arXiv subjects

Gunnar K. Pálsson

Publications and source records attributed to Gunnar K. Pálsson.

10 recordsLinked to original sources

arXiv2026

Probing deuterium-induced magnetic phase transitions in TbCo alloys with in-situ polarized neutron reflectometry

Hydrogen-based magneto-ionics is a promising approach for rapid magnetoelectric control of spintronic devices. Most investigations so far into the magneto-ionic manipulation of rare-earth transition-metal alloys have used electrochemical methods for evaluating the magnetoelectric properties, but this technique makes it difficult to discriminate between the effects of competing ionic species. In this work, we use atmospheric loading to evaluate the effect of an isotope of hydrogen, deuterium, on the magnetic properties of TbCo films using in-situ polarized neutron reflectometry. With this approach, we are able to simultaneously measure the magnetization, thickness expansion and deuterium concentration of TbCo films. We quantitatively observe the deuterium concentrations at which the paramagnetic phase transition occurs for a Tb-rich film, and the weakening of out-of-plane magnetic anisotropy for a Co-rich film. For the Tb-rich film the expansion of the film thickness is the primary mechanism identified for the paramagnetic phase transition, while for the Co-rich film no thickness expansion is observed. We also find that an oxidized interface is insensitive to deuterium loading, but remains exchange coupled to the rest of the film and can be indirectly manipulated by loading of deuterium in the alloy. We expect these results to be directly translatable to that of hydrogen.

cond-mat.mtrl-sci↗

arXiv2026

GenL: An extensible fitting program for Laue oscillations and whole pattern fitting

GenL is a flexible program that can be used to simulate and/or fit X-ray reflectivity and X-ray diffraction data from epitaxial thin films exhibiting, for example, Laue oscillations. It utilizes a differential evolution within a genetic algorithm for fitting data and uses a modular approach based on either the kinematic theory of diffraction or the dynamic theory. Effects of polarization, absorption, the Lorentz factor, as well as instrumental resolution and lattice vibrations are taken into account. Useful parameters that can be extracted after fitting include atomic interplanar spacings, number of coherently scattering atomic planes, strain profiles along the film thickness, and crystal roughness. The program has been developed in MATLAB and employs a graphical user interface. The deployment strategy is twofold, whereby the software can either be obtained in source code form and executed within the MATLAB environment, or as a pre-compiled binary for those who prefer not to run it within MATLAB. Finally, GenL can be easily extended to simulate multilayered film systems, superlattices, and films with atomic steps. The program is released under the GNU General Public License.

cond-mat.mtrl-sci↗

arXiv2024

Inverse spillover and dimensionality effects on interstitial hydrogen

Nanoscaling interstitial metal hydrides offers opportunities for hydrogenation applications by enhancing kinetics, increasing surface area, and allowing for tunable properties. The introduction of interfaces impacts hydrogen absorption properties and distribution heterogeneously, making it however challenging to examine the multiple concurrent mechanisms, especially at the atomic level. Here we demonstrate the effect of proximity on interstitial hydrogen in ultrathin single crystalline vanadium films, by comparing hydride formation in identically strained Fe/V- and Cr/V-superlattices. Pressure concentration and excess resistivity isotherms show higher absolute solubility of hydrogen, higher critical temperature and concentration in the Cr/V-superlattice. Direct measurements of hydrogen site location and thermal vibrations show identical occupation of octahedral z sites at room temperature with a vibrational amplitude of 0.20-0.25 Å over a wide range of hydrogen concentrations. Our findings are consistent with a more extended region of hydrogen depletion in the vicinity of Fe compared to Cr, which showcases an inverse of the hydrogen spillover effect. Advancing the understanding of interface effects resolves previously puzzling differences in the hydrogen loading of Fe/V- and Cr/V-superlattices and is relevant for advancing both catalysis and storage.

cond-mat.mtrl-sci↗

arXiv2024

The influence of hydrogen on the electronic structure in transition metallic glasses

We investigate the influence of hydrogen on the electronic structure of a binary transition metallic glass of V$_{80}$Zr$_{20}$. We examine the hybridization between the hydrogen and metal atoms with the aid of hard x-ray photoelectron spectroscopy. Combined with ab initio density functional theory, we are able to show and predict the formation of $s$-$d$ hybridized energy states. With optical transmission and resistivity measurements, we investigate the emergent electronic properties formed out of those altered energy states, and together with the theoretical calculations of the frequency-dependent conductivity tensor, we qualitatively support the observed strong wavelength-dependency of the hydrogen-induced changes on the optical absorption and a positive parabolic change in resistivity with hydrogen concentration.

cond-mat.mtrl-sci↗

arXiv2024

One-shot pair distribution functions of thin films using lab-based x-ray sources

We demonstrate the feasibility of obtaining accurate pair distribution functions of thin amorphous films down to 80 nm, using modern laboratory-based x-ray sources. The pair distribution functions are obtained using a single diffraction scan (one-shot) without the requirement of additional scans of the substrate or of the air. By using a crystalline substrate combined with an oblique scattering geometry, most of the Bragg scattering of the substrate is avoided, rendering the substrate Compton scattering the primary contribution. By utilizing a discriminating energy filter, available in the latest generation of modern detectors, we demonstrate that the Compton intensity can further be reduced to negligible levels at higher wavevector values. We minimize scattering from the sample holder and the air by the systematic selection of pixels in the detector image based on the projected detection footprint of the sample and the use of a 3D printed sample holder. Finally, x-ray optical effects in the absorption factors and the ratios between the Compton intensity of the substrate and film are taken into account by using a theoretical tool that simulates the electric field inside the film and the substrate, which aids in planning both the sample design and measurement protocol.

cond-mat.mtrl-sci↗

arXiv2024

Boundary-induced phase in epitaxial iron layers

We report the discovery of a boundary-induced body-centered tetragonal (bct) iron phase in thin films deposited on MgAl$_{2}$O$_{4}$ ($001$) substrates. We present evidence for this phase using detailed x-ray analysis and ab-initio density functional theory calculations. A lower magnetic moment and a rotation of the easy magnetisation direction are observed, as compared to body-centered cubic (bcc) iron. Our findings expand the range of known crystal and magnetic phases of iron, providing valuable insights for the development of heterostructure devices using ultra-thin iron layers.

cond-mat.mtrl-sci↗

arXiv2023

Epitaxy enhancement in oxide/tungsten heterostructures by harnessing the interface adhesion

The conditions whereby epitaxy is achieved are commonly believed to be mostly governed by misfit strain. We report on a systematic investigation of growth and interface structure of single crystalline tungsten thin films on two different metal oxide substrates, Al$_{2}$O$_{3}$ ($11\bar{2}0$) and MgO ($001$). We demonstrate that despite a significant mismatch, enhanced crystal quality is observed for tungsten grown on the sapphire substrates. This is promoted by stronger adhesion and chemical bonding with sapphire compared to magnesium oxide, along with the restructuring of the tungsten layers close to the interface. The latter is supported by ab initio calculations using density functional theory. Finally, we demonstrate the growth of magnetic heterostructures consisting of high-quality tungsten layers in combination with ferromagnetic CoFe layers, which are relevant for spintronic applications.

cond-mat.mtrl-sci↗

arXiv2022

Influence of misfit strain on the physical properties of Fe thin films

We investigate the growth of thin Fe layers on MgAl$_2$O$_4$~(001) and MgO~(001) substrates using dc magnetron sputtering. The crystal quality of Fe layers deposited on MgAl$_2$O$_4$ is found to be substantially higher as compared to Fe grown on MgO substrates. The effects of the crystal quality on the magnetic and electronic transport properties are discussed.

cond-mat.mtrl-sci↗

arXiv2022

Emergent tri-criticality in magnetic metamaterials

Metallic discs engineered on the 100 nm scale have an internal magnetic texture which varies from a fully magnetized state to a vortex state with zero moment. The interplay between this internal structure and the inter-disc interactions is studied in magnetic metamaterials made of square arrays of the magnetic discs. The texture is modeled by a mesospin of varying length with O(2) symmetry and the inter-disc interaction by a nearest neighbour coupling between mesospins. The thermodynamic properties of the model are studied numerically and an ordering transition is found which varies from Kosterlitz-Thouless to first order via an apparent tri-critical point. The effective critical exponent characterising the finite size magnetization evolves from the value for the 2D-XY model to less than half this value at the tri-critical point. The consequences for future experiments both in and out of equilibrium are discussed.

cond-mat.mes-hall↗

arXiv2012

Hydrogen site occupancy and strength of forces in nano-sized metal hydrides

The dipole force components in nano-sized metal hydrides are quantitatively determined with curvature and x-ray diffraction measurements. Ab-initio density functional theory is used to calculate the dipole components and the symmetry of the strain field. The hydrogen occupancy in a 100 nm thick V film is shown to be tetrahedral with a slight asymmetry at low concentration and a transition to octahedral occupancy is shown to take place at around 0.07 [H/V] at 360 K. When the thickness of the V layer is reduced to 3 nm and biaxially strained, in a Fe_0.5V_0.5/V superlattice, the hydrogen unequivocally occupies octahedral z-like sites, even at and below concentrations of 0.02 [H/V].

cond-mat.mes-hall↗