coherent phonon dynamics in real and reciprocal space, overcoming frequency resolution limits imposed by the real-space resolution of Bragg-peak tracking. Using Takagi-Taupin dynamical diffraction formalism, we establish the spatial and reciprocal space resolution achievable for studying the coherent phonon dynamics and evaluate conditions for observing long-lived intensity oscillations. We close by proposing experimental strategies to optimize excitation bandwidths and reciprocal-space selectivity. The formalism in the paper enables the design of DFXM experiments for quantitative, frequency-resolved measurements of acoustic phonon decay and phonon-defect interactions in bulk crystalline materials.

cond-mat.mes-hall↗

arXiv2026

Depth-resolved magnetization dynamics in Fe thin films after ultrafast laser excitation

We performed time-resolved x-ray resonant magnetic reflectivity measurements on a laser-excited ferromagnetic Fe thin film to simultaneously probe the transient structural and magnetic depth profiles with nanometer spatial and femtosecond temporal resolution. Our results show that during the first picoseconds after optical excitation, the magnetization of the Fe layer is strongly inhomogeneous, especially in the vicinity of the buried interface. By comparing our experimental results to predictions based on the microscopic three-temperature model and simulations of laser-induced spin-currents, we demonstrate that local and non-local angular momentum transfer phenomena take place simultaneously. After a few picoseconds, the magnetization relaxes back to equilibrium while the total thin film thickness starts oscillating periodically, with a maximum dilation of approximately 1.3% of the entire thin film thickness due to laser-induced stresses.

cond-mat.mtrl-sci↗

arXiv2026

Anisotropic skyrmion liquid phase

The nature of the melting transition in two-dimensional systems of particles has attracted considerable research attention since the development of Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) theory. The hexatic phase proposed by this theory has been recently identified experimentally in ensembles of magnetic skyrmions, quasiparticles formed in a magnetically ordered crystal. Here, we use quasiparticle dynamical simulations to study how the anisotropy of the skyrmion-skyrmion interactions induced by the atomic lattice influences the melting transition. For isotropic interactions, we find a transition from a solid phase through a hexatic phase stable in a narrow temperature range to an isotropic liquid phase. However, if the interactions between skyrmions are forced to be anisotropic by the atomic lattice, then a direct solid-liquid transition can be observed with orientational order persisting up to temperatures of 30 K in the liquid phase.

cond-mat.other↗

arXiv2025

Experimental evidence of dominant ultrafast diffusive energy transport by hot electrons in Cu

When the dimensions of structures shrink to the order of the inelastic mean free path of the energy-carrying quasi-particles, the character of energy transport changes from diffusive to ballistic. However, the point of transition remains a matter of debate. Here, we determine the dominant channel of energy transport through a nanoscale Cu layer as a function of its thickness. The energy rapidly transferred across Cu via hot electrons from a photo-excited Pt layer into a buried Ni detection layer translates into a rapid expansion of the Ni layer probed via ultrafast x-ray diffraction. The non-linear dependence of the Ni strain amplitude on the absorbed laser fluence indicates that the transport through Cu becomes more efficient with increasing fluence. This fluence-dependent transport efficiency is reproduced by a diffusive energy transport model and serves as a generally applicable experimental approach to distinguish diffusion from ballistic transport. Following this approach, we identify diffusive electronic energy transport to govern the spatial energy distribution for Cu layer thicknesses larger than twice the electronic inelastic mean free path.

cond-mat.mtrl-sci↗

arXiv2025

Ultrafast X-ray sonography reveals the spatial heterogeneity of the laser-induced magneto-structural phase transition in FeRh

Phase transitions are governed by both intrinsic and extrinsic heterogeneities, yet capturing their spatio-temporal dynamics remains a challenge. While ultrafast techniques track phase changes on femtosecond timescales, the spatial complexity and stochastic nature of the processes often remain hidden. Here, we present an experimental approach that combines well-established ultrafast hard-X-ray diffraction with a propagating strain pulse as a universal and non-invasive probe. This ultrafast X-ray sonography can capture the spatio-temporal phase heterogeneity in great detail by resolving the phase-specific strain response. We apply this approach to the antiferromagnetic-to-ferromagnetic magneto-structural phase transition in FeRh and identify the ferromagnetic phase to nucleate at the surface as narrow columnar domains of approximately $30\,\text{nm}$ diameter. Besides reconciling the diverse experimental results in the literature on FeRh, X-ray sonography offers a versatile platform for investigating a wide range of phase transitions accompanied by structural changes.

cond-mat.mtrl-sci↗

arXiv2025

X-ray View of Light-Induced Spin Reorientation in TmFeO$_{3}$: Direct Observation of a 90$^\circ$ N\'eel Vector Rotation

Using time-resolved X-ray magnetic linear dichroism in reflection, we provide a direct probe of the N\'eel vector dynamics in TmFeO$_3$ on a ultrafast timescale. Our measurements reveal that, following optical excitation, the N\'eel vector undergoes a spin reorientation transition primarily within the a-c plane, completing a full 90{\deg} rotation within approximately 20 ps. This study highlights the ability to probe dynamics of antiferromagnets at its intrinsic timescale in reflection geometry, paving the way for investigations of a wide range of antiferromagnets grown on application relevant substrates.

cond-mat.mtrl-sci↗

arXiv2025

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↗

arXiv2025

Fluence dependent delay of Ni in an FeNi alloy supports an exchange based origin

The delayed demagnetization in Ni relative to Fe in the ultrafast demagnetization studies in FeNi alloy has led to two competing theoretical explanations: The Inhomogeneous Magnon Generation (IMG) and the Optically Induced Spin Transfer (OISTR) model. The IMG attributes the delay to the preferential magnon generation at the Fe sites and its subsequent propagation to Ni, while OISTR proposes direct spin transfer from Ni to Fe. In this study, we employ element-resolved extreme ultraviolet spectroscopy to investigate the effect of excitation strength on this delay, aiming to resolve the controversy. The data indicate a significant reduction in the delay with increasing fluence, which is inconsistent with the theoretical predictions of OISTR. These findings, in conjunction with the observation of a saturation of Fe demagnetization at the onset of Ni demagnetization, indicate that a spin-wave instability within the IMG framework may provide a potential explanation for the experimental results.

cond-mat.mtrl-sci↗

arXiv2025

Spectroscopic probe of ultrafast magnetization dynamics in the extreme ultraviolet spectral range

The development of spectroscopic techniques in the extreme ultraviolet (XUV) spectral range has significantly advanced the understanding of ultrafast interactions in magnetic systems triggered by optical excitation. In this work, we introduce a previously missing geometry that facilitates the observation of the ultrafast magnetization dynamics of magnetic systems with an out-of-plane magnetization grown on XUV opaque substrates. This approach to probing ultrafast magnetization dynamics combines the magneto-optical Kerr effect with the strong dependence of a sample's reflectance near its Brewster angle. It therefore works with linearly polarized light and does not require any additional polarizing optics. We provide a comprehensive analysis of the technique by presenting both simulations and experimental data as a function of the energy and the polarization of the XUV probe radiation as well as of the delay time after optical excitation.

cond-mat.mtrl-sci↗

arXiv2025

Laser-driven resonant soft-X-ray scattering for probing picosecond dynamics of nanometre-scale order

X-ray scattering has been an indispensable tool in advancing our understanding of matter, from the first evidence of the crystal lattice to recent discoveries of nuclei's fastest dynamics. In addition to the lattice, ultrafast resonant elastic scattering of soft X-rays provides a sensitive probe of charge, spin, and orbital order with unparalleled nanometre spatial and femto- to picosecond temporal resolution. However, the full potential of this technique remains largely unexploited due to its high demand on the X-ray source. Only a selected number of instruments at large-scale facilities can deliver the required short-pulsed and wavelength-tunable radiation, rendering laboratory-scale experiments elusive so far. Here, we demonstrate time-resolved X-ray scattering with spectroscopic contrast at a laboratory-based instrument using the soft-X-ray radiation emitted from a laser-driven plasma source. Specifically, we investigate the photo-induced response of magnetic domains emerging in a ferrimagnetic heterostructure with 9$\,$ps temporal resolution. The achieved sensitivity allows for tracking the reorganisation of the domain network on pico- to nanosecond time scales in great detail. This instrumental development and experimental demonstration break new ground for studying material dynamics in a wide range of laterally ordered systems in a flexible laboratory environment.

cond-mat.mes-hall↗

arXiv2024

Ultrafast energy-dispersive soft-x-ray diffraction in the water window with a laser-driven source

Time-resolved soft-x-ray-diffraction experiments give access to microscopic processes in a broad range of solid-state materials by probing ultrafast dynamics of ordering phenomena. While laboratory-based high-harmonic generation (HHG) light sources provide the required photon energies, their limited photon flux is distributed over a wide spectral range, rendering typical monochromatic diffraction schemes challenging. Here, we present a scheme for energy-dispersive soft-x-ray diffraction with femtosecond temporal resolution and photon energies across the water window from 200 to 600 eV. The experiment utilizes the broadband nature of the HHG emission to efficiently probe large slices in reciprocal space. As a proof-of-concept, we study the laser-induced structural dynamics of a Mo/Si superlattice in an ultrafast, non-resonant soft-x-ray diffraction experiment. We extract the underlying strain dynamics from the measured shift of its first order superlattice Bragg peak in reciprocal space at photon energies around 500 eV via soft-x-ray scattering simulations.

physics.optics↗

arXiv2022

Picosecond x-ray magnetic circular dichroism spectroscopy at the Fe L-edges with a laser-driven plasma source

Time-resolved x-ray magnetic circular dichroism (XMCD) enables a unique spectroscopic view on complex spin and charge dynamics in multi-elemental magnetic materials. So far, its application in the soft-x-ray range has been limited to synchrotron-radiation sources and free-electron lasers. By combining a laser-driven plasma source with a magnetic thin-film polarizer, we generate circularly polarized photons in the soft x-ray regime, enabling the first XMCD spectroscopy at the Fe L edges in a laser laboratory. Our approach can be readily adapted to other transition metal L and rare earth M absorption edges and with a temporal resolution of < 10 ps, a wide range of ultrafast magnetization studies can be realized.

cond-mat.mtrl-sci↗

arXiv2022

Ultrafast behavior of induced and intrinsic magnetic moments in CoFeB/Pt bilayers probed by element-specific measurements in the extreme ultraviolet spectral range

The ultrafast and element-specific response of magnetic systems containing ferromagnetic 3d transition metals and 4d/5d heavy metals is of interest both from a fundamental as well as an applied research perspective. However, to date no consensus about the main microscopic processes describing the interplay between intrinsic 3d and induced 4d/5d magnetic moments upon femtosecond laser excitation exist. In this work, we study the ultrafast response of CoFeB/Pt bilayers by probing element-specific, core-to-valence band transitions in the extreme ultraviolet spectral range using high harmonic radiation. We show that the combination of magnetic scattering simulations and analysis of the energy- and time-dependent magnetic asymmetries allows to accurately disentangle the element-specific response in spite of overlapping Co and Fe M$_{2,3}$ as well as Pt O$_{2,3}$ and N$_7$ resonances. We find a considerably smaller demagnetization time constant as well as much larger demagnetization amplitudes of the induced moment of Pt compared to the intrinsic moment of CoFeB. Our results are in agreement with enhanced spin-flip probabilities due to the high spin-orbit coupling localized at the heavy metal Pt, as well as with the recently formulated hypothesis that a laser generated, incoherent magnon population within the ferromagnetic film leads to an overproportional reduction of the induced magnetic moment of Pt.

cond-mat.mtrl-sci↗

arXiv2022

All-optical switching on the nanometer scale excited and probed with femtosecond extreme ultraviolet pulses

Ultrafast control of magnetization on the nanometer length scale, in particular all-optical switching, is key to putting ultrafast magnetism on the path towards future technological application in data storage technology. However, magnetization manipulation with light on this length scale is challenging due to the wavelength limitations of optical radiation. Here, we excite transient magnetic gratings in a GdFe alloy with a periodicity of 87 nm by interference of two coherent femtosecond light pulses in the extreme ultraviolet spectral range at the free electron laser facility FERMI. The subsequent ultrafast evolution of the magnetization pattern is probed by diffraction of a third, time-delayed pulse tuned to the Gd N-edge at a wavelength of 8.3 nm. By examining the simultaneously recorded first and second diffraction orders and by performing reference real-space measurements with a wide-field magneto-optical microscope with femtosecond time resolution, we can conclusively demonstrate the ultrafast emergence of all-optical switching on the nanometer length scale.

cond-mat.mtrl-sci↗

arXiv2021

Probing electron and hole co-localization by resonant four-wave mixing in the extreme-ultraviolet

The extension of nonlinear spectroscopic techniques into the x-ray domain is in its infancy but holds the promise to provide unique insight into the dynamics of charges in photoexcited processes, which are of fundamental as well as applied interest. We report on the observation of a third order nonlinear process in lithium fluoride at a free-electron laser. Exploring the yield of four wave mixing (FWM) in resonance with transitions to strongly localized core exciton states vs. delocalized Bloch states, we find resonant FWM to be a sensitive probe for the degree of charge localization: substantial sum- and difference-frequency generation is observed exclusively when in a one- or three-photon resonance with a LiF core exciton, with a dipole forbidden transition affecting details of the nonlinear response. Our reflection-geometry-based approach to detect FWM signals enables the study of a wide variety of condensed matter sample systems, provides atomic selectivity via resonant transitions and can be easily scaled to shorter wavelengths at free electron x-ray lasers.

physics.app-ph↗

arXiv2021

Ultrafast element- and depth-resolved magnetization dynamics probed by transverse magneto-optical Kerr effect spectroscopy in the soft x-ray range

We report on time- and angle-resolved transverse magneto-optical Kerr effect spectroscopy in the soft x-ray range that, by analysis via polarization-dependent magnetic scattering simulations, allows us to determine the spatio-temporal and element-specific evolution of femtosecond laser-induced spin dynamics in nanostructured magnetic materials. In a ferrimagnetic GdFe thin film system, we correlate a reshaping spectrum of the magneto-optical Kerr signal to depth-dependent magnetization dynamics and disentangle contributions due to non-equilibrium electron transport and nanoscale heat diffusion on their intrinsic time scales. Our work provides a quantitative insight into light-driven spin dynamics occurring at buried interfaces of complex magnetic heterostructures, which can be tailored and functionalized for future opto-spintronic devices.

cond-mat.mtrl-sci↗

arXiv2021

Wide-field magneto-optical microscope to access quantitative magnetization dynamics with femtosecond temporal and sub-micrometer spatial resolution

We introduce a wide-field magneto-optical microscope to probe magnetization dynamics with femtosecond temporal and sub-micrometer spatial resolution. We carefully calibrate the non-linear dependency between the magnetization of the sample and the detected light intensity by determining the absolute values of the magneto-optical polarization rotation. With that, an analytical transfer function is defined to directly map the recorded intensity to the corresponding magnetization, which results in significantly reduced acquisition times and relaxed computational requirements. The performance of the instrument is characterized by probing the magnetic all-optical switching dynamics of GdFe in a pump-probe experiment. The high spatial resolution of the microscope allows for accurately subdividing the laser-excited area into different fluence-regions in order to capture the strongly non-linear magnetization dynamics as a function of the optical pump intensity in a single measurement.

physics.optics↗

arXiv2021

udkm1Dsim -- A Python toolbox for simulating 1D ultrafast dynamics in condensed matter

The udkm1Dsim toolbox is a collection of Python classes and routines to simulate the thermal, structural, and magnetic dynamics after laser excitation as well as the according X-ray scattering response in one-dimensional sample structures. The toolbox provides the capabilities to define arbitrary layered structures on the atomic level including a rich database of element-specific physical properties. The excitation of dynamics is represented by an $N$-temperature-model which is commonly applied in ultrafast physics. Structural dynamics due to thermal stresses are calculated by a linear-chain model of masses and springs. The implementation of specific magnetic dynamics can be easily accomplished by the user employing a generalized magnetization interface class. The resulting X-ray diffraction response is computed by kinematical or dynamical X-ray theory which can also include polarization-dependent magnetic scattering. The udkm1Dsim toolbox is highly modular and allows for injecting user-defined inputs at any step within the simulation procedure.

physics.comp-ph↗