arXiv subjects

Daniel Primetzhofer

Publications and source records attributed to Daniel Primetzhofer.

At least 37 records · Page 2Linked to original sources

arXiv2024

Magnetic metamaterials by ion-implantation

We present a method for the additive fabrication of planar magnetic nanoarrays with minimal surface roughness. Synthesis is accomplished by combining electron-beam lithography, used to generate nanometric patterned masks, with ion implantation in thin films. By implanting $^{56}$Fe$^{+}$ ions, we are able to introduce magnetic functionality in a controlled manner into continuous Pd thin films, achieving 3D spatial resolution down to a few tens of nanometers. Our results demonstrate the application of this technique in fabricating square artificial spin ice lattices, which exhibit well-defined magnetization textures and interactions among the patterned magnetic elements.

cond-mat.mtrl-sci↗

arXiv2024

Sputter Yields of the Lunar Surface: Experimental Validation and Numerical Modelling of Solar Wind Sputtering of Apollo 16 Soils

Sputtering by solar wind ions is a key process driving the ejection of high-energy particles into the exospheres of airless bodies like asteroids, Mercury and the Moon. In view of upcoming missions which will deliver new in-situ data on these exospheres like the Artemis program at the Moon and BepiColombo at Mercury, a deeper understanding of sputtering effects is crucial. In this work, we combine sensitive quartz crystal microbalance measurements and numerical simulations to quantify sputter yields of Apollo soil 68501 under solar wind relevant conditions. We find that none of the commonly used simulation codes can reliably predict laboratory sputter yields without experimental benchmarks. All of the employed packages significantly overestimate the sputter yields of flat samples by at least a factor of 2 for the case of hydrogen. When accounting for surface roughness and regolith-like porosity, sputter yields are decreased even further to 7.3E-3 atoms\ion and 7.6E-2 atoms\ion for H and He at solar wind energies of 1 keV\amu, respectively. The reduced yields of porous regolith structures are largely independent of the ion incidence angle, making them applicable across a wide range of lunar latitudes. This study highlights the need for experimental validation of sputtering models to ensure accurate predictions for space weathering and lunar exosphere composition.

astro-ph.EP↗

arXiv2024

All epitaxial self-assembly of vertically-confined silicon color centers using ultra-low temperature epitaxy

Silicon-based color-centers (SiCCs) have recently emerged as quantum-light sources that can be combined with telecom-range Si Photonics platforms. Unfortunately, using current SiCC fabrication, deterministic control over the vertical emitter position is impossible due to ion-implantation's stochastic nature. To overcome this bottleneck towards high-yield integration, we demonstrate a radically innovative creation method for various SiCCs, solely relying on epitaxial growth of Si and C-doped Si at atypically-low temperatures in a ultra-clean growth environment. These telecom emitters can be confined within sub-1nm thick layers embedded at arbitrary vertical positions within a highly crystalline Si matrix. Tuning growth conditions and doping, different SiCC types, e.g., W-centers, T-centers, G-centers, or derivatives like G'-centers can be created, which are particularly promising as Si-based single-photon sources and spin-photon interfaces. The zero-phonon emission from G'-centers can be conveniently tuned by the C-concentration, leading to a systematic wavelength shift and linewidth narrowing towards low emitter densities.

cond-mat.mes-hall↗

arXiv2024

Local electronic excitations by slow light ions in tungsten

Accurately predicting the electronic energy deposition of ions in materials is an important challenge in both fundamental and applied research. While employing ab initio simulations to investigate electronic stopping of ions in matter holds promise, its combined use with experimental measurements paves the way for obtaining reliable data. In this paper, we present a collaborative study using real-time time-dependent density functional theory and experimental methods to determine the electronic stopping power of hydrogen and helium ions in tungsten, a primary candidate material for future nuclear fusion devices. While calculated stopping powers in hyperchanneling trajectories are significantly lower than the experimental data, off-center and random geometries demonstrate a better agreement. We show that the deviation from velocity proportionality for both projectiles traversing the hyperchanneling directions can be explained through the existence of a threshold velocity leading to the activation of semicore states. Additionally, we analyse the pseudopotential and the trajectory dependence of computed electronic energy losses. It is demonstrated that the role of including inner-shell electrons varies depending on the velocity range. While these states play a crucial role at high projectile velocities by introducing additional dissipation channels, their impact diminishes in the low-velocity range. Finally, we introduce a simple expression that links electronic energy losses in different trajectories to local electron density, and we show that utilizing this formula allows for quite accurate predictions of stopping powers around the Bragg peak.

cond-mat.mtrl-sci↗

arXiv2023

Valence electron concentration- and N vacancy-induced elasticity in cubic early transition metal nitrides

Motivated by frequently reported deviations from stoichiometry in cubic transition metal nitride (TMNx) thin films, the effect of N-vacancy concentration on the elastic properties of cubic TiNx, ZrNx, VNx, NbNx, and MoNx (0.72<x<1.00) is systematically studied by density functional theory (DFT) calculations. The predictions are validated experimentally for VNx (0.77<x<0.97). The DFT results indicate that the elastic behavior of the TMNx depends on both the N-vacancy concentration and the valence electron concentration (VEC) of the transition metal: While TiNx and ZrNx exhibit vacancy-induced reductions in elastic modulus, VNx and NbNx show an increase. These trends can be rationalized by considering vacancy-induced changes in elastic anisotropy and bonding. While introduction of N-vacancies in TiNx results in a significant reduction of elastic modulus along all directions and a lower average bond strength of Ti-N, the vacancy-induced reduction in [001] direction of VNx is overcompensated by the higher stiffness along [011] and [111] directions, resulting in a higher average bond strength of V-N. To validate the predicted vacancy-induced changes in elasticity experimentally, close-to-single-crystal VNx (0.77<x<0.97) are grown on MgO(001) substrates. As the N-content is reduced, the relaxed lattice parameter a0, as probed by X-ray diffraction, decreases from 4.128 A to 4.096 A. This reduction in lattice parameter is accompanied by an anomalous 11% increase in elastic modulus, as determined by nanoindentation. As the experimental data agree with the predictions, the elasticity enhancement in VNx upon N-vacancy formation can be understood based on the concomitant changes in elastic anisotropy and bonding.

cond-mat.mtrl-sci↗

arXiv2023

On the determination of the thermal shock parameter of MAX phases: A combined experimental-computational study

Thermal shock resistance is one of the performance-defining properties for applications where extreme temperature gradients are required. The thermal shock resistance of a material can be described by means of the thermal shock parameter RT. Here, the thermo-mechanical properties required for the calculation of RT are quantum-mechanically predicted, experimentally determined, and compared for Ti3AlC2 and Cr2AlC MAX phases. The coatings are synthesized utilizing direct current magnetron sputtering without additional heating, followed by vacuum annealing. It is shown that the RT of both Ti3AlC2 and Cr2AlC obtained via simulations are in good agreement with the experimentally obtained ones. Comparing the MAX phase coatings, both experiments and simulations indicate superior thermal shock behavior of Ti3AlC2 compared to Cr2AlC, attributed primarily to the larger linear coefficient of thermal expansion of Cr2AlC. The results presented herein underline the potential of ab initio calculations for predicting the thermal shock behavior of ionically-covalently bonded materials.

cond-mat.mtrl-sci↗

arXiv2023

Room-temperature spin glass behavior in zinc ferrite epitaxial thin films

Zinc ferrite (ZnFe$_{\text{2}}$O$_{\text{4}}$) epitaxial thin films were grown by reactive magnetron sputtering on MgAl$_{\text{2}}$O$_{\text{4}}$ and Al$_{\text{2}}$O$_{\text{3}}$ substrates varying a range of preparation parameters. The resulting structural and magnetic properties were investigated using a range of experimental techniques confirming epitaxial growth of ZnFe$_{\text{2}}$O$_{\text{4}}$ with the nominal stoichiometric composition and long range magnetic order at and above room temperature. The main preparation parameter influencing the temperature $T_{\text{f}}$ of the bifurcation between $M(T)$ curves under field cooled and zero-field cooled conditions was found to be the growth rate of the films, while growth temperature or the Ar:O$_2$ ratio did not systematically influence $T_{\text{f}}$. Furthermore $T_{\text{f}}$ was found to be systematically higher for MgAl$_{\text{2}}$O$_{\text{4}}$ as substrate and $T_{\text{f}}$ extends to above room temperature. While in some samples $T_{\text{f}}$ seems to be more likely correlated with superparamagentism, the highest $T_{\text{f}}$ occurs in ZnFe$_{\text{2}}$O$_{\text{4}}$ epitaxial films where experimental signatures of magnetic glassiness can be found. Element-selective X-ray magnetic circular dichroism measurements aim at associating the magnetic glassiness with the occurrence of a different valence state and lattice site incorporation of Fe pointing to a complex interplay of various competing magnetic interactions in ZnFe$_{\text{2}}$O$_{\text{4}}$.

cond-mat.other↗

arXiv2023

Trajectory dependence of electronic energy-loss straggling at keV ion energies

We have measured the electronic energy-loss straggling of protons, helium, boron and silicon ions in silicon using a transmission time-of-flight approach. Ions with velocities between 0.25 and 1.6 times the Bohr velocity were transmitted through single-crystalline Si(100) nanomembranes in either channelling or random geometry to study the impact parameter dependence of energy-loss straggling. Nuclear and path length contributions to the straggling were determined with the help of Monte Carlo simulations. Our results exhibit an increase in straggling with increasing ion velocity for channelled trajectories for all projectiles as well as for protons and helium in random geometry. In contrast for heavier ions, electronic straggling at low velocities does not decrease further but plateaus and even seems to increase again. We compare our experimental results with transport cross section calculations. The satisfying agreement for helium shows that electronic stopping for light ions is dominated by electron-hole pair excitations, and that the previously observed trajectory dependence can indeed be attributed to a higher mean charge state for random trajectories. No agreement is found for boron and silicon indicating the breakdown of models based solely on electron-hole pair excitations, and that local electron-promotion and charge-exchange events significantly contribute to energy loss at low velocities.

cond-mat.mtrl-sci↗

arXiv2022

Phase formation in CrFeCoNi nitride thin films

As a single-phase alloy, CrFeCoNi is a face centered cubic (fcc) material related to the archetypical high-entropy Cantor alloy CrFeCoNiMn. For thin films, CrFeCoNi of approximately equimolar composition tends to assume an fcc structure when grown at room temperature by magnetron sputtering. However, the single-phase solid solution state is typically not achieved for thin films grown at higher temperatures. The same holds true for Cantor alloy-based ceramics (nitrides and oxides), where phase formation is extremely sensitive to process parameters such as the amount of reactive gas. This study combines theoretical and experimental methods to understand the phase formation in nitrogen-containing CrFeCoNi thin films. Density functional theory calculations considering three competing phases (CrN, Fe-Ni and Co) show that the free energy of mixing, delta G of (CrFeCoNi)1-xNx solid solutions has a maximum at x = 0.20-0.25, and delta G becomes lower when x less than 0.20, greater than 0.25. Thin films of (CrFeCoNi)1-xNx (x = 0.14-0.41) grown by magnetron sputtering show stabilization of the metallic fcc when x lesser than or equal to 0.22 and the stabilization of the NaCl B1 structure when x is greater than 0.33, consistent with the theoretical prediction. In contrast, films with intermediate amounts of nitrogen (x = 0.22) grown at higher temperatures show segregation into multiple phases of CrN, Fe-Ni-rich and Co. These results offer an explanation for the requirement of kinetically limited growth conditions at low temperature for obtaining single-phase CrFeCoNi Cantor-like nitrogen-containing thin films and are of importance for understanding the phase-formation mechanisms in multicomponent ceramics.

cond-mat.mtrl-sci↗

arXiv2022

Yttrium incorporation in Cr2AlC: On the metastable phase formation and decomposition of (Cr,Y)2AlC MAX phase thin films

Herein we report on the synthesis of a metastable (Cr,Y)2AlC MAX phase solid solution by co-sputtering from a composite Cr-Al-C and elemental Y target, at room temperature, followed by annealing. While direct high-temperature synthesis resulted in multiphase films, as evidenced by X-ray diffraction analyses, room temperature depositions, followed by annealing to 760 °C led to the formation of phase pure (Cr,Y)2AlC by diffusion. Higher annealing temperatures caused decomposition of the metastable phase into Cr2AlC, Y5Al3 , and Cr-carbides. In contrast to pure Cr2AlC, the Y-containing phase crystallizes directly in the MAX phase structure instead of first forming a disordered solid solution. Furthermore, the crystallization temperature was shown to be Y-content dependent and was increased by ~200 °C for 5 at.% Y compared to Cr2AlC. Calculations predicting the metastable phase formation of (Cr,Y)2AlC and its decomposition are in excellent agreement with the experimental findings.

cond-mat.mtrl-sci↗

arXiv2021

Control of site occupancy by variation of the Zn and Al content in NiZnAl ferrite epitaxial films with low magnetic damping

The structural and magnetic properties of Zn/Al doped nickel ferrite thin films can be adjusted by changing the Zn and Al content. The films are epitaxially grown by reactive magnetron sputtering using a triple cluster system to sputter simultaneously from three different targets. Upon the variation of the Zn content the films remain fully strained with similar structural properties, while the magnetic properties are strongly affected. The saturation magnetization and coercivity as well as resonance position and linewidth from ferromagnetic resonance (FMR) measurements are altered depending on the Zn content in the material. The reason for these changes can be elucidated by investigation of the x-ray magnetic circular dichroism spectra to gain site and valence specific information with elemental specificity. Additionally, from a detailed investigation by broadband FMR a minimum in g-factor and linewidth could be found as a function of film thickness. Furthermore, the results from a variation of the Al content using the same set of measurement techniques is given. Other than for Zn, the variation of Al affects the strain and even more pronounced changes to the magnetic properties are apparent.

cond-mat.mtrl-sci↗

arXiv2021

On the correlation of angular distributions of keV ions and trajectory-dependent electronic excitations in transmission channelling geometry

We use energy discrimination of keV ions transmitted through a thin, single-crystalline silicon membrane to correlate specific angular distribution patterns formed in channelling geometry with trajectory-dependent electronic energy loss. The integral energy and intensity distribution of transmitted ions can thus be dissected into on one side axially channelled projectiles travelling along rather straight trajectories and on the other side dechannelled projectiles predominantly experiencing blocking. Angular distributions of transmitted ions are further simulated with two different Monte-Carlo codes.

physics.app-ph↗

arXiv2021

$\textit{Ab initio}$-guided X-ray photoelectron spectroscopy quantification of Ti vacancies in Ti$_{1-δ}$O$_x$N$_{1-x}$ thin films

$\textit{Ab initio}$ calculations were employed to investigate the effect of oxygen concentration dependent Ti vacancies formation on the core electron binding energy shifts in cubic titanium oxynitride (Ti$_{1-δ}$O$_x$N$_{1-x}$). It was shown, that the presence of a Ti vacancy reduces the 1s core electron binding energy of the first N neighbors by $\sim$0.6 eV and that this effect is additive with respect to the number of vacancies. Hence it is predicted that the Ti vacancy concentration can be revealed from the intensity of the shifted components in the N 1s core spectra region. This notion was critically appraised by fitting the N 1s region obtained via X-ray photoelectron spectroscopy (XPS) measurements of Ti$_{1-δ}$O$_x$N$_{1-x}$ thin films deposited by high power pulsed magnetron sputtering. A model to quantify the Ti vacancy concentration based on the intensity ratio between the N 1s signal components, corresponding to N atoms with locally different Ti vacancy concentration, was developed. Herein a random vacancy distribution was assumed and the influence of surface oxidation from atmospheric exposure after deposition was considered. The so estimated vacancy concentrations are consistent with a model calculating the vacancy concentration based on the O concentrations determined by elastic recoil detection analysis and text book oxidation states and hence electroneutrality. Thus, we have unequivocally established that the formation and population of Ti vacancies in cubic Ti$_{1-δ}$O$_x$N$_{1-x}$ thin films can be quantified by XPS measurements from N 1s core electron binding energy shifts.

cond-mat.mtrl-sci↗

arXiv2021

Tailoring magnetic order via atomically stacking 3d/5d electrons

The ability to tune magnetic orders, such as magnetic anisotropy and topological spin texture, is desired in order to achieve high-performance spintronic devices. A recent strategy has been to employ interfacial engineering techniques, such as the introduction of spin-correlated interfacial coupling, to tailor magnetic orders and achieve novel magnetic properties. We chose a unique polar-nonpolar LaMnO3/SrIrO3 superlattice because Mn (3d)/Ir (5d) oxides exhibit rich magnetic behaviors and strong spin-orbit coupling through the entanglement of their 3d and 5d electrons. Through magnetization and magnetotransport measurements, we found that the magnetic order is interface-dominated as the superlattice period is decreased. We were able to then effectively modify the magnetization, tilt of the ferromagnetic easy axis, and symmetry transition of the anisotropic magnetoresistance of the LaMnO3/SrIrO3 superlattice by introducing additional Mn (3d) and Ir (5d) interfaces. Further investigations using in-depth first-principles calculations and numerical simulations revealed that these magnetic behaviors could be understood by the 3d/5d electron correlation and Rashba spin-orbit coupling. The results reported here demonstrate a new route to synchronously engineer magnetic properties through the atomic stacking of different electrons, contributing to future applications.

cond-mat.str-el↗

arXiv2020

Photochromic response of encapsulated oxygen-containing yttrium hydride thin films

Photochromic oxygen$-$containing yttrium$-$hydride thin films are synthesized by argon$-$magnetron sputtering on microscope slides. Some of them are encapsulated with a thin, transparent and non$-$photochromic diffusion-barrier layer of either Al2O3 or Si3N4. Ion beam-based methods prove that these protective diffusion barriers are stable and free from pinholes, with thicknesses of only a few tens of nanometers. Optical spectrophotometry reveals that the photochromic response and relaxation time for both $-$ protected and unprotected $-$ samples are almost identical. Ageing effects in the unprotected films lead to degradation of the photochromic performance (self$-$delamination) while the photochromic response for the encapsulated films is stable. Our results show that the environment does not play a decisive role for the photochromic process and encapsulation of oxygen containing rare-earth hydride films with transparent and non-organic thin diffusion barrier layers provides long-time stability of the films, mandatory for applications as photochromic coatings on e.g., smart windows.

cond-mat.mtrl-sci↗

arXiv2020

Trajectory-dependent electronic excitations by light and heavy ions around and below the Bohr velocity

We present experiments demonstrating trajectory-dependent electronic excitations at low ion velocities, where ions are expected to primarily interact with delocalized valence electrons. The energy loss of H$^+$, H$_2 ^+$, He$^+$, B$^+$, N$^+$, Ne$^+$, $^{28/29}$Si$^+$ and Ar$^+$ in self-supporting silicon membranes was analysed along channelled and random trajectories in a transmission approach. For all ions, we observe a difference in electronic stopping dependent on crystal orientation. For heavier ions, the energy-loss difference between channelling and random geometry is generally found more pronounced, and, in contrast to protons, increases for decreasing ion energy. Due to the inefficiency of core-electron excitations at employed ion velocities, we explain these results by reionization events occurring in close collisions of ions with target atoms, which are heavily suppressed for channelled trajectories. These processes result in trajectory-dependent mean charge states, which strongly affects the energy loss. The strength of the effect seems to exhibit a Z$_1$ oscillation with an observed minimum for Ne. We, furthermore, demonstrate that the simplicity of our experimental geometry leads to results that can serve as excellent benchmark systems for dynamic calculations of the electronic systems of solids using time-dependent density functional theory.

cond-mat.mtrl-sci↗

arXiv2020

Unprecedented Severe Atomic Redistribution in Germanium Induced by MeV Self-Irradiation

We present a pronounced unprecedented surface modification of a crystalline Ge layer under heavy ion irradiation with a Ge ion beam at high energy of 2.5 MeV. Under the irradiation conditions, the Ge layer did not become porous as observed for other projectiles and lower energies but develops into an uneven ripple morphology in which the roughness monotonically increases with the irradiation doses. We show that this phenomenon is caused neither by surface erosion effect nor by a non-uniform volumetric expansion. Rather, atomic redistribution in the bulk of the material is the only drive for the ripple surface. Furthermore, the deformation of the Ge layer likely occurs to largest extend after irradiation, as indicated by the very flat interface around the end-of-range region. The observed morphology modification is discussed based on irradiation-induced plastic flow, coupled with a larger contribution of the electronic component in the ion-solid interactions.

cond-mat.mtrl-sci↗

arXiv2020

In-situ Nanoscale Characterization of Composition and Structure during Formation of Ultrathin Nickel Silicide

We characterize composition and structure of ultrathin nickel silicide during formation from 3 nm Ni films on Si(100) using in-situ high resolution ion scattering and high resolution transmission electron microscopy. We show the transition to occur in discrete steps, in which an intermediate phase is observed within a narrow range of temperature from 230 oC to 290 oC. The film composition of this intermediate phase is found to be 50% Ni:50% Si, without evidence for long-range structure, indicating the film to be a homogeneous monosilicide NiSi phase. The final phase is resemblant of the cubic disilicide NiSi2, but with slightly off-stoichiometric composition of 38% Ni and 62% Si. Along the [100] axis, the lattices of the film and the substrate are found in perfect alignment. Due to the epitaxial growth of the silicide, a contraction of the c lattice constant of the film by 0.7-1% is detected.

cond-mat.mtrl-sci↗