arXiv subjects

Snorri Ingvarsson

Publications and source records attributed to Snorri Ingvarsson.

15 recordsLinked to original sources

arXiv2025

Spinor formulation of the Landau-Lifshitz-Gilbert equation with geometric algebra

The Landau-Lifshitz-Gilbert equation for magnetization dynamics is recast into spinor form using the real-valued Clifford algebra (geometric algebra) of three-space. We show how the undamped case can be explicitly solved to obtain component-wise solutions, with clear geometrical meaning. Generalizations of the approach to include damping are formulated. The implications of the axial property of the magnetization vector are briefly discussed.

physics.class-ph↗

arXiv2023

Epitaxial growth and characterization of (001) [NiFe/M]$_{20}$ (M = Cu, CuPt and Pt) superlattices

We present optimization of [(15 $\unicode{x212B}$) Ni$_{80}$Fe$_{20}$/(5 $\unicode{xC5}$) M]$_{20}$ single crystal multilayers on (001) MgO, with M being Cu, Cu$_{50}$Pt$_{50}$ and Pt. These superlattices were characterized by high-resolution X-ray reflectivity (XRR) and diffraction (XRD) as well as polar mapping of important crystal planes. It is shown that cube on cube epitaxial relationship can be obtained when depositing at the substrate temperature of 100 $^\circ$C regardless of the lattice mismatch (5% and 14% for Cu and Pt, respectively). At lower substrate temperatures poly-crystalline multilayers were obtained while at higher substrate temperatures {111} planes appear at $\sim$10$^\circ$ off normal to the film plane. It is also shown that as the epitaxial strain increases, the easy magnetization axis rotates towards the direction that previously was assumed to be harder, i.e. from [110] to [100], and eventually further increase in the strain makes the magnetic hysteresis loops isotropic in the film plane. Higher epitaxial strain is also accompanied with increased coercivity values. Thus, the effect of epitaxial strain on the magnetocrystalline anisotropy is much larger than what was observed previously in similar, but polycrystalline samples with uniaxial anisotropy (Kateb et al. 2021).

cond-mat.mtrl-sci↗

arXiv2022

Piezoresistance characterization of silicon nanowires in uniaxial and isostatic pressure variation

Silicon nanowires (SiNWs) are known to exhibit large piezoresistance (PZR) effect, making it suitable for various sensing applications. Here, we report the results of a PZR investigation on randomly distributed and interconnected vertical silicon nanowire arrays as a pressure sensor. The samples were produced from p-type (100) Si wafers using a silver catalysed top-down etching process. The piezoresistance response of these SiNW arrays was analysed by measuring their I-V characteristics under applied uniaxial as well as isostatic pressure. The interconnected SiNWs exhibit increased mechanical stability in comparison with separated or periodic nanowires. The repeatability of the fabrication process and statistical distribution of measurements were also tested on several samples from different batches. A sensing resolution down to roughly \SI{1}{\milli\bar} pressure was observed with uniaxial force application, and more than two orders of magnitude resistance variation was determined for isostatic pressure below atmospheric pressure.

physics.app-ph↗

arXiv2021

On the role of ion potential energy in low energy HiPIMS deposition: An atomistic simulation

We study the effect of the so-called ion potential or non-kinetic energies of bombarding ions during ionized physical vapor deposition of Cu using molecular dynamics simulations. In particular we focus on low energy HiPIMS deposition, in which the potential energy of ions can be comparable to their kinetic energy. The ion potential, as a short-ranged repulsive force between the ions of the film-forming material and the surface atoms (substrate and later deposited film), is defined by the Ziegler-Biersack-Littmark potential. Analyzing the final structure indicates that, including the ion potential leads to a slightly lower interface mixing and fewer point defects (such as vacancies and interstitials), but resputtering and twinning have increased slightly. However, by including the ion potential the collision pattern changes. We also observed temporary formation of a ripple/pore with 5~nm height when the ion potential is included. The latter effect can explain the pores in HiPIMS deposited Cu thin films observed experimentally by atomic force microscopy.

cond-mat.mtrl-sci↗

arXiv2021

Tailoring interface mixing and magnetic properties in (111) Permalloy/Pt multilayers

We present deposition and characterization of multilayers consisting of 20 repetitions of 15 $Å$ thick permalloy Ni$_{80}$Fe$_{20}$ at. \% (Py) and 5 $Å$ Pt. The samples were prepared by two different sputter deposition methods, namely dc magnetron sputtering (dcMS) and high power impulse magnetron sputtering (HiPIMS), that represent low and moderate ionized flux fraction of the film forming material, respectively, for deposition of the Py layers. The effect of substrate roughness, working gas pressure and sputter power on the in-plane uniaxial magnetic anisotropy of the films are studied. The multilayers were characterized by X-ray reflectivity and diffraction, and by magneto-optical Kerr effect (MOKE). It is shown that HiPIMS deposition produces multilayers with unique surface roughness regardless of the substrate surface roughness. Multilayers prepared by both dcMS and HiPIMS deposition present a strong (111) texture normal to the film plane. The results show that utilizing HiPIMS for deposition of the Py layer leads to a minimum interface mixing between individual layers compared to dcMS deposition performed at sputter power. This is associated with the smooth surface of Py deposited by HiPIMS. However, this sharp interface results in higher coercivity and an opening in the hard axis hysteresis loops while multilayers with intermixing present well defined in-plane uniaxial anisotropy i.e. a linear hard axis. Comparison with Py/Cu and Py/CuPt multilayers, prepared under identical conditions using HiPIMS, suggests that poor in-plane uniaxial anisotropy is obtained in the Py/Pt case, caused by the inverse magnetostriction arising from the large lattice mismatch between Py and Pt. The Py/Pt multilayers that exhibit interface mixing have a more relaxed interface and thus presents negligible inverse magnetostriction and have better defined anisotropy.

cond-mat.mtrl-sci↗

arXiv2021

Correlation of uniaxial magnetic anisotropy axes and principal resistivities in polycrystalline ferromagnetic films

In the present study, we demonstrate the measurement of resistivity tensor ($ρ$) along the magnetic axes of a polycrystalline film of ferromagnetic permalloy (Py). To this end, conventional Hall-bar and a more recent extended van der Pauw methods were utilized for determining 2D $ρ$ in the film plane. The samples were prepared by normal incidence sputter deposition within an in-situ magnetic field to induce in-plane uniaxial magnetic anisotropy in the film. Since $ρ$ might be affected by the internal magnetization of the film, we performed measurements by rotation of a saturating magnetic field in the film plane. Both methods indicate that the average resistivity is lower along the easy axis of the film compared to the hard axis. Since X-ray diffraction results indicated no dominating texture in the film, we concluded that there is a correlation between uniaxial magnetic axes and principal resistivity axes. This is an important finding that allows determining the direction of magnetic anisotropy axes without magnetometry. The results also verify atomic or pair ordering to be the origin of uniaxial magnetic anisotropy in the Py since resistivity is sensitive to the level of order in solids. The extended van der Pauw utilized here can be easily performed on the as-received samples which is of practical interest.

cond-mat.mtrl-sci↗

arXiv2020

Effect of substrate bias on microstructure of epitaxial film grown by HiPIMS: An atomistic simulation

We explore combination of high power impulse magnetron sputtering (HiPIMS) and substrate bias for the epitaxial growth of Cu film on Cu (111) substrate by molecular dynamics simulation. A fully ionized deposition flux was used to represent the high ionization fraction in the HiPIMS process. To mimic different substrate bias, we assumed the deposition flux with a flat energy distribution in the low, moderate and high energy ranges. We also compared the results of fully ionized flux with results assuming a completely neutral flux, in analogy with thermal evaporation. It is confirmed that in the low energy regime, HiPIMS presents a slightly smoother surface and more interface mixing compared to that of thermal evaporation. In the moderate energy HiPIMS, however, an atomically smooth surface was obtained with a slight increase in the interface mixing compared to low energy HiPIMS. In the high energy regime, HiPIMS presents severe interface mixing with a smooth surface but limited growth due to resputtering from the surface. The results also indicate that fewer crystal defects appear in the film for moderate energy HiPIMS. We attribute this behavior to the repetition frequency of collision events. In particular high energy HiPIMS suffers from high repetition of collision events which does not allow reconstruction of the film. While in the low energy HiPIMS there are not enough events to overcome island growth. At moderate energy, collision events repeat in a manner that provides enough time for reconstruction which results in a smooth surface, fewer defects and limited intermixing.

cond-mat.mtrl-sci↗

arXiv2020

Size and shape-dependent melting mechanism of Pd nanoparticles

Molecular dynamics simulation is employed to understand the thermodynamic behavior of cuboctahedron (cub) and icosahedron (ico) nanoparticles with 2-20 number of full shells. The original embedded atom method (EAM) was compared to the more recent highly optimized version as inter-atomic potential. The thermal stability of clusters were probed using potential energy and specific heat capacity as well as structure analysis by radial distribution function, G(r), and common neighbor analysis (CNA), simultaneously, to make a comprehensive picture of the solid state and melting transitions. The result shows ico is the only stable shape of small clusters (Pd55-Pd309 using original EAM and Pd55 using optimized version) those are melting uniformly due to their small diameter. An exception is cub Pd309 modeled via optimized EAM that transforms to ico at elevated temperatures. A similar cub to ico transition was predicted by original EAM for Pd923-Pd2075 clusters while for the larger clusters both cub and ico are stable up to the melting point. As detected by G(r) and CNA, moderate and large cub clusters were showing surface melting by nucleation of the liquid phase at (100) planes and growth of liquid phase at the surface before inward growth. While diagonal (one corner to another) melting was dominating over ico clusters owing to their partitioned structure which retarded the growth of the liquid phase. The large ico cluster, using optimized EAM, presented a combination of surface and diagonal melting due to the simultaneous diagonal melting started from different corners. Finally, the melting temperature as well as latent heat of fusion were calculated and compared with available models and previous studies which showed, unlike the present result, the models failed to predict size-dependent motif crossover.

cond-mat.mtrl-sci↗

arXiv2019

The role of ionization fraction on the surface roughness, density and interface mixing of the films deposited by thermal evaporation, dc magnetron sputtering and HiPIMS: An atomistic simulation

We explore the effect of ionization fraction on the epitaxial growth of Cu film on Cu (111) substrate at room temperature. We compare thermal evaporation, dc magnetron sputtering (dcMS) and high power impulse magnetron sputtering (HiPIMS). Three deposition conditions i.e. fully neutral, 50% ionized and 100% ionized flux were considered as thermal evaporation, dcMS and HiPIMS, respectively, for ~20000 adatoms. It is shown that higher ionization fraction of the deposition flux leads to smoother surfaces by two major mechanisms i.e. decreasing clustering in the vapor phase and bi-collision of high energy ions at the film surface. The bi-collision event consists of local amorphization which fills the gaps between islands followed by crystallization due to secondary collisions. We found bi-collision events to be very important to prevent island growth to become dominant and increase the surface roughness. Regardless of the deposition method, epitaxial Cu thin films suffer from stacking fault areas (twin boundaries) in agreement with recent experimental results. In addition, HiPIMS deposition presents considerable interface mixing while it is negligible in thermal evaporation and dcMS deposition, those present less adhesion accordingly.

cond-mat.mtrl-sci↗

arXiv2019

Comparison of magnetic and structural properties of permalloy Ni80Fe20 grown by dc and high power impulse magnetron sputtering

We study the microstructure and magnetic properties of Ni80Fe20 thin films grown by high power impulse magnetron sputtering (HiPIMS), and compare with films grown by dc magnetron sputtering (dcMS). The films were grown under a tilt angle of 35° to identical thickness of 37 nm using both techniques, at different pressure (0.13-0.73 Pa) and substrate temperature (room temperature and 100 °C). All of our films display effective in-plane uniaxial anisotropy with square easy axis and linear hard axis magnetization traces. X-ray diffraction reveals that there is very little change in grain size within the pressure and temperature ranges explored. However, variations in film density, obtained by X-ray reflectivity measurements, with pressure have a significant effect on magnetic properties such as anisotropy field (Hk) and coercivity (Hc). Depositions where adatom energy is high produce dense films, while low adatom energy results in void-rich films with higher Hk and Hc. The latter applies to our dcMS deposited films at room temperature and high pressure. However, the HiPIMS deposition method gives higher adatom energy than the dcMS and results in dense films with low Hk and Hc. The surface roughness is found to increase with increased pressure, in all cases, however it showed negligible contribution to the increase in Hk and Hc.

cond-mat.mtrl-sci↗

arXiv2019

Effect of atomic ordering on the magnetic anisotropy of single crystal Ni80Fe20

We investigate the effect of atomic ordering on the magnetic anisotropy of Ni80Fe20 at.% (Py). To this end, Py films were grown epitaxially on MgO (001) using dc magnetron sputtering (dcMS) and high power impulse magnetron sputtering (HiPIMS). Aside from twin boundaries observed in the latter case, both methods present high quality single crystals with cube-on-cube epitaxial relationship as verified by the polar mapping of important crystal planes. However, X-ray diffraction results indicate higher order for the dcMS deposited film towards L12 Ni3Fe superlattice. This difference can be understood by the very high deposition rate of HiPIMS during each pulse which suppresses adatom mobility and ordering. We show that the dcMS deposited film presents biaxial anisotropy while HiPIMS deposition gives well defined uniaxial anisotropy. Thus, higher order achieved in the dcMS deposition behaves as predicted by magnetocrystalline anisotropy i.e. easy axis along the [111] direction that forced in the plane along the [110] direction due to shape anisotropy. The uniaxial behaviour in HiPIMS deposited film then can be explained by pair ordering or more recent localized composition non-uniformity theories. Further, we studied magnetoresistance of the films along the [100] directions using an extended van der Pauw method. We find that the electrical resistivities of the dcMS deposited film are lower than in their HiPIMS counterparts verifying the higher order in the dcMS case.

cond-mat.mtrl-sci↗

arXiv2018

Application of an extended van der Pauw method to anisotropic magnetoresistance measurements of ferromagnetic films

We demonstrate anisotropic resistivity measurements using the extended van der Pauw (vdP) method in ferromagnetic Ni80Fe20 (Py) films. We apply it to measure anisotropic magnetoresistance (AMR) and compare the results of the vdP method with the more conventional Hall-bar method along the hard and easy axis of the film and show that the vdP method gives more reliable AMR result. For instance, the AMR result along the hard and easy axis of the film are in close agreement. Further, we applied the vdP method to study AMR in a series of Py films with thicknesses ranging between 10-250 nm. The films were grown by sputtering deposition at an angle with respect to the substrate normal and with an in-situ magnetic field, both conditions assisting in the definition of in-plane uniaxial anisotropy. The microstructure of Py films was characterized using X-ray reflectivity, diffraction and polar mapping of (111) planes. We detected no off-normal texture and negligible surface roughness, which indicates that self-shadowing is not dominating in our growth. Yet the films have well defined uniaxial anisotropy. Abrupt changes in the average resistivity vs. film thickness were observed, which cannot be explained by the models accounting for the thickness and grain size but strongly correlate with the changes in (111) texture in the films. We compared our results with the literature and show that independent of growth method, substrate and deposition temperature, the AMR value presents a saturation behavior with thickness at about 100 nm.

cond-mat.mtrl-sci↗

arXiv2018

Power regulation and electromigration in platinum microwires

We introduce a new experimental setup with a biasing circuit and computer control for electrical power regulation under reversing polarity in Pt microwires with dimensions of $1\times10$ μm$^2$. The circuit is computer controlled via a data acquisition board. It amplifies a control signal from the computer and drives current of alternating polarity through the sample in question. Time-to-failure investigations under DC and AC current stress are performed. We confirm that AC current stress can improve the life time of microwires at least by a factor of $10^3$ compared to the corresponding time-to-failure under DC current stress.

physics.app-ph↗

arXiv2018

Grain growth in Pt microheaters subjected to high current density under constant power

When $50$ nm thick Pt microheaters of lateral dimensions $1\times10$ $μ$m$^2$ are subjected to high electric power their resistance $R$ rises, as expected. Following an initial rise however there is a gradual decrement in $R$ while constant electric power dissipation is maintained. We find that this lowering in $R$ is accompanied by grain growth in the polycrystalline thin Pt film of our heaters. This is confirmed by XRD measurements and SEM imaging. Similar growth in grain size is observed in thin Pt films that are oven-annealed at high temperatures. Thus we argue that maintaining high power dissipation in a microheater has the same effect on its material structure as post-annealing. We observe the in-plane grain size of a $50$ nm thick as-grown Pt film/heater to be $D_\parallel=15$ nm. When post-annealed at a temperature of $T=600^\circ$C for 30 min, $D_\parallel=30$ nm, compared with when electric current is run through a heater we estimate the mean crystalline length to be $D_\parallel=35$ nm.

physics.app-ph↗

arXiv2008

Coherence properties of infrared thermal emission from heated metallic nanowires

Coherence properties of the infrared thermal radiation from individual heated nanowires are investigated as function of nanowire dimensions. Interfering the thermally induced radiation from a heated nanowire with its image in a nearby moveable mirror, well-defined fringes are observed. From the fringe visibility, the coherence length of the thermal emission radiation from the narrowest nanowires was estimated to be at least 20 um which is much larger than expected from a classical blackbody radiator. A significant increase in coherence and emission efficiency is observed for smaller nanowires.

cond-mat.mtrl-sci↗