arXiv subjects

Jörg Raabe

Publications and source records attributed to Jörg Raabe.

At least 19 recordsLinked to original sources

arXiv2026

Supersampled scanning transmission X-ray microscopy for high-resolution vibration-independent time-resolved imaging

Scanning transmission X-ray microscopy (STXM) is a nanoscale imaging technique that can utilize several powerful contrast mechanisms for the quantitative mapping of chemical and physical materials properties. Spatial resolutions down to 7~nm at the soft X-ray energy range have been demonstrated. A limiting factor for high-resolution STXM imaging is given by the positioning precision of the sample with respect to the focusing optic, with the current state-of-the-art leading to significant overheads, especially at low pixel dwell times, and being vulnerable to unavoidable external vibrations sources. In this work, we present a method, called supersampled scanning microscopy, that allows for a significant reduction of overhead times while simultaneously removing the effects of vibrational noise by sampling the position of the sample at a rate significantly higher than the vibration spectrum and reconstructing the sample transmission image from the recorded list of positions and detector counts. We demonstrate the performance of the technique with a set of proof-of-concept high-resolution imaging experiments.

physics.ins-det↗

arXiv2026

Scanning through space and time: past, present, and future of time-resolved scanning transmission soft X-ray microscopy

Time-resolved microscopy with the pump-probe protocol is one of the most important techniques for the investigation of dynamical processes at the nanoscale, thanks to the possibility of combining nanometric resolution imaging with sub-nanosecond temporal resolutions. Amongst the ensemble of time-resolved microscopy techniques, time-resolved scanning transmission X-ray microscopy has been, since its inception in 2006, extensively utilized for the study of magneto-dynamical processes. In this review, an overview of the concept and experimental implementations of the pump-probe protocol in time-resolved scanning transmission X-ray microscopy imaging will be presented together with some examples of recent applications of the technique. Possible future developments aimed at meeting the new opportunities and challenges offered by the upgrade of synchrotrons to diffraction limited lightsources will also be discussed.

physics.ins-det↗

arXiv2026

In-situ Straining of Epitaxial Freestanding Ferroic Films by a MEMS Device

Mechanical strain can be used to control physical properties in materials. The experimental investigation of strain-induced effects at the nanoscale is of importance not only for its fundamental aspects, but also for the development of device applications. Transmission X-ray microscopy is a particularly well-suited technique for nanoscale imaging of magnetic materials, but its compatibility with in-situ mechanical straining of samples is limited. In this work, we present a setup for applying tailored in-situ mechanical strains to freestanding thin films by means of a micro electromechanical system (MEMS) actuator. We then present a proof-of-concept experiment in which a freestanding 80 nm thick (001) BiFeO3 multiferroic thin film is strained with the MEMS device, allowing us to control the coupled ferroelectric/spin cycloidal configuration.

cond-mat.mes-hall↗

arXiv2025

A fluctuation-free pathway for a topological magnetic phase transition

Topological magnetic textures are particle-like spin configurations stabilized by competing interactions. Their formation is commonly attributed to fluctuation-driven, first-order nucleation processes requiring activation over a topological energy barrier. Here, we demonstrate an alternative barrier- and fluctuation-free pathway for nucleating topological magnetic textures, triggered in our experiments by an excitation-induced spin reorientation transition. By combining x-ray imaging, scattering and micromagnetic simulations, we show that the system follows a deterministic cascade of symmetry-breaking phase transitions after excitation. First, the system undergoes a second-order phase transition from a homogeneous state to weak stripe domains, then a first-order transition to topologically trivial bubbles, and finally a topological switching event into skyrmionic textures. Through simulations, we generalize our findings and demonstrate that this pathway is active in a vast range of low-anisotropy materials. This previously unrecognized, spontaneous transition pathway suggests strategies for rapid, low-energy generation of topological spin textures and points to a general role of intrinsic modulational instabilities in phase transitions beyond magnetism.

cond-mat.mtrl-sci↗

arXiv2025

Soft x-ray ptychography with SOPHIE: Guide and instrumentation

Soft x-ray ptychography is becoming a key synchrotron microscopy technique in the fields of condensed matter physics, chemistry, and environmental and life sciences. Its attractiveness across broad disciplinary fields is owed to the favorable combination of high spatial resolution and strong contrast mechanisms. The SOft X-ray Ptychography Highly Integrated Endstation (SOPHIE) at the Swiss Light Source (SLS) was developed to accommodate soft x-ray ptychography experiments requiring high spatial resolution, in addition to high chemical and ferroic sensitivities. An introduction to soft x-ray ptychography with SOPHIE aimed at prospective users is provided. Furthermore, an overview of the instrumentation of SOPHIE is given along with an example of the imaging capabilities, which demonstrate the achievement of a sub-10 nm spatial resolution at a photon energy of 706 eV.

physics.ins-det↗

arXiv2025

Room temperature control of axial and basal antiferromagnetic anisotropies using strain

Antiferromagnetic materials are promising platforms for the development of ultra-fast spintronics and magnonics due to their robust magnetism, high-frequency relativistic dynamics, low-loss transport, and the ability to support topological textures. However, achieving deterministic control over antiferromagnetic order in thin films is a major challenge, due to the formation of multi-domain states stabilised by competing magnetic and destressing interactions. Thus, the successful implementation of antiferromagnetic materials necessitates careful engineering of their anisotropy. Here, we demonstrate strain-based robust control over multiple antiferromagnetic anisotropies and nanoscale domains in the promising spintronic candidate a-Fe2O3, at room temperature. By applying isotropic and anisotropic in-plane strains across a broad temperature-strain phase space, we systematically tune the interplay between magneto-crystalline and magneto-elastic interactions. We discover that strain-driven control steers the system towards an aligned antiferromagnetic state, whilst preserving topological spin textures, such as merons, antimerons and bimerons. We directly map the nanoscale antiferromagnetic order using linear dichroic scanning transmission X-ray microscopy integrated with in situ strain and temperature control. A Landau model and micromagnetic simulations reveal how strain reshapes the magnetic energy landscape. These findings suggest that strain could serve as a versatile control mechanism to reconfigure equilibrium or dynamic antiferromagnetic states on demand in a-Fe2O3, paving the way for next-generation spintronic and magnonic devices.

cond-mat.mtrl-sci↗

arXiv2025

Observation of a non-reciprocal skyrmion Hall effect of hybrid chiral skyrmion tubes in synthetic antiferromagnetic multilayers

A hybrid chiral skyrmion tube is a well-known example of a 3D topological spin texture, exhibiting an intriguing chirality transition along the thickness direction. This transition progresses from left-handed to right-handed Néel-type chirality, passing through a Bloch-type intermediate state. Such an exotic spin configuration potentially exhibits distinctly different dynamics from that of the common skyrmion tube that exhibits a homogeneous chirality; yet these dynamics have not been ascertained so far. Here we reveal the distinct features of current-induced dynamics that result from the hybrid chiral skyrmion tube structure in synthetic antiferromagnetic (SyAFM) multilayers. Strikingly, the SyAFM hybrid chiral skyrmion tubes exhibit a non-reciprocal skyrmion Hall effect in the flow regime. The non-reciprocity can even be tuned by the degree of magnetic compensation in the SyAFM systems. Our theoretical modeling qualitatively corroborates that the non-reciprocity stems from the dynamic oscillation of skyrmion helicity during its current-induced motion. The findings highlight the critical role of the internal degrees of freedom of these complex skyrmion tubes for their current-induced dynamics.

cond-mat.mes-hall↗

arXiv2025

Single-photon counting pixel detector for soft X-rays

Soft X-ray experiments at synchrotron light sources are essential for a wide range of research fields. However, commercially available detectors for this energy range often cannot deliver the necessary combination of quantum efficiency, signal-to-noise ratio, dynamic range, speed, and radiation hardness within a single system. While hybrid detectors have addressed these challenges effectively in the hard X-ray regime, specifically with single photon counting pixel detectors extensively used in high-performance synchrotron applications, similar solutions are desired for energies below 2 keV. In this work, we introduce the first single-photon-counting hybrid pixel detector capable of detecting X-ray energies as low as 550 eV, utilizing the internal amplification of Low-Gain Avalanche Diode (LGAD) sensors. This detector is thoroughly characterized in terms of Signal-to-Noise Ratio and Detective Quantum Efficiency. We demonstrate its capabilities through ptychographic imaging at MAX IV 4th generation synchrotron light source at the Fe L$_3$-edge (707 eV), showcasing the enhanced detection performance of the system. This development sets a new benchmark for soft X-ray applications at synchrotrons, paving the way for significant advancements in imaging and analysis at lower photon energies.

physics.ins-det↗

arXiv2025

Nanoscale Domain Wall Dynamics in Micromagnetic Structures with Weak Perpendicular Anisotropy

Time-resolved pump-probe soft X-ray ptychography and Scanning Transmission X-ray Microscopy (STXM) were employed to study the magnetic domain wall dynamics in microstructures of permalloy (Ni$_{81}$Fe$_{19}$; Py) with a weak growth-induced perpendicular magnetic anisotropy. The X-ray magnetic circular dichroism (XMCD) images of a micrometer-sized Py square (160 nm thickness) and an elliptical disk (80 nm thickness) show flux-closure patterns with domain walls that fall into alternating out-of-plane (OOP) magnetization states precipitated by the perpendicular anisotropy, which is a precursor of the nucleation of stripe domains at higher thicknesses. An oscillating magnetic field at frequencies from tens of MHz to GHz and up to 4 mT magnitude excited dynamic modes in the domain walls along with the vortex core gyration. The domain wall dynamics include the translation of inversion points of the OOP magnetization and nucleation of one-dimensional spin waves.

cond-mat.mtrl-sci↗

arXiv2025

Unidirectional motion of topological defects mediating continuous rotation processes

Topological defects play a critical role across many fields, mediating phase transitions and macroscopic behaviors as they move through space. Their role as robust information carriers has also generated much attention. However, controlling their motion remains challenging, especially towards achieving motion along well-defined paths which typically require predefined structural patterning. Here we demonstrate the tunable, unidirectional motion of topological defects, specifically magnetic dislocations in a weak magnetic stripe pattern, induced by external magnetic field in a laterally unconfined thin film. This motion is shown to mediate the overall continuous rotation of the stripe pattern. We determine the connection between the unidirectional motion of dislocations and the underlying three-dimensional (3D) magnetic structure by performing 3D magnetic vectorial imaging with in situ magnetic fields. A minimal model for dislocations in stripe patterns that encodes the symmetry breaking induced by the external magnetic field reproduces the motion of dislocations that facilitate the 2D rotation of the stripes, highlighting the universality of the phenomenon. This work establishes a framework for studying the field-driven behavior of topological textures and designing materials that enable well defined, controlled motion of defects in unconfined systems, paving the way to manipulate information carriers in higher-dimensional systems.

cond-mat.mes-hall↗

arXiv2025

Imaging ferroelectric domains with soft X-ray ptychography at the oxygen K-edge

The ferroelectric domain structure of a freestanding BiFeO$_3$ film was visualized by ptychographic dichroic imaging with linearly polarized X-rays at the O K-edge around 530 eV. The dichroic contrast is maximized at the energy of the hybridization of the O 2p state and the Fe 3d orbitals, which is split by the octahedral crystal field of the perovskite structure. The microscopy images thus obtained complement the ptychographic imaging of the antiferromagnetic contribution at the Fe L$_3$-edge. The approach can be extended to the separation of different ferroic contributions in other multiferroic oxides.

cond-mat.mtrl-sci↗

arXiv2024

Ptychographic nanoscale imaging of the magnetoelectric coupling in freestanding BiFeO$_3$

Understanding the magnetic and ferroelectric ordering of magnetoelectric multiferroic materials at the nanoscale necessitates a versatile imaging method with high spatial resolution. Here, soft X-ray ptychography is employed to simultaneously image the ferroelectric and antiferromagnetic domains in an 80 nm thin freestanding film of the room-temperature multiferroic BiFeO$_3$ (BFO). The antiferromagnetic spin cycloid of period 64 nm is resolved by reconstructing the corresponding resonant elastic X-ray scattering in real space and visualized together with mosaic-like ferroelectric domains in a linear dichroic contrast image at the Fe L$_3$ edge. The measurements reveal a near perfect coupling between the antiferromagnetic and ferroelectric ordering by which the propagation direction of the spin cycloid is locked orthogonally to the ferroelectric polarization. In addition, the study evinces both a preference for in-plane propagation of the spin cycloid and changes of the ferroelectric polarization by 71° between multiferroic domains in the epitaxial strain-free, freestanding BFO film. The results provide a direct visualization of the strong magnetoelectric coupling in BFO and of its fine multiferroic domain structure, emphasizing the potential of ptychographic imaging for the study of multiferroics and non-collinear magnetic materials with soft X-rays.

cond-mat.mtrl-sci↗

arXiv2024

Three-dimensional spin-wave dynamics, localization and interference in a synthetic antiferromagnet

Spin waves are collective perturbations in the orientation of the magnetic moments in magnetically ordered materials. Their rich phenomenology is intrinsically three-dimensional; however, the three-dimensional imaging of spin waves has so far not been possible. Here, we image the three-dimensional dynamics of spin waves excited in a synthetic antiferromagnet, with nanoscale spatial resolution and sub-ns temporal resolution, using time-resolved magnetic laminography. In this way, we map the distribution of the spin-wave modes throughout the volume of the structure, revealing unexpected depth-dependent profiles originating from the interlayer dipolar interaction. We experimentally demonstrate the existence of complex three-dimensional interference patterns and analyze them via micromagnetic modelling. We find that these patterns are generated by the superposition of spin waves with non-uniform amplitude profiles, and that their features can be controlled by tuning the composition and structure of the magnetic system. Our results open unforeseen possibilities for the study and manipulation of complex spin-wave modes within nanostructures and magnonic devices.

cond-mat.mes-hall↗

arXiv2023

Quantifying the Topology of Magnetic Skyrmions in three Dimensions

Magnetic skyrmions have so far been treated as two-dimensional spin structures characterized by a topological winding number describing the rotation of spins across the skyrmion. However, in real systems with a finite thickness of the material being larger than the magnetic exchange length, the skyrmion spin texture extends into the third dimension and cannot be assumed as homogeneous. Using soft x-ray laminography we reconstruct with about 20nm spatial (voxel) resolution the full three-dimensional spin texture of a skyrmion in an 800 nm diameter and 95 nm thin disk patterned into a trilayer [Ir/Co/Pt] thin film structure. A quantitative analysis finds that the evolution of the radial profile of the topological skyrmion number and the chirality is non-uniform across the thickness of the disk. Estimates of local micromagnetic energy densities suggest that the changes in topological profile are related to non-uniform competing energetic interactions. Theoretical calculations and micromagnetic simulations are consistent with the experimental findings. Our results provide the foundation for nanoscale magnetic metrology for future tailored spintronics devices using topology as a design parameter, and have the potential to reverse-engineer a spin Hamiltonian from macroscopic data, tying theory more closely to experiment.

cond-mat.mtrl-sci↗

arXiv2023

Spatially reconfigurable topological textures in freestanding antiferromagnetic nanomembranes

Antiferromagnets hosting real-space topological spin textures are promising platforms to model fundamental ultrafast phenomena and explore spintronics. However, to date, they have only been fabricated epitaxially on specific symmetry-matched crystalline substrates, to preserve their intrinsic magneto-crystalline order. This curtails their integration with dissimilar supports, markedly restricting the scope of fundamental and applied investigations. Here, we circumvent this limitation by designing detachable crystalline antiferromagnetic nanomembranes of $α$-Fe$_{2}$O$_{3}$, that can be transferred onto other desirable supports after growth. We develop transmission-based antiferromagnetic vector-mapping to show that these nanomembranes harbour rich topological phenomenology at room temperature. Moreover, we exploit their extreme flexibility to demonstrate three-dimensional reconfiguration of antiferromagnetic properties, driven locally via flexure-induced strains. This allows us to spatially design antiferromagnetic states outside their typical thermal stability window. Integration of such freestanding antiferromagnetic layers with flat or curved nanostructures could enable spin texture designs tailored by magnetoelastic-/geometric-effects in the quasi-static and dynamical regimes, opening new explorations into curvilinear antiferromagnetism and unconventional computing.

cond-mat.mtrl-sci↗

arXiv2022

Collective skyrmion motion under the influence of an additional interfacial spin-transfer torque

Here we study the effect of an additional interfacial spin-transfer torque, as well as the well established spin-orbit torque and bulk spin-transfer torque, on skyrmion collections - group of skyrmions dense enough that they are not isolated from on another - in ultrathin heavy metal / ferromagnetic multilayers, by comparing modelling with experimental results. Using a skyrmion collection with a range of skyrmion diameters and landscape disorder, we study the dependence of the skyrmion Hall angle on diameter and velocity, as well as the velocity as a function of diameter. We show the experimental results are in good agreement with modelling when including the interfacial spin-transfer torque, and cannot be reproduced by using the spin-orbit torque alone. We also show that for skyrmion collections the velocity is approximately independent of diameter, in marked contrast to the motion of isolated skyrmions, as the group of skyrmions move together at an average group velocity. Moreover, the calculated skyrmion velocities are comparable to those obtained in experiments when the interfacial spin-transfer torque in included, whilst modelling using the spin-orbit torque alone shows large discrepancies with the experimental data. Our results thus show the significance of the interfacial spin-transfer torque in ultrathin magnetic multilayers, which is of similar strength to the spin-orbit torque, and both significantly larger than the bulk spin-transfer torque. Due to the good agreement with experiments, we conclude that the interfacial spin-transfer torque should be included in numerical modelling for correct reproduction of experimental results.

cond-mat.mes-hall↗

arXiv2022

Domain wall motion at low current density in a synthetic antiferromagnet nanowire

The current-driven motion of magnetic domain walls (DWs) is the working principle of magnetic racetrack memories. In this type of spintronic technology, high current densities are used to propel DW motion in magnetic nanowires, causing significant wire heating. Synthetic antiferromagnets are known to show very fast DW motion at high current densities, but lower current densities around onset of motion have received less attention. Here we use scanning transmission x-ray microscopy to study the response of DWs in a SAF multilayer to currents. We observe that the DWs depin at $\sim 3 \times 10^{11}$~A/m$^2$ and move more quickly in response to 5~ns duration current pulses than in comparable conventional multilayers. The results suggest that DWs in SAF structures are superior to conventional Néel DWs for low energy consumption racetrack technologies.

cond-mat.mes-hall↗

arXiv2021

Three-dimensional Resonant Magnetization Dynamics Unraveled by Time-Resolved Soft X-ray Laminography

The imaging of magneto-dynamical processes has been, so far, mostly a two-dimensional business, due to the constraints of the available experimental techniques. In this manuscript, building on the recent developments of soft X-ray magnetic laminography, we present an experimental setup where magneto-dynamical processes can be resolved in all three spatial dimensions and in time, with the possibility to freely tune the frequency of the dynamical process. We then employ this setup to investigate the three-dimensional dynamics of two resonant magneto-dynamical modes in a CoFeB microstructure occurring at different frequencies, namely the fundamental vortex gyration mode and a magnetic field-induced domain wall excitation mode. This new technique provides much needed capabilities for the experimental investigation of the magnetization dynamics of three-dimensional magnetic systems.

cond-mat.mes-hall↗