arXiv subjects

Gregor Hlawacek

Publications and source records attributed to Gregor Hlawacek.

At least 19 recordsLinked to original sources

arXiv2026

Effect of Near-surface Thermal Spikes on Radiation Hardness of Gallium Oxide

Gallium oxide (Ga$_{2}$O$_{3}$) stands out as an extraordinary high radiation-tolerant semiconductor, because its lattice displacements induce polymorph transitions, holding material crystalline instead of leading to amorphization. Meanwhile, under extremely severe irradiation conditions many crystals become amorphous, often starting from surfaces where the translation symmetry breaks. Here, we show that the surface amorphization may prevail over the crystallisation in Ga$_{2}$O$_{3}$, however only if the mass and energy of irradiated ions produce sufficiently dense heat spikes in the immediate vicinity of the free surface. Applying machine-learned molecular dynamics simulations together with experimental broad-beam and focused ion-beam irradiations, we conclude that the presence of the free surface enables asymmetric displacements of Ga and O atoms, leading to a local non-stoichiometry. Consequently, when the affected cascade volume is sufficiently large, this compositional imbalance suppresses recrystallization and promotes amorphization. As such, our results are ready to use for tailoring irradiation conditions to either prevent or induce surface amorphization, depending on the requirements of the intended applications in Ga$_{2}$O$_{3}$ or other compound semiconductors.

cond-mat.mtrl-sci↗

arXiv2026

TetMaG-Guided Design and Operando Electron Holography Validation of Current-Induced Domain-Wall Motion in 3D Curved and Cornered Fe Nanobridges

Three-dimensional (3D) magnetic nanostructures offer new opportunities for controlling domain-wall (DW) configurations beyond the limitations of planar systems, providing promising architectures. However, the realization of reliable 3D magnetic devices requires precise control of geometry-dependent DW behaviour and quantitative experimental validation of the resulting magnetic states. Here, we combine TetMaG micromagnetic simulations, focused electron beam induced deposition (FEBID), and off-axis electron holography to investigate the influence of curvature and corner geometries on DW behaviour in 3D magnetic nanobridges. TetMaG simulations predict fundamentally different magnetic properties for curved and cornered geometries. Cornered nanobridges act as preferential DW pinning sites, stabilizing localized magnetic configurations and enabling controlled switching between neighbouring pinning positions. While curved nanobridges promote gradual magnetization rotation, reduced pinning, and smoother DW motion. These optimized geometries were fabricated with high structural fidelity using FEBID and subsequently characterized by quantitative electron holography. Electron holography measurements revealed magnetic induction maps that matched the simulated magnetization configurations, providing direct experimental validation of the TetMaG predictions. Curved and cornered geometries exhibited distinct DW characteristics governed by their local structural features, demonstrating the critical role of geometry in tailoring magnetic behaviour in 3D systems. Operando current-biasing experiments further revealed current-induced DW motion, including the displacement of a tail-to-tail DW into a head-to-tail configuration within corner structures.

cond-mat.mes-hall↗

arXiv2026

Interplay of Cl Substitution and He$^{+}$ Irradiation in CrSBr$_{1-x}$Cl$_{x}$

Two-dimensional magnetic semiconductors provide a promising platform for exploring the interplay between disorder, lattice dynamics, and resonant light--matter interactions. Among them, CrSBr exhibits strong in-plane anisotropy and pronounced resonance-enhanced Raman scattering. Here, we investigate the effects of Cl substitution and He$^{+}$ irradiation on the vibrational response of CrSBr using polarization-resolved Raman spectroscopy. Cl substitution activates additional phonon modes associated with local symmetry breaking, while He$^{+}$ irradiation introduces distinct defect-related scattering channels and enhanced phonon broadening. The combined effects of alloy disorder and externally introduced defects lead to strong anisotropic reconstruction of the Raman spectra and modification of the nonlinear Raman response under near-resonant 1.96 eV excitation. Power-dependent measurements reveal robust superlinear scaling of both intrinsic and substitution-induced phonon modes, indicating persistent resonance-enhanced electron--phonon coupling even in defect-engineered samples.

cond-mat.mes-hall↗

arXiv2025

Defect analysis of the $β$- to $γ$-Ga$_{2}$O$_{3}$ phase transition

In this study, we investigate the ion-irradiation-induced phase transition in gallium oxide (Ga2O3) from the $β$ to the $γ$ phase, the role of defects during the transformation, and the quality of the resulting crystal structure. Using a multi-method analysis approach including X-ray diffraction (XRD), transmission electron microscopy (TEM), Rutherford backscattering spectrometry in channeling mode (RBS/c), Doppler broadening variable energy positron annihilation spectroscopy (DB-VEPAS) and variable energy positron annihilation lifetime spectroscopy (VEPALS) supported by density functional theory (DFT) calculations, we have characterized defects at all the relevant stages before, during, and after the phase transition. Reduction in backscattering yield was observed in RBS/c spectra after the transition to the $γ$ phase. This is corroborated by a significant decrease in the positron trapping center density due to generation of embedded vacancies intrinsic for the $γ$-Ga2O3 but too shallow in order to trap positrons. A comparison of the observed positron lifetime of $γ$-Ga2O3 with different theoretical models shows good agreement with the three-site $γ$ phase approach. A characteristic increase in the effective positron diffusion length and the positron lifetime at the transition point from $β$-Ga2O3 to $γ$-Ga2O3 enables visualization of the phase transition with positrons for the first time. Moreover, a subsequent reduction of these quantities with increasing irradiation fluence was observed, which we attribute to further evolution of the $γ$-Ga2O3 and changes in the gallium vacancy density as well as relative occupation in the crystal lattice.

cond-mat.mtrl-sci↗

arXiv2025

Control of magnon frequency combs in magnetic rings

Using Brillouin light scattering microscopy, we study the rich dynamics in magnetic disks and rings governed by non-linear interactions, focusing on the role of vortex core dynamics on the spin-wave eigenmode spectrum. By strongly exciting quantized magnon modes in magnetic vortices, self-induced magnon Floquet states are populated by the intrinsic nonlinear coupling of magnon modes to the vortex core gyration. In magnetic rings, however, this generation is suppressed even when exciting the system over a large power range. To retrieve the rich nonlinear dynamics in rings, we apply external in-plane magnetic fields by which the vortex core is restored. Our findings demonstrate how to take active control of the nonlinear processes in magnetic structures of different topology.

cond-mat.mes-hall↗

arXiv2023

Roadmap for focused ion beam technologies

The focused ion beam (FIB) is a powerful tool for the fabrication, modification and characterization of materials down to the nanoscale. Starting with the gallium FIB, which was originally intended for photomask repair in the semiconductor industry, there are now many different types of FIB that are commercially available. These instruments use a range of ion species and are applied broadly in materials science, physics, chemistry, biology, medicine, and even archaeology. The goal of this roadmap is to provide an overview of FIB instrumentation, theory, techniques and applications. By viewing FIB developments through the lens of the various research communities, we aim to identify future pathways for ion source and instrumentation development as well as emerging applications, and the scope for improved understanding of the complex interplay of ion-solid interactions. We intend to provide a guide for all scientists in the field that identifies common research interests and will support future fruitful interactions connecting tool development, experiment and theory. While a comprehensive overview of the field is sought, it is not possible to cover all research related to FIB technologies in detail. We give examples of specific projects within the broader context, referencing original works and previous review articles throughout.

physics.ins-det↗

arXiv2023

Universal radiation tolerant semiconductor

Radiation tolerance is determined as the ability of crystalline materials to withstand the accumulation of the radiation induced disorder. Nevertheless, for sufficiently high fluences, in all by far known semiconductors it ends up with either very high disorder levels or amorphization. Here we show that gamma/beta double polymorph Ga2O3 structures exhibit remarkably high radiation tolerance. Specifically, for room temperature experiments, they tolerate a disorder equivalent to hundreds of displacements per atom, without severe degradations of crystallinity; in comparison with, e.g., Si amorphizable already with the lattice atoms displaced just once. We explain this behavior by an interesting combination of the Ga- and O- sublattice properties in gamma-Ga2O3. In particular, O-sublattice exhibits a strong recrystallization trend to recover the face-centered-cubic stacking despite the stronger displacement of O atoms compared to Ga during the active periods of cascades. Notably, we also explained the origin of the beta-to-gamma Ga2O3 transformation, as a function of the increased disorder in beta-Ga2O3 and studied the phenomena as a function of the chemical nature of the implanted atoms. As a result, we conclude that gamma/beta double polymorph Ga2O3 structures, in terms of their radiation tolerance properties, benchmark a class of universal radiation tolerant semiconductors.

cond-mat.mtrl-sci↗

arXiv2023

Deterministic multi-level spin orbit torque switching using He+ microscopy patterning

He$^+$ ion irradiation is used to pattern multiple areas of Pt/Co/W films with different irradiation doses in Hall bars. The resulting perpendicular magnetic anisotropy landscape enables selective multilevel current-induced switching, with full deterministic control of the position and order of the individual switching elements. Key pattern design parameters are specified, opening a way to scalable multilevel switching devices.

cond-mat.mes-hall↗

arXiv2022

Tailoring crosstalk between localized 1D spin-wave nanochannels using focused ion beams

1D spin-wave conduits are envisioned as nanoscale components of magnonics-based logic and computing schemes for future generation electronics. `A-la-carte methods of versatile control of the local magnetization dynamics in such nanochannels are highly desired for efficient steering of the spin waves in magnonic devices. Here, we present a study of localized dynamical modes in 1-$μ$m-wide Permalloy conduits probed by microresonator ferromagnetic resonance technique. We clearly observe the lowest-energy edge mode in the microstrip after its edges were finely trimmed by means of focused Ne$^+$ ion irradiation. Furthermore, after milling the microstrip along its long axis by focused ion beams, creating consecutively $\sim$50 and $\sim$100 nm gaps, additional resonances emerge and are attributed to modes localized at the inner edges of the separated strips. To visualize the mode distribution, spatially resolved Brillouin light scattering microscopy was used showing an excellent agreement with the ferromagnetic resonance data and confirming the mode localization at the outer/inner edges of the strips depending on the magnitude of the applied magnetic field. Micromagnetic simulations confirm that the lowest-energy modes are localized within $\sim$15-nm-wide regions at the edges of the strips and their frequencies can be tuned in a wide range (up to 5 GHz) by changing the magnetostatic coupling (i.e. spatial separation) between the microstrips.

cond-mat.mes-hall↗

arXiv2020

Helium Ion Microscopy for Reduced Spin Orbit Torque Switching Currents

Spin orbit torque driven switching is a favourable way to manipulate nanoscale magnetic objects for both memory and wireless communication devices. The critical current required to switch from one magnetic state to another depends on the geometry and the intrinsic properties of the materials used, which are difficult to control locally. Here we demonstrate how focused helium ion beam irradiation can modulate the local magnetic anisotropy of a Co thin film at the microscopic scale. Real-time in-situ characterisation using the anomalous Hall effect showed up to an order of magnitude reduction of the magnetic anisotropy under irradiation, and using this, multi-level switching is demonstrated. The result is that spin-switching current densities, down to 800 kA cm$^{-2}$, can be achieved on predetermined areas of the film, without the need for lithography. The ability to vary critical currents spatially has implications not only for storage elements, but also neuromorphic and probabilistic computing.

cond-mat.mes-hall↗

arXiv2020

An atomic force microscope integrated with a helium ion microscope for correlative nanocharacterization

In this work, we report the integration of an atomic force microscope (AFM) into a helium ion microscope (HIM). The HIM is a powerful instrument, capable of sub-nanometer resolution imaging and machining of nanoscale structures, while the AFM is a well-established versatile tool for multiparametric nanoscale characterization. Combining the two techniques opens the way for unprecedented, in situ, correlative analysis at the nanoscale. Nanomachining and analysis can be performed without contamination of the sample and environmental changes between processing steps. The practicality of the resulting tool lies in the complementarity of the two techniques. The AFM offers not only true 3D topography maps, something the HIM can only provide in an indirect way but also allows for nanomechanical property mapping, as well as for electrical and magnetic characterization of the sample after focused ion beam materials modification with the HIM. The experimental setup is described and evaluated through a series of correlative experiments, demonstrating the feasibility of the integration.

physics.ins-det↗

arXiv2019

Site-controlled formation of single Si nanocrystals in a buried SiO2 matrix using ion beam mixing

For future nanoelectronic devices - such as room-temperature single electron transistors - the site-controlled formation of single Si nanocrystals (NCs) is a crucial prerequisite. Here, we report an approach to fabricate single Si NCs via medium-energy Si+ or Ne+ ion beam mixing of Si into a buried SiO2 layer followed by thermally activated phase separation. Binary collision approximation and kinetic Monte Carlo methods are conducted to gain atomistic insight into the influence of relevant experimental parameters on the Si NC formation process. Energy-filtered transmission electron microscopy is performed to obtain quantitative values on the Si NC size and distribution in dependence of the layer stack geometry, ion fluence and thermal budget. Employing a focused Ne+ beam from a helium ion microscope, we demonstrate site-controlled self-assembly of single Si NCs. Line irradiation with a fluence of 3000 Ne+/nm2 and a line width of 4 nm leads to the formation of a chain of Si NCs, and a single NC with 2.2 nm diameter is subsequently isolated and visualized in a few nanometer thin lamella prepared by a focused ion beam (FIB). The Si NC is centered between the SiO2 layers and perpendicular to the incident Ne+ beam.

cond-mat.mes-hall↗

arXiv2019

Morphology modifcation of Si nanopillars under ion irradiation at elevated temperatures: plastic deformation and controlled thinning to 10 nm

Si nanopillars of less than 50 nm diameter have been irradiated in a helium ion microscope with a focused Ne$^+$ beam. The morphological changes due to ion beam irradiation at room temperature and elevated temperatures have been studied with the transmission electron microscope. We found that the shape changes of the nanopillars depend on irradiation-induced amorphization and thermally driven dynamic annealing. While at room temperature, the nanopillars evolve to a conical shape due to ion-induced plastic deformation and viscous flow of amorphized Si, simultaneous dynamic annealing during the irradiation at elevated temperatures prevents amorphization which is necessary for the viscous flow. Above the critical temperature of ion-induced amorphization, a steady decrease of the diameter was observed as a result of the dominating forward sputtering process through the nanopillar sidewalls. Under these conditions the nanopillars can be thinned down to a diameter of 10 nm in a well-controlled manner. A deeper understanding of the pillar thinning process has been achieved by a comparison of experimental results with 3D computer simulations based on the binary collision approximation.

physics.app-ph↗

arXiv2015

Nanometer scale elemental analysis in the helium ion microscope using time of flight spectrometry

Time of flight backscattering spectrometry (ToF-BS) was successfully implemented in a helium ion microscope (HIM). Its integration introduces the ability to perform laterally resolved elemental analysis as well as elemental depth profiling on the nm scale. A lateral resolution of $\leq$ 54 nm and a time resolution of $Δt \leq$ 17 ns $(Δt/t \leq 5.4\%)$ are achieved. By using the energy of the backscattered particles for contrast generation, we introduce a new imaging method to the HIM allowing direct elemental mapping as well as local spectrometry. In addition laterally resolved time of flight secondary ion mass spectrometry (ToF-SIMS) can be performed with the same setup. Time of flight is implemented by pulsing the primary ion beam. This is achieved in a cost effective and minimal invasive way that does not influence the high resolution capabilities of the microscope when operating in standard secondary electron (SE) imaging mode. This technique can thus be easily adapted to existing devices. The particular implementation of ToF-BS and ToF-SIMS techniques are described, results are presented and advantages, difficulties and limitations of this new techniques are discussed.

physics.ins-det↗

arXiv2015

Visualization of steps and surface reconstructions in Helium Ion Microscopy with atomic precision

Helium Ion Microscopy is known for its surface sensitivity and high lateral resolution. Here, we present results of a Helium Ion Microscopy based investigation of a surface confined alloy of Ag on Pt(111). Based on a change of the work function of 25\,meV across the atomically flat terraces we can distinguish Pt rich from Pt poor areas and visualize the single atomic layer high steps between the terraces. Furthermore, dechanneling contrast has been utilized to measure the periodicity of the hcp/fcc pattern formed in the 2--3 layers thick Ag/Pt alloy film. A periodicity of 6.65\,nm along the $\langle\overline{11}2\rangle$ surface direction has been measured. In terms of crystallography a hcp domain is obtained through a lateral displacement of a part of the outermost layer by $1/\sqrt{3}$ of a nearest neighbour spacing along $\langle\overline{11}2\rangle$. This periodicity is measured with atomic precision: coincidence between the Ag and the Pt lattices is observed for 23 Ag atoms on 24 Pt atoms. The findings are perfectly in line with results obtained with Low Energy Electron Microscopy and Phase Contrast Atomic Force Microscopy.

cond-mat.mtrl-sci↗

arXiv2014

Helium Ion Microscopy

Helium Ion Microcopy (HIM) based on Gas Field Ion Sources (GFIS) represents a new ultra high resolution microscopy and nano-fabrication technique. It is an enabling technology that not only provides imagery of conducting as well as uncoated insulating nano-structures but also allows to create these features. The latter can be achieved using resists or material removal due to sputtering. The close to free-form sculpting of structures over several length scales has been made possible by the extension of the method to other gases such as Neon. A brief introduction of the underlying physics as well as a broad review of the applicability of the method is presented in this review.

cond-mat.mtrl-sci↗

arXiv2013

To see or not to see: Imaging surfactant coated nano--particles using HIM and SEM

Nano--particles are of great interest in fundamental and applied research. However, their accurate visualization is often difficult and the interpretation of the obtained images can be complicated. We present a comparative scanning electron microscopy and helium ion microscopy study of cetyltrimethylammonium--bromide (CTAB) coated gold nano--rods. Using both methods we show how the gold core as well as the surrounding thin CTAB shell can selectively be visualized. This allows for a quantitative determination of the dimensions of the gold core or the CTAB shell. The obtained CTAB shell thickness of 1.0 nm--1.5 nm is in excellent agreement with earlier results using more demanding and reciprocal space techniques.

cond-mat.mtrl-sci↗

arXiv2013

Nucleation and growth of thin films of rod--like conjugated molecules

Thin films formed from small molecules rapidly gain importance in different technological fields. To explain their growth, methods developed for zero--dimensional atoms as the film forming particles are applied. However, in organic thin film growth the dimensionality of the building blocks comes into play. Using the special case of the model molecule para--Sexiphenyl, we will emphasize the challenges that arise from the anisotropic and one--dimensional nature of building blocks. Differences or common features with other rodlike molecules will be discussed. The typical morphologies encountered for this group of molecules and the relevant growth modes will be investigated. Special attention is given to the transition between flat lying and upright orientation of the building blocks during nucleation. We will further discuss methods to control the molecular orientation and describe the involved diffusion processes qualitatively and quantitatively.

cond-mat.mtrl-sci↗