arXiv subjects

Jens Birch

Publications and source records attributed to Jens Birch.

At least 19 recordsLinked to original sources

arXiv2026

Local coordination, structural softening, and polarization-switching energetics in Sc-alloyed GaN

Sc-alloyed wurtzite nitrides exhibit strongly tunable electromechanical and ferroelectric properties, yet the relationship between their lattice evolution and local bonding environment remains insufficiently established, particularly in Sc$_x$Ga$_{1-x}$N. Here, we investigate the structural and local bonding evolution of Sc$_x$Ga$_{1-x}$N across the dilute-to-intermediate composition range by combining X-ray diffraction, Sc K-edge X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure (EXAFS) and first-principles calculations. Sc incorporation produces an anisotropic lattice expansion and a progressive reduction in the c/a ratio. XANES and EXAFS reveal a concurrent modification of the local environment around Sc, with the effective Sc-N coordination number increasing from 4.1(4) to 4.5(2) and the average Sc-N bond length increasing from 2.045(7) to 2.081(8){\AA} over x=0.06-0.26. The local response is accompanied by a reduction in the Sc K-edge pre-edge intensity, consistent with a gradual reduction of the local tetrahedral asymmetry. First-principles calculations show that these structural changes are associated with site-selective distortions around Sc and progressive flattening of the structural energy landscape. The calculated intrinsic polarization-switching barrier decreases from 24.2 to 19.0 meV/{\AA}$^3$ with increasing Sc content, while the calculated piezoelectric stress coefficient (e$_{33}$) increases from 0.82 to 1.66~C/m$^2$ and elastic constant (C$_{33}$) decreases from 380 to 227~GPa, resulting in an increase of piezoelectric strain coefficient (d$_{33}$) from 2.99 to 12.46~pC/N. These results show that Sc incorporation progressively modifies the local coordination environment and structural energetics of ScGaN while the long-range wurtzite structure remains preserved over the investigated composition range.

cond-mat.mtrl-sci↗

arXiv2025

Optimization of magnetic contrast layer for neutron reflectometry

Neutron reflectivity is a powerful technique for probing density profiles in films, with applications across Physics, Chemistry, and Biology. However, challenges arise when dealing with samples characterized by high roughness, unknown scattering length density (SLD) with low contrast, very thin layers, or complex multi-layered structures, that cannot be uniquely resolved due to the phase problem. Incorporating a magnetic reference layer (MRL) and using polarized neutron reflectivity improves sensitivity and modeling accuracy by providing complementary information. In this study, we introduce a quantitative way to compare MRL systems in a model-free way. We apply this approach to demonstrate that CoTi alloys offer a superior solution as an MRL compared to the commonly used Fe or Ni-based MRLs. The low nuclear and magnetic scattering length densities of CoTi significantly enhance sensitivity, making it particularly advantageous for soft matter research. Furthermore, the tunable Co vs Ti ratio allows for optimization of the SLDs to achieve maximum sensitivity, establishing CoTi as a highly effective choice for MRL applications. The applied simulation framework for optimizing MRL sensitivity to a specific materials system and research question is a generic approach that can be used prior to growing the MRL for a given experiment.

cond-mat.mtrl-sci↗

arXiv2025

Magnetic hysteresis control in thin film Fe/Si multilayers by incorporation of B4C

Magnetic hysteresis properties in Fe/Si multilayers have been studied as a function of the B4C content to control magnetization amplitude, coercivity, and hysteresis tilt, properties that are beneficial to tune for advancing applications in e.g. data storage, spintronics, and sensors. With an ion-assisted magnetron sputtering technique, 35 distinct thin film multilayer samples were prepared and their magnetic and structural properties were characterized by vibrating sample magnetometry, X-ray photoelectron spectroscopy, near edge X-ray absorption fine structure spectroscopy, and X-ray and neutron scattering methods. Key findings indicate that adding B4C lowers the coercivity and can decrease the saturation magnetization, demonstrating the tunability of magnetic responses based on composition. For samples with =30{\AA} periodicity, 10-15% of B4C addition produces antiferromagnetically (AF) coupled multilayers, and such AF coupling strength increases with the B4C content. Our findings reveal that B atoms do not chemically bind within the Fe atoms but instead occupy interstitial positions, disrupting medium- to long-range crystallinity thereby inducing the amorphization. Thereon, the observed effects on magnetic properties are directly attributed to this amorphization process caused by the presence of B4C. The demonstrated ability to finely adjust magnetic properties by varying the B4C content offers a promising approach to overcome challenges in magnetic device performance and efficiency.

cond-mat.mtrl-sci↗

arXiv2025

The Role of 11B4C Interlayers in Enhancing Fe/Si Multilayer Performance for Polarized Neutron Mirrors

This study investigates the effects of incorporating 11B4C interlayers into Fe/Si multilayers, with a focus on interface quality, reflectivity, polarization, and magnetic properties for polarized neutron optics. It is found that the introduction of 1 {\AA} and 2 {\AA} 11B4C interlayers significantly improves the interface sharpness, reducing interface width and preventing excessive Si diffusion into the Fe layers. X-ray reflectivity and polarized neutron reflectivity measurements show enhanced reflectivity and polarization, with a notable increase in polarization for 30 {\AA} period multilayers. The inclusion of interlayers also helps prevent the formation of iron-silicides, improving both the magnetic properties and neutron optical performance. However, the impact of interlayers is less pronounced in thicker-period multilayers (100 {\AA}), primarily due to the ratio between layer and interface widths. These results suggest that 11B4C interlayers offer a promising route for optimizing Fe/Si multilayer performance in polarized neutron mirrors.

cond-mat.mtrl-sci↗

arXiv2025

Diminished spin-flip reflectivity in stacked multilayers with varying period thicknesses of Fe/Si by incorporating 11B4C

This study investigates the effects of 11B4C co-sputtering on the structural and optical properties of Fe/Si stacked multilayers, with a focus on neutron supermirror applications. X-ray and neutron reflectivity techniques reveal that 11B4C incorporation improves interface sharpness, reduces roughness, and enhances reflectivity for all multilayer periods. Neutron reflectivity measurements show reduced spin-flip intensities, while wafer-curvature measurements indicate a 50% reduction in internal stress, allowing for higher mechanical stability of the multilayers. These improvements are attributed to the amorphization of Fe layers, which also suppress the formation of structural and magnetic domains responsible for stress and spin-flip scattering. In contrast, the pure Fe/Si sample exhibits a persistent half-order Bragg peak, indicating residual antiferromagnetic coupling. The results demonstrate that 11B4C enhances neutron optics by reducing spin-flip effects, increasing reflectivity and polarization, and alleviating stress, enabling the use of polarizers at reduced external fields compared to pure Fe/Si multilayers. These findings establish 11B4C as a transformative material for advancing neutron supermirror technology, paving the way for more efficient, stable, and high-performance polarizers in next-generation neutron optics.

cond-mat.mtrl-sci↗

arXiv2025

Persistent Uncorrelated Magnetic Domains in Fe/Si Multilayers and their suppression by incorporating 11B4C

This study investigates magnetic domains in Fe/Si and Fe/Si + B4C multilayers using spin flip off-specular polarized neutron reflectometry. The results show that Fe/Si multilayers exhibit pronounced spin flip off-specular scattering originating from magnetic domains that are uncorrelated out of plane. With increasing external magnetic field the domains progressively coalesce and their magnetization rotates toward alignment with the applied field, approaching a homogeneous magnetic state at higher fields. In contrast, Fe/Si + B4C multilayers exhibit no detectable spin flip off-specular scattering already at low fields, indicating that the multilayer reaches magnetic saturation at significantly lower applied fields. The scattering patterns are interpreted using distorted wave Born approximation simulations in BornAgain, enabled by our added code for simulating magnetic domains and magnetic ordering. To further probe the magnetic behavior, low-energy mu+SR measurements were performed, representing the first mu+SR investigation of polarizing neutron optics multilayers. Together with comparison to previously reported VSM data, these measurements provide insight into the magnetic behavior across short range, medium range, and long range length scales. The results show that incorporating approximately 15 vol.% B4C makes the magnetic configuration highly responsive to external magnetic fields, with clear sensitivity to both in-plane and out-of-plane field geometries. These results show that B4C suppresses magnetic domains and spin flip off-specular scattering, improving Fe/Si coatings for neutron polarization optics in regards to off-specular scattering, and other applications requiring easy magnetic manipulation.

cond-mat.mtrl-sci↗

arXiv2024

Morphology of buried interfaces in ion-assisted magnetron sputter deposited 11B4C-containing Ni/Ti multilayer neutron optics investigated by grazing incidence small angle scattering

Multilayer neutron optics require precise control of interface morphology for optimal performance. In this work, we investigate the effects of different growth conditions on the interface morphology of Ni/Ti based multilayers, with a focus on incorporating low-neutron-absorbing 11B4C and using different ion assistance schemes. Grazing incidence small angle X-ray scattering was used to probe the structural and morphological details of buried interfaces, revealing that the layers become more strongly correlated and the interfaces form mounds with increasing amounts of 11B4C. Applying high flux ion assistance during growth can reduce mound formation but lead to interface mixing, while a high flux modulated ion assistance scheme with an initial buffer layer grown at low ion energy and the top layer at higher ion energy prevents intermixing. The optimal condition was found to be adding 26.0 at.% 11B4C combined with high flux modulated ion assistance. A multilayer with a period of 48.2 Å and 100 periods was grown under these conditions, and coupled fitting to neutron and X-ray reflectivity data revealed an average interface width of only 2.7 Å, a significant improvement over the current state-of-the-art commercial Ni/Ti multilayers. Overall, our study demonstrates that the addition of 11B4C and the use of high flux modulated ion assistance during growth can significantly improve the interface morphology of Ni/Ti multilayers, leading to improved neutron optics performance.

cond-mat.mtrl-sci↗

arXiv2024

Spectral Tuning of Polarization Selective Reflections Bands in GLAD deposited HfAlN chiral sculptured thin films

We present the first report on fabrication of Hafnium aluminum nitride chiral sculptured thin films (CSTFs) using reactive magnetron sputtering in a glancing angle deposition configuration, and the analysis of its optical polarization properties. The resulting CSTFs were designed to give interference extrema or so-called circular Bragg (CB) resonances at desired wavelengths in the region from 370 to 690 nm. This was achieved by tailoring the growth of the chiral thin films to obtain a dielectric pitch between 87 and 260.9 nm. The spectral positions of the obtained CB resonances were compared to values from analytical expressions. Contrary to the common case where the dielectric pitch is half of the growth-related rotational pitch due to a 180° symmetry, this pitch was shown to be the same as the rotational pitch. It is concluded that this is due to the c-axis of the CSTF being tilted about 45° from the substrate normal. The morphology and crystallographic characterizations were done using scanning electron microscopy and X-ray diffraction, respectively, while the tilt of the crystal lattice was corroborated using X-ray diffraction pole figures. The optical response from the CSTFs was analyzed using Mueller matrix spectroscopic ellipsometry from which the degree of circular polarization at the CB resonances was obtained. In addition, a strong non-reciprocal reflection was observed which could be attributed to the helicoidal morphology and the intrinsic crystal tilt. An optical layered model of the chiral structure including azimuthal twist and using the Cauchy dispersion relations was used to simulate the Mueller matrix elements and compare with the ellipsometry measurements. The correlation between the simulated and experimental data gave information of the morphological parameters of the CSTF and its optical properties.

physics.optics↗

arXiv2024

Material design optimization for large-m 11B4C-based Ni/Ti supermirror neutron optics

State-of-the-art Ni/Ti supermirror neutron optics have limited reflected intensity and a restricted neutron energy range due to the interface width. Incorporating low-neutron-absorbing 11B4C enhances reflectivity and allows for thinner layers to be deposited, with which more efficient supermirrors with higher m-values can be realized. However, incorporating 11B4C reduces the optical contrast, limiting the attainable reflectivity at low scattering vectors, making this approach infeasible. This study explores various approaches to optimize the material design of 11B4C-containing Ni/Ti supermirrors to maintain high reflectivity at low scattering vectors and achieve low interface widths at large scattering vectors. The scattering length density contrast versus interface width is investigated for multilayer periods of 30 Å, 48 Å, and 84 Å, for designs involving pure Ni/Ti multilayers, multilayers with 11B4C co-deposited in Ni and Ti layers, multilayers with 11B4C co-deposited only in Ni layers, and multilayers with 11B4C as thin interlayers between Ni and Ti layers. Our results suggest that a depth-graded hybrid material design by incorporating 11B4C inside the Ni and Ti layers, below approximately 26 Å, and introducing 1.5 Å 11B4C interlayers between the thicker Ni and Ti layers can achieve a higher reflectivity than state-of-the-art Ni/Ti multilayers over the entire scattering vector range.

cond-mat.mtrl-sci↗

arXiv2023

Dynamic evolution of internal stress, grain growth, and crystallographic texture in arc-evaporated AlTiN thin films using in-situ synchrotron x-ray diffraction

Understanding the nucleation and growth of polycrystalline thin films is a long-standing goal. Polycrystalline films have many grains with different orientations that affect thin-film properties. Numerous studies have been done to determine these grain size and their preferred crystallographic orientation as well as stress in films. However most past studies have either employed an ex-situ methodology or only monitor the development of macroscopic stress in real-time. There has never been any research done on the simultaneous determination of crystallographic texture, grain size, and microscopic stress in polycrystalline thin films. In this study, we simultaneously monitored the generation and temporal evolution of texture, grain size, and internal stress in cathodic arc evaporated Al0.50Ti0.50N thin films using a bespoke deposition apparatus designed for use with 2-dimensional synchrotron x-ray diffraction technique. The influence of the substrate temperature is investigated in terms of the emergence and development of texture, grain size and stress evolution. A dynamic evolution of the crystallographic texture is observed as the overall film thickness varies. We clearly resolved two regime of films growth based on stress evolution. Beyond a threshold grain size (~ 14 nm), the stress scales inversely to the average grain sizes, and as the film thickness increases, immediate compressive stress relaxation was seen. An extensive ex-situ evaluation of thin films using electron microscopies and electron diffraction was performed to support the in-situ x-ray diffraction results.

cond-mat.mtrl-sci↗

arXiv2023

Electronic and optical properties of core-shell InAlN nanorods: a comparative study via LDA, LDA-1/2, mBJ and $G_0W_0$ methods

Currently, self-induced InAlN core-shell nanorods enjoy an advanced stage of accumulation of experimental data from their growth and characterization as well as a comprehensive understanding of their formation mechanism by the ab initio modeling based on Synthetic Growth Concept. However, their electronic and optical properties, on which most of their foreseen applications are expected to depend, have not been investigated comprehensively. $G_0W_0$ is currently regarded as a gold-standard methodology with quasi-particle corrections to calculate electronic properties of materials in general. It is also the starting point for higher-order methods that study excitonic effects, such as those based on the Bethe-Salpeter equation. One major drawback of $G_0W_0$, however, is its computational cost, much higher than density-functional theory (DFT). Therefore, in many applications, it is highly desirable to answer the question of how well approaches based on DFT, such as e. g. LDA, LDA-1/2, and mBJ, can approximately reproduce $G_0W_0$ results with respect to the electronic and optical properties. Thus, the purpose of the present paper is to investigate how the DFT-based methodologies LDA, LDA-1/2, and mBJ can be used as tools to approximate $G_0W_0$ in studies of the electronic and optical properties of scaled down models of core-shell InAlN nanorods. For these systems, we observed that band gaps, density of states, dielectric functions, refractive indexes, absorption and reflectance coefficients are reasonably well described by LDA-1/2 and mBJ when compared to $G_0W_0$, however, at a much more favorable computational cost.

cond-mat.mtrl-sci↗

arXiv2023

Effects of deposition temperature on the mechanical and structural properties of amorphous Al-Si-O thin films prepared by RF magnetron sputtering

Aluminosilicate (Al-Si-O) thin films containing up to 31 at. % Al and 23 at. % Si were prepared by reactive RF magnetron co-sputtering. Mechanical and structural properties were measured by indentation and specular reflectance infrared spectroscopy at varying Si sputtering target power and substrate temperature in the range 100 to 500 °C. It was found that an increased substrate temperature and Al/Si ratio give denser structure and consequently higher hardness (7.4 to 9.5 GPa) and reduced elastic modulus (85 to 93 GPa) while at the same time lower crack resistance (2.6 to 0.9 N). The intensity of the infrared Si-O-Si/Al asymmetric stretching vibrations shows a linear dependence with respect to Al concentration. The Al-O-Al vibrational band (at 1050 cm-1) shifts towards higher wavenumbers with increasing Al concentration which indicates a decrease of the bond length, evidencing denser structure and higher residual stress, which is supported by the increased hardness. The same Al-O-Al vibrational band (at 1050 cm-1) shifts towards lower wavenumber with increasing substrate temperature indicating an increase in the of the average coordination number of Al.

cond-mat.mtrl-sci↗

arXiv2023

In-situ real-time evolution of intrinsic stresses and microstructure during growth of cathodic arc deposited (Al,Ti)N coatings

The residual stress plays a vital role in determination of the device performance that uses thin films coating and thus the accurate determination of stress and its optimization with process parameters is an ongoing research work for many decades. In line with this, the microscopic origin of the stress at the atomic scale and its development during the thin film deposition is a matter of major scientific interests. The development of stress is a complex phenomenon and has a complex dependence to process parameters, film microstructure and its morphology. In this work, by utilizing a custom-designed cathodic arc deposition system and synchrotron radiation based 2D x-ray diffraction (XRD) technique, we determine the real-time evolution of stress, crystallite sizes and their preferential orientations of Aluminum-Titanium-Nitride (AlxTi1-xN) films with varied Al-content (x=0.0, 0.25, 0.50, and 0.67) on Si-100 substrate. The energies of incoming ions and hence stress in the films is tuned by applying different direct current substrate bias (Vs = floating potential, -20, -40, -60, -80, and -100 V). The instantaneous stress is evaluated by the well-known d vs. sin2ψ technique, while crystallite sizes are determined by analyzing line profiles of x-ray diffractograms. The evolution of stress and crystallite sizes are modelled with multiple numerical models from which kinetic parameters associated with the thin film depositions are extracted. The ex-situ microstructure characterizations of AlxTi1-xN coatings are carried out by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The formation of ex-situ microstructure of the films is discussed considering the results obtained from in-situ XRD data. Finally, we demonstrate that the method utilized here is a powerful approach towards estimation of the fracture toughness of thin film coatings.

cond-mat.mtrl-sci↗

arXiv2022

Effect of substrate roughness and material selection on the microstructure of sputtering deposited boron carbide thin films

Amorphous boron carbide (B4C) thin films are by far the most popular form for the neutron converting layers in the 10B-based neutron detectors, which are a rising trend in detector technologies in response to the increasing scarcity and price of 3He, the standard material for neutron detection. The microstructure of the B4C films is closely related to the important properties, e.g. density and adhesion, for the converting layers, which eventually affect the detection efficiency and the long-term stability of the detectors. To study the influence from substrates of different roughness and materials, the B4C films were deposited on polished Si substrates with Al, Ti, and Cu buffer layers and unpolished Si, Al, Ti, and Cu substrates by direct current magnetron sputtering at a substrate temperature of 623 K. The tapered columnar grains and nodular defects, generally observed in SEM images, indicated a strong shadowing effect where voids were introduced around the grains. The change in the grain size did not show a direct dependence to the substrate roughness, acquired from the surface profile, nor to the mass density of the films, obtained from reflectivity patterns. However, films with non-uniform size of columnar grains were deposited on substrates with high skewness, leading to a drop of mass density from ~95 % down to ~70 % of tabulated bulk density. On the other hand, similar microstructures and mass density were obtained from the films deposited on Al, Ti, and Cu of different roughness and good adhesion were observed from cross-cut adhesion tests, showing the reliability of sputtering deposited B4C films on common structural materials in neutron detectors.

cond-mat.mtrl-sci↗

arXiv2020

Microscopic Origin of Structural Disorder in $δ$-NbN: Correlation of Superconductivity and Electronic Structure

Rock-salt type niobium nitride ($δ$-NbN) is a well-known superconductor having superconducting transition temperature (Tc) $\approx$ 18\,K and a large superconducting gap $\approx$3\,meV. The Tc of $δ$-NbN thin film exhibits a large scattering irrespective of the growth conditions and lattice parameter. In this work, we investigate the atomic origin of suppression of Tc in $δ$-NbN thin film by employing combined methods of experiments and ab-initio simulations. Sputtered $δ$-NbN thin films with different disorder were analyzed through electrical resistivity and x-ray absorption spectroscopy. A strong correlation between the superconductivity and the atomic distortion induced electronic reconstruction was observed. The theoretical analysis revealed that under N-rich growth conditions, atomic and molecular N-interstitial defects assisted by cation vacancies form spontaneously and are responsible for the suppression of Tc in $δ$-NbN by smearing its electronic densities of states around Fermi level.

cond-mat.supr-con↗

arXiv2019

Optical and mechanical properties of amorphous Mg-Si-O-N thin films deposited by reactive magnetron sputtering

In this work, amorphous thin films in Mg-Si-O-N system were prepared in order to investigate the dependence of optical and mechanical properties on Mg composition. Reactive RF magnetron co-sputtering from magnesium and silicon targets were used for the deposition of Mg-Si-O-N thin films. Films were deposited on float glass, silica wafers and sapphire substrates in an Ar, N2 and O2 gas mixture. X-ray photoelectron spectroscopy, atomic force microscopy, scanning electron microscopy, spectroscopic ellipsometry, and nanoindentation were employed to characterize the composition, surface morphology, and properties of the films.

physics.app-ph↗

arXiv2016

First Measurements with New High-Resolution Gadolinium-GEM Neutron Detectors

European Spallation Source instruments like the macromolecular diffractometer, NMX, require an excellent neutron detection efficiency, high-rate capabilities, time resolution, and an unprecedented spatial resolution in the order of a few hundred micrometers over a wide angular range of the incoming neutrons. For these instruments solid converters in combination with Micro Pattern Gaseous Detectors (MPGDs) are a promising option. A GEM detector with gadolinium converter was tested on a cold neutron beam at the IFE research reactor in Norway. The μTPC analysis, proven to improve the spatial resolution in the case of $^{10}$B converters, is extended to gadolinium based detectors. For the first time, a Gd-GEM was successfully operated to detect neutrons with a measured efficiency of 11.8% at a wavelength of 2 Å and a position resolution better than 250 μm.

physics.ins-det↗

arXiv2015

The uTPC Method: Improving the Position Resolution of Neutron Detectors Based on MPGDs

Due to the Helium-3 crisis, alternatives to the standard neutron detection techniques are becoming urgent. In addition, the instruments of the European Spallation Source (ESS) require advances in the state of the art of neutron detection. The instruments need detectors with excellent neutron detection efficiency, high-rate capabilities and unprecedented spatial resolution. The Macromolecular Crystallography instrument (NMX) requires a position resolution in the order of 200 um over a wide angular range of incoming neutrons. Solid converters in combination with Micro Pattern Gaseous Detectors (MPGDs) are proposed to meet the new requirements. Charged particles rising from the neutron capture have usually ranges larger than several millimetres in gas. This is apparently in contrast with the requirements for the position resolution. In this paper, we present an analysis technique, new in the field of neutron detection, based on the Time Projection Chamber (TPC) concept. Using a standard Single-GEM with the cathode coated with 10B4C, we extract the neutron interaction point with a resolution of better than sigma = 200 um.

physics.ins-det↗