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Hjörvarsson, BjörgvinORCID iD iconorcid.org/0000-0003-1803-9467
Alternative names
Publications (10 of 262) Show all publications
Ali, H., Warnatz, T., Xie, L., Hjörvarsson, B. & Leifer, K. (2019). Quantitative EMCD by use of a double aperture for simultaneous acquisition of EELS. Ultramicroscopy, 196, 192-196
Open this publication in new window or tab >>Quantitative EMCD by use of a double aperture for simultaneous acquisition of EELS
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2019 (English)In: Ultramicroscopy, ISSN 0304-3991, E-ISSN 1879-2723, Vol. 196, p. 192-196Article in journal (Refereed) Published
Abstract [en]

The weak signal strength in electron magnetic circular dichroism (EMCD) measurements remains one of the main challenges in the quantification of EMCD related EELS spectra. As a consequence, small variations in peak intensity caused by changes of background intervals, choice of method for extraction of signal intensity and equally differences in sample quality can cause strong changes in the EMCD signal. When aiming for high resolution quantitative EMCD, an additional difficulty consists in the fact that the two angular resolved EELS spectra needed to obtain the EMCD signal are taken at two different instances and it cannot be guaranteed that the acquisition conditions for these two spectra are identical.  Here, we present an experimental setup where we use a double hole aperture in the transmission electron microscope to obtain the EMCD signal in a single acquisition. This geometry allows for the parallel acquisition of the two electron energy loss spectra (EELS) under exactly the same conditions. We also compare the double aperture acquisition mode with the qE acquisition mode which has been previously used for parallel acquisition of EMCD. We show that the double aperture mode not only offers better signal to noise ratio as compared to qE mode but also allows for much higher acquisition times to significantly improve the signal quality which is crucial for quantitative analysis of the magnetic moments.

National Category
Other Materials Engineering
Research subject
Materials Science
Identifiers
urn:nbn:se:uu:diva-364715 (URN)10.1016/j.ultramic.2018.10.012 (DOI)000451180800026 ()30439606 (PubMedID)
Funder
Swedish Research Council, C0367901Swedish Research Council, 2016-05259Knut and Alice Wallenberg Foundation
Available from: 2018-10-31 Created: 2018-10-31 Last updated: 2019-01-28Bibliographically approved
Huang, W., Brischetto, M. & Hjörvarsson, B. (2019). Size effect on deuterium behavior in nano-sized vanadium layers. SCIENCE CHINA-PHYSICS MECHANICS & ASTRONOMY, 62(11), Article ID 117011.
Open this publication in new window or tab >>Size effect on deuterium behavior in nano-sized vanadium layers
2019 (English)In: SCIENCE CHINA-PHYSICS MECHANICS & ASTRONOMY, ISSN 1674-7348, Vol. 62, no 11, article id 117011Article in journal (Refereed) Published
Abstract [en]

Size effect on thermodynamics and diffusion of deuterium in nano-sized vanadium (V) layers is studied. Critical temperature (T-c) for deuterium phase transition is found to decrease with the inverse thickness of V layers and the thermodynamic factor increases as V thickness decreases. These effects are related to the deuterium-deuterium (D-D) interaction change versus V thickness, which experimentally proves that the D-D interaction plays the main contribution to the previously observed V size effect on deuterium chemical diffusion coefficients (D-c). The self-diffusion coefficients (D-s) are obtained through correcting D-c with the thermodynamic factors. It is found that the D-s are similar in 14 and 28 monolayers of V while slightly larger D-s are observed at high concentrations in 14 atomic layers. The weak site blocking effect in the interface is argued to be the main contribution to the observed size effect on D-s.

Keywords
size effect, D-D interaction, deuterium diffusion, Fe/V superlattice, optical transmission
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-383494 (URN)10.1007/s11433-018-9381-8 (DOI)000466398100001 ()
Funder
Swedish Energy AgencySwedish Research Council
Available from: 2019-05-17 Created: 2019-05-17 Last updated: 2019-05-17Bibliographically approved
Holmberg, M., Dancila, D., Rydberg, A., Hjörvarsson, B., Jansson, U., Marattukalam, J. J., . . . Andersson, J. (2018). Direct metal laser sintering printed millimeter and submillimeter waveguides. In: : . Paper presented at GigaHertz Symposium, Lund, May 24-25 2018.
Open this publication in new window or tab >>Direct metal laser sintering printed millimeter and submillimeter waveguides
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2018 (English)Conference paper, Poster (with or without abstract) (Other academic)
National Category
Telecommunications
Identifiers
urn:nbn:se:uu:diva-369172 (URN)
Conference
GigaHertz Symposium, Lund, May 24-25 2018
Funder
Swedish Foundation for Strategic Research , GMT14-0048
Available from: 2018-12-11 Created: 2018-12-11 Last updated: 2018-12-11Bibliographically approved
Palonen, H., Magnus, F. & Hjörvarsson, B. (2018). Double magnetic proximity in Fe/Fe0.32V0.68 superlattices. Physical Review B, 98(14), Article ID 144419.
Open this publication in new window or tab >>Double magnetic proximity in Fe/Fe0.32V0.68 superlattices
2018 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 98, no 14, article id 144419Article in journal (Refereed) Published
Abstract [en]

The conventional magnetic proximity effect and double-proximity effects were studied in a set of fully coherent high-quality Fe/Fe0.32V0.68 superlattices. Applying a simple model to the saturation magnetization, it is seen that the magnetic proximity effect is gigantic in magnitude in the alloy-the magnetization is enhanced by 20-450 % and the ordering temperature is enhanced by a factor of 2. The magnitude of the effect can be explained by the large susceptibility of the alloy above its intrinsic ordering temperature. Additionally, a strong dependence of the ordering temperature of single monolayers of Fe on the interlayer distance is observed. The results give insight into new ways of using alloying and large magnetic susceptibility combined with magnetic proximity effects to enhance the functionality of materials that are of interest for spintronic devices.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2018
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-368444 (URN)10.1103/PhysRevB.98.144419 (DOI)000447183100005 ()
Available from: 2018-12-07 Created: 2018-12-07 Last updated: 2018-12-07Bibliographically approved
Qviller, A. J., Marstein, E. S., You, C. C., Haug, H., Webster, J. R. P., Hjörvarsson, B., . . . Hauback, B. C. (2018). Hydrogen Concentration in Photovoltaic a-Si:H Annealed at Different Temperatures Measured by Neutron Reflectometry. IEEE Journal of Photovoltaics, 8(4), 1098-1101
Open this publication in new window or tab >>Hydrogen Concentration in Photovoltaic a-Si:H Annealed at Different Temperatures Measured by Neutron Reflectometry
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2018 (English)In: IEEE Journal of Photovoltaics, ISSN 2156-3381, E-ISSN 2156-3403, Vol. 8, no 4, p. 1098-1101Article in journal (Refereed) Published
Abstract [en]

Amorphous hydrogenated silicon (a-Si:H) is an important material for surface defect passivation of photovoltaic silicon (Si) wafers in order to reduce their recombination losses. The material is, however, unstable with regard to hydrogen (H) desorption at elevated temperatures, which can be an issue during processing and device manufacturing. In this work, we determine the temperature stability of a-Si:H by structural characterization of a-Si:H/Si bilayers with neutron reflectometry and X-ray reflectometry combined with photoconductance measurements, yielding the minority carrier lifetime. The neutrons are sensitive to light elements such as H, while the X-rays, which are insensitive to the H concentration, provide an independent constraint on the layer structure. It is shown that H desorption takes place at a temperature of approximately T = 425 degrees C, and that the H content and minority carrier lifetimes have a strongly correlated linear relationship, which can be interpreted as one H atom passivating one defect.

Place, publisher, year, edition, pages
IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2018
Keywords
Amorphous materials, silicon, solar energy
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-359995 (URN)10.1109/JPHOTOV.2018.2838447 (DOI)000436007400027 ()
Available from: 2018-09-14 Created: 2018-09-14 Last updated: 2018-09-14Bibliographically approved
Östman, E., Stopfel, H., Chioar, I.-A., Arnalds, U. B., Stein, A., Kapaklis, V. & Hjörvarsson, B. (2018). Interaction modifiers in artificial spin ices. Nature Physics, 14(4), 375-379
Open this publication in new window or tab >>Interaction modifiers in artificial spin ices
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2018 (English)In: Nature Physics, ISSN 1745-2473, E-ISSN 1745-2481, Vol. 14, no 4, p. 375-379Article in journal (Refereed) Published
Abstract [en]

The modification of geometry and interactions in two-dimensional magnetic nanosystems has enabled a range of studies addressing the magnetic order(1-6), collective low-energy dynamics(7,8) and emergent magnetic properties(5,9,10) in, for example, artificial spin-ice structures. The common denominator of all these investigations is the use of Ising-like mesospins as building blocks, in the form of elongated magnetic islands. Here, we introduce a new approach: single interaction modifiers, using slave mesospins in the form of discs, within which the mesospin is free to rotate in the disc plane(11). We show that by placing these on the vertices of square artificial spin-ice arrays and varying their diameter, it is possible to tailor the strength and the ratio of the interaction energies. We demonstrate the existence of degenerate ice-rule-obeying states in square artificial spin-ice structures, enabling the exploration of thermal dynamics in a spin-liquid manifold. Furthermore, we even observe the emergence of flux lattices on larger length scales, when the energy landscape of the vertices is reversed. The work highlights the potential of a design strategy for two-dimensional magnetic nano-architectures, through which mixed dimensionality of mesospins can be used to promote thermally emergent mesoscale magnetic states.

Place, publisher, year, edition, pages
NATURE PUBLISHING GROUP, 2018
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-352710 (URN)10.1038/s41567-017-0027-2 (DOI)000429434100018 ()
Funder
Knut and Alice Wallenberg Foundation, 2015.0060Swedish Research CouncilEU, Horizon 2020, 737093
Available from: 2018-06-07 Created: 2018-06-07 Last updated: 2018-09-14Bibliographically approved
Droulias, S. A., Pálsson, G. K., Hjörvarsson, B. & Wolff, M. (2018). Limitations of the kinematic approximation in neutron reflectivity measurements for the analysis of bilayers. Journal of applied crystallography, 51, 1556-1563
Open this publication in new window or tab >>Limitations of the kinematic approximation in neutron reflectivity measurements for the analysis of bilayers
2018 (English)In: Journal of applied crystallography, ISSN 0021-8898, E-ISSN 1600-5767, Vol. 51, p. 1556-1563Article in journal (Refereed) Published
Abstract [en]

The limitations of a phenomenological fitting approach compared to simulations of the optical model including reflection and refraction at all interfaces are demonstrated using the example of hydrogen loading in ultra-thin vanadium layers. Fe/V superlattices are loaded with deuterium and the lattice expansion and deuterium concentration are extracted from neutron reflectivity data. A noticeable difference is found between the extraction of concentrations and bilayer thicknesses directly from the superlattice peaks and fits of the density profile using the Parratt formalism. The results underline the importance of carefully considering the limitations of phenomenological approaches, in order to obtain robust results. The limitations of the kinematic approximation for the analysis are discussed in detail.

Place, publisher, year, edition, pages
INT UNION CRYSTALLOGRAPHY, 2018
Keywords
neutron reflectivity, superlattices, deuterium, volume expansion, Parratt's algorithm
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-372377 (URN)10.1107/S1600576718013092 (DOI)000451833600006 ()
Available from: 2019-01-08 Created: 2019-01-08 Last updated: 2019-01-08Bibliographically approved
Rodrigues, D. C. M., B Klautau, A., Edström, A., Rusz, J., Nordström, L., Pereiro, M., . . . Eriksson, O. (2018). Magnetic anisotropy in permalloy: hidden quantum mechanical features. Physical Review B, 97(22), Article ID 224402.
Open this publication in new window or tab >>Magnetic anisotropy in permalloy: hidden quantum mechanical features
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2018 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 22, article id 224402Article in journal (Refereed) Published
Abstract [en]

By means of relativistic, first principles calculations, we investigate the microscopic origin of the vanishingly low magnetic anisotropy of Permalloy, here proposed to be intrinsically related to the local symmetries of the alloy. It is shown that the local magnetic anisotropy of individual atoms in Permalloy can be several orders of magnitude larger than that of the bulk sample and 5–10 times larger than that of elemental Fe or Ni. We furthermore show that locally there are several easy axis directions that are favored, depending on local composition. The results are discussed in the context of perturbation theory, applying the relation between magnetic anisotropy and orbital moment. Permalloy keeps its pronounced soft ferromagnetic nature due to the exchange energy to be larger than the magnetocrystalline anisotropy. Our results shine light on the magnetic anisotropy of permalloy and of magnetic materials in general, and in addition enhance the understanding of pump-probe measurements and ultrafast magnetization dynamics.

Keywords
Permalloy, Orbital Anisotropy, Anisotropy Energy
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-319925 (URN)10.1103/PhysRevB.97.224402 (DOI)000434015300004 ()
Funder
Knut and Alice Wallenberg Foundation, 2012.0031Knut and Alice Wallenberg Foundation, 2013.0020Swedish Research CouncileSSENCE - An eScience CollaborationStandUp
Available from: 2017-04-11 Created: 2017-04-11 Last updated: 2018-09-14Bibliographically approved
Stopfel, H., Östman, E., Chioar, I.-A., Greving, D., Arnalds, U., Hase, T. P. A., . . . Kapaklis, V. (2018). Magnetic order and energy-scale hierarchy in articial spin ice structures. Physical Review B, 98, Article ID 014435.
Open this publication in new window or tab >>Magnetic order and energy-scale hierarchy in articial spin ice structures
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2018 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 98, article id 014435Article in journal (Refereed) Published
Abstract [en]

In order to explain and predict the properties of many physical systems, it is essential to understand the interplay of different energy scales. Here we present investigations of the magnetic order in thermalized artificial spin-ice structures, with different activation energies of the interacting Ising-like elements. We image the thermally equilibrated magnetic states of the nanostructures using synchrotron-based magnetic microscopy. By comparing results obtained from structures with one or two different activation energies, we demonstrate a clear impact on the resulting magnetic order. The differences are obtained by the analysis of the magnetic spin structure factors, in which the role of the activation energies is manifested by distinct short-range order. These results highlight the potential of artificial spin-ice structures to serve as model systems for designing various energy-scale hierarchies and investigating their impact on the collective dynamics and magnetic order.

Keywords
artificial spin ice, magnetic nano-structures, mesoscopic spin systems, mesospins, model systems, Shakti, engineering of energy-landscape, multiple energy-scales
National Category
Condensed Matter Physics Nano Technology
Research subject
Physics
Identifiers
urn:nbn:se:uu:diva-329016 (URN)10.1103/PhysRevB.98.014435 (DOI)000440137800003 ()
Funder
The Swedish Foundation for International Cooperation in Research and Higher Education (STINT)Knut and Alice Wallenberg FoundationSwedish Research Council
Note

Title in thesis list of papers: Magnetic order and energy-scale hierarchy in articial spin ice

Available from: 2017-10-10 Created: 2017-10-10 Last updated: 2018-10-08Bibliographically approved
Holmberg, M., Dancila, D., Rydberg, A., Hjörvarsson, B., Jansson, U., Marattukalam, J. J., . . . Andersson, J. (2018). On Surface Losses in Direct Metal Laser Sintering Printed Millimeter and Submillimeter Waveguides. Journal of Infrared, Millimeter and Terahertz Waves, 39(6), 535-545
Open this publication in new window or tab >>On Surface Losses in Direct Metal Laser Sintering Printed Millimeter and Submillimeter Waveguides
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2018 (English)In: Journal of Infrared, Millimeter and Terahertz Waves, ISSN 1866-6892, E-ISSN 1866-6906, Vol. 39, no 6, p. 535-545Article in journal (Refereed) Published
Abstract [en]

Different lengths of WR3 (220-330 GHz) and WR10 (75-110 GHz) waveguides are fabricated through direct metal laser sintering (DMLS). The losses in these waveguides are measured and modelled using the Huray surface roughness model. The losses in WR3 are around 0.3 dB/mm and in WR10 0.05 dB/mm. The Huray equation model is accounting relatively good for the attenuation in the WR10 waveguide but deviates more in the WR3 waveguide. The model is compared to finite element simulations of the losses assuming an approximate surface structure similar to the resulting one from the DMLS process.

Place, publisher, year, edition, pages
SPRINGER, 2018
Keywords
Millimeterwave, 3D-metal-printed, Waveguide loss
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-356385 (URN)10.1007/s10762-018-0470-x (DOI)000431255500005 ()
Funder
Swedish Foundation for Strategic Research
Available from: 2018-07-25 Created: 2018-07-25 Last updated: 2018-07-25Bibliographically approved
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Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0003-1803-9467

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