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Gunnarsson, K
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Publications (10 of 58) Show all publications
Jana, S., Panda, S. K., Phuyal, D., Pal, B., Mukherjee, S., Dutta, A., . . . Sarma, D. D. (2019). Charge disproportionate antiferromagnetism at the verge of the insulator-metal transition in doped LaFeO3. Physical Review B, 99(7), Article ID 075106.
Open this publication in new window or tab >>Charge disproportionate antiferromagnetism at the verge of the insulator-metal transition in doped LaFeO3
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2019 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 7, article id 075106Article in journal (Refereed) Published
Abstract [en]

We explore the effects of electron doping in lanthanum ferrite, LaFeO3 by doping Mo at the Fe sites. Based on magnetic, transport, scanning tunneling spectroscopy, and x-ray photoelectron spectroscopy measurements, we find that the large gap, charge-transfer, antiferromagnetic (AFM) insulator LaFeO3 becomes a small gap AFM band insulator at low Mo doping. With increasing doping concentration, Mo states, which appear around the Fermi level, is broadened and become gapless at a critical doping of 20%. Using a combination of calculations based on density functional theory plus Hubbard U (DFT+U) and x-ray absorption spectroscopy measurements, we find that the system shows charge disproportionation (CD) in Fe ions at 25% Mo doping, where two distinct Fe sites, having Fe2+ and Fe3+ nominal charge states appear. A local breathing-type lattice distortion induces the charge disproportionation at the Fe site without destroying the antiferromagnetic order. Our combined experimental and theoretical investigations establish that the Fe states form a CD antiferromagnet at 25% Mo doping, which remains insulating, while the appearance of Mo states around the Fermi level is showing an indication towards the insulator-metal transition.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2019
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-378537 (URN)10.1103/PhysRevB.99.075106 (DOI)000458168300001 ()
Funder
Swedish Research Council, 2016-4524Knut and Alice Wallenberg Foundation, 2012.0031Swedish Energy Agency, P43294-1EU, European Research Council, CorrelMat-617196Swedish Research Council, 2016-03278Swedish Research CouncilSwedish Foundation for Strategic Research Carl Tryggers foundation , CTS-17:376eSSENCE - An eScience CollaborationStandUp
Available from: 2019-03-21 Created: 2019-03-21 Last updated: 2019-03-21Bibliographically approved
Fang, H., Li, J., Shafeie, S., Hedlund, D., Cedervall, J., Ekström, F., . . . Sahlberg, M. (2019). Insights into phase transitions and magnetism of MnBi crystals synthesized from self-flux. Journal of Alloys and Compounds, 781, 308-314
Open this publication in new window or tab >>Insights into phase transitions and magnetism of MnBi crystals synthesized from self-flux
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2019 (English)In: Journal of Alloys and Compounds, ISSN 0925-8388, E-ISSN 1873-4669, Vol. 781, p. 308-314Article in journal (Refereed) Published
Abstract [en]

To effectively synthesize high purity ferromagnetic low temperature phase (LTP) MnBi with optimal microstructure is still a challenge that needs to be overcome for the system to reach its full potential. Here, the phase transitions and magnetic properties of MnBi crystals are reported. The phase transition between the low and high temperature structure of MnBi was systematically investigated at different heating/cooling rates using in situ synchrotron radiation X-ray diffraction. The material crystallizes in a layered hexagonal structure giving a platelike microstructure. The magnetic characterization of the crystals reveal that the saturation magnetization varies from 645 kA/m at 50 K to 546 kA/m at 300 K. Magnetization measurements also show that the sample upon heating becomes non-magnetic and transforms to the high temperature phase (HTP) at similar to 640 K, and that it regains ferromagnetic properties and transforms back to the LTP at similar to 610 K upon subsequent cooling.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE SA, 2019
Keywords
In situ synchrotron radiation X-ray diffraction, Phase transitions, Rare earth free permanent magnet, MnBi, Single crystals
National Category
Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-378612 (URN)10.1016/j.jallcom.2018.12.146 (DOI)000457845900034 ()
Funder
Swedish Energy AgencySwedish Foundation for Strategic Research
Available from: 2019-03-11 Created: 2019-03-11 Last updated: 2019-03-13Bibliographically approved
Kontos, S., Fang, H., Li, J., Delczeg-Czirjak, E. K., Shafeie, S., Svedlindh, P., . . . Gunnarsson, K. (2019). Measured and calculated properties of B-doped τ-phase MnAl: A rare earth free permanent magnet. Journal of Magnetism and Magnetic Materials, 474, 591-598
Open this publication in new window or tab >>Measured and calculated properties of B-doped τ-phase MnAl: A rare earth free permanent magnet
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2019 (English)In: Journal of Magnetism and Magnetic Materials, Vol. 474, p. 591-598Article in journal (Refereed) Published
Abstract [en]

The metastable tetragonal τ-phase MnAl has been doped interstitially with B through a drop synthesis method creating the (Mn0.55Al0.45)B0.02 compound. The as-casted samples were annealed, quenched and thereafter ball-milled and relaxed in order to decrease grain size and reduce the number of crystallographic defects. The Curie temperature of the quenched sample was estimated to 655 K. The magnetization, coercivity and anisotropy were analyzed with respect to flash-milling time, relaxation time and temperature. The results show that (Mn0.55Al0.45)B0.02 could be directly obtained from drop synthesis. The highest measured saturation magnetization of 393 kA/m (measured at ±1440kA/m) was achieved with a relaxation process after 1.5h milling, giving a theoretical maximum energy product of 48 kJ/m3. The highest value of the coercivity was 355 kA/m achieved by flash-milling for 10 h. However, the high coercivity was achieved at an expense of low saturation magnetization.

Keywords
Permanent magnets; Rare-Earth-free; Diffraction; Magnetometry; Computational modeling
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:uu:diva-368265 (URN)10.1016/j.jmmm.2018.11.006 (DOI)000459494600086 ()
Funder
Swedish Energy AgencySweGRIDS - Swedish Centre for Smart Grids and Energy Storage
Available from: 2018-12-03 Created: 2018-12-03 Last updated: 2019-03-21Bibliographically approved
Legaria, E. P., Saldan, I., Svedlindh, P., Wetterskog, E., Gunnarsson, K., Kessler, V. G. & Seisenbaeva, G. A. (2018). Coordination of rare earth element cations on the surface of silica-derived nanoadsorbents. Dalton Transactions, 47(4), 1312-1320
Open this publication in new window or tab >>Coordination of rare earth element cations on the surface of silica-derived nanoadsorbents
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2018 (English)In: Dalton Transactions, ISSN 1477-9226, E-ISSN 1477-9234, Vol. 47, no 4, p. 1312-1320Article in journal (Refereed) Published
Abstract [en]

Silica (SiO2)-derived nanoadsorbents are a powerful and attractive tool for the extraction and separation of rare earth elements (REE) from many perspectives such as reusability, efficiency and minimum impact on the environment. In the present work, we investigated two different methods of adsorption down to the molecular level: (1) the mechanism of the coordination of different groups of REE (light, medium, heavy) with iminodiacetic acid (IDA) was revealed by exploiting models obtained from X-ray crystallography, explaining the selectivity of this type of ligand, and (2) the mechanism of the seeding of RE(OH)(3) initiated by SiO2-based nanoadsorbents was investigated by EXAFS, both individually and in combination with mechanism (1), showing the coexistence of both mechanisms. The REE loaded nanoadsorbents possess a high magnetic susceptibility. This property was studied by magnetometry to quantify the REE adsorption efficiency and compared with the values obtained from complexometry.

Place, publisher, year, edition, pages
ROYAL SOC CHEMISTRY, 2018
National Category
Chemical Sciences Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-346224 (URN)10.1039/c7dt04388k (DOI)000423465200036 ()29300064 (PubMedID)
Funder
EU, FP7, Seventh Framework Programme, 309373
Available from: 2018-03-15 Created: 2018-03-15 Last updated: 2018-03-28Bibliographically approved
Cedervall, J., Nonnet, E., Hedlund, D., Häggström, L., Ericsson, T., Werwinski, M., . . . Sahlberg, M. (2018). Influence of cobalt substitution on the magnetic properties of Fe5PB2. Inorganic Chemistry, 57(2), 777-784
Open this publication in new window or tab >>Influence of cobalt substitution on the magnetic properties of Fe5PB2
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2018 (English)In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 57, no 2, p. 777-784Article in journal (Refereed) Published
Abstract [en]

In this study the effects of cobalt substitutions in Fe5PB2 have been studied. An increased cobalt content reduces the magnetic exchange interactions. This has been concluded from a large, linear decrease in both the Curie temperature as well as the saturated magnetic moment. At high cobalt concentrations, cobalt prefers to order at the M(2) position in the crystal structure. A tunable Curie transition like this shows some prerequisites for magnetic cooling applications.

The substitutional effects of cobalt in (Fe1–xCox)5PB2 have been studied with respect to crystalline structure and chemical order with X-ray diffraction and Mössbauer spectroscopy. The magnetic properties have been determined from magnetic measurements, and density functional theory calculations have been performed for the magnetic properties of both the end compounds, as well as the chemically disordered intermediate compounds. The crystal structure of (Fe1–xCox)5PB2 is tetragonal (space group I4/mcm) with two different metal sites, with a preference for cobalt atoms in the M(2) position (4c) at higher cobalt contents. The substitution also affects the magnetic properties with a decrease of the Curie temperature (TC) with increasing cobalt content, from 622 to 152 K for Fe5PB2 and (Fe0.3Co0.7)5PB2, respectively. Thus, the Curie temperature is dependent on composition, and it is possible to tune TC to a temperature near room temperature, which is one prerequisite for magnetic cooling materials.

National Category
Inorganic Chemistry Condensed Matter Physics Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-331758 (URN)10.1021/acs.inorgchem.7b02663 (DOI)000422810900030 ()29298054 (PubMedID)
Funder
Swedish Research CouncilSwedish Energy AgencyEuropean Regional Development Fund (ERDF)
Available from: 2017-10-19 Created: 2017-10-19 Last updated: 2018-10-19Bibliographically approved
Keshavarz, S., Kontos, S., Wardecki, D., Kvashnin, Y., Pereiro, M., Panda, S. K., . . . Svedlindh, P. (2018). Magnetic properties of Ruddlesden-Popper phases Sr3-&: A combined experimental and theoretical investigation. Physical Review Materials, 2(4), Article ID 044005.
Open this publication in new window or tab >>Magnetic properties of Ruddlesden-Popper phases Sr3-&: A combined experimental and theoretical investigation
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2018 (English)In: Physical Review Materials, ISSN 2475-9953, Vol. 2, no 4, article id 044005Article in journal (Refereed) Published
Abstract [en]

We present a comprehensive study of the magnetic properties of Sr3-xYx(Fe1.25Ni0.75)O-7(-delta )(0 <= x <= 0.75). Experimentally, the magnetic properties are investigated using superconducting quantum interference device (SQUID) magnetometry and neutron powder diffraction (NPD). This is complemented by a theoretical study based on density functional theory as well as the Heisenberg exchange parameters. Experimental results show an increase in the Ned temperature (T-N) with an increase of Y concentrations and O occupancy. The NPD data reveal that all samples are antiferromagnetically ordered at low temperatures, which has been confirmed by our theoretical simulations for the selected samples. Our first-principles calculations suggest that the three-dimensional magnetic order is stabilized due to finite interlayer exchange couplings. The latter give rise to finite interlayer spin-spin correlations, which disappear above T-N.

National Category
Condensed Matter Physics Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-354116 (URN)10.1103/PhysRevMaterials.2.044005 (DOI)000430385300001 ()
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Available from: 2018-06-19 Created: 2018-06-19 Last updated: 2018-08-24Bibliographically approved
Werwinski, M., Edstroem, A., Rusz, J., Hedlund, D., Gunnarsson, K., Svedlindh, P., . . . Sahlberg, M. (2018). Magnetocrystalline anisotropy of Fe5PB2 and its alloys with Co and 5d elements: A combined first-principles and experimental study. Physical Review B, 98(21), Article ID 214431.
Open this publication in new window or tab >>Magnetocrystalline anisotropy of Fe5PB2 and its alloys with Co and 5d elements: A combined first-principles and experimental study
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2018 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 98, no 21, article id 214431Article in journal (Refereed) Published
Abstract [en]

The Fe5PB2 compound offers tunable magnetic properties via the possibility of various combinations of substitutions on the Fe and P sites. Here, we present a combined computational and experimental study of the magnetic properties of (Fe1-xCox)(5)PB2. Computationally, we are able to explore the full concentration range, while the real samples were only obtained for 0 <= x <= 0.7. The calculated magnetic moments, Curie temperatures, and magnetocrystalline anisotropy energies (MAEs) are found to decrease with increasing Co concentration. Co substitution allows for tuning the Curie temperature in a wide range of values, from about six hundred to zero kelvins. As the MAE depends on the electronic structure in the vicinity of the Fermi energy, the geometry of the Fermi surface of Fe5PB2 and the k-resolved contributions to the MAE are discussed. Low-temperature measurements of an effective anisotropy constant for a series of (Fe1-xCox)(5)PB2 samples determined the highest value of 0.94 MJ m(-3) for the terminal Fe5PB2 composition, which then decreases with increasing Co concentration, thus confirming the computational result that Co alloying of Fe5PB2 is not a good strategy to increase the MAE of the system. However, the relativistic version of the fixed spin moment method reveals that a reduction in the magnetic moment of Fe5PB2, by about 25%, produces a fourfold increase of the MAE. Furthermore, calculations for (Fe0.95X0.05)(5)PB2 (X = 5d element) indicate that 5% doping of Fe5PB2 with W or Re should double the MAE. These are results of high interest for, e.g., permanent magnet applications, where a large MAE is crucial.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-373233 (URN)10.1103/PhysRevB.98.214431 (DOI)000454163400003 ()
Funder
Swedish Research CouncilSwedish Foundation for Strategic Research
Available from: 2019-01-14 Created: 2019-01-14 Last updated: 2019-01-14Bibliographically approved
Shafeie, S., Fang, H., Hedlund, D., Nyberg, A., Svedlindh, P., Gunnarsson, K. & Sahlberg, M. (2018). One step towards MnAl-based permanent magnets: Differences in magnetic, and microstructural properties from an intermediate annealing step during synthesis. Journal of Solid State Chemistry
Open this publication in new window or tab >>One step towards MnAl-based permanent magnets: Differences in magnetic, and microstructural properties from an intermediate annealing step during synthesis
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2018 (English)In: Journal of Solid State Chemistry, ISSN 0022-4596, E-ISSN 1095-726XArticle in journal (Refereed) Submitted
Abstract [en]

The influence of an additional annealing step during synthesis on the preparation of MnAl based permanent magnet alloys has been investigated. Bulk samples of Mn55Al45C2 alloys were synthesized using induction heating through drop synthesis from 1400 °C. Samples produced using cooling directly from 1400 °C (from the melt), and from 1400 °C to an intermediate annealing step at 1200 °C for ~ 30 min before cooling were compared with respect to differences in phase purity, microstructure and magnetic properties. We found that the phase purity was significantly enhanced using the route with an intermediate annealing step at 1200 °C. From XRD the phase purity of the tau-phase was improved from ~ 91 wt% for the sample cooled directly from 1400 °C to ~ 95.1 - 99.5 wt% for the sample exposed to an intermediate annealing step before cooling. Additionally, EBSD, and SEM with EDS indicates a clear difference in the phase composition and differences in the distribution of the magnetic tau phase and the non-magnetic epsilon-, beta-, and gamma-phases. Magnetic properties also indicate, an improvement in saturation magnetization for the sample exposed to the extra annealing step during synthesis. Our results suggest that an intermediate annealing step in the production of MnAl based alloys will provide a simple way of achieving better phase purity and magnetic properties in the bulk alloy.

Keywords
permanent magnet, rare earth free, microstructure
National Category
Natural Sciences Engineering and Technology Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-368283 (URN)
Available from: 2018-12-03 Created: 2018-12-03 Last updated: 2019-03-13Bibliographically approved
Fang, H., Cedervall, J., Hedlund, D., Shafeie, S., Deledda, S., Olsson, F., . . . Sahlberg, M. (2018). Structural, microstructural and magnetic evolution in cryo milled carbon doped MnAl. Scientific Reports, 8(1), Article ID 2525.
Open this publication in new window or tab >>Structural, microstructural and magnetic evolution in cryo milled carbon doped MnAl
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2018 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 8, no 1, article id 2525Article in journal (Refereed) Published
Abstract [en]

The low cost, rare earth free τ-phase of MnAl has high potential to partially replace bonded Nd2Fe14B rare earth permanent magnets. However, the τ-phase is metastable and it is experimentally difficult to obtain powders suitable for the permanent magnet alignment process, which requires the fine powders to have an appropriate microstructure and high τ-phase purity. In this work, a new method to make high purity τ-phase fne powders is presented. A high purity τ-phase Mn0.55Al0.45C0.02 alloy was synthesized by the drop synthesis method. The drop synthesized material was subjected to cryo milling and followed by a fash heating process. The crystal structure and microstructure of the drop synthesized, cryo milled and flash heated samples were studied by X-ray in situ powder diffraction, scanning electron microscopy, X-ray energy dispersive spectroscopy and electron backscatter diffraction. Magnetic properties and magnetic structure of the drop synthesized, cryo milled, flash heated samples were characterized by magnetometry and neutron powder diffraction, respectively. The results reveal that the 2 and 4hours cryo milled and flash heated samples both exhibit high τ-phase purity and micron-sized round particle shapes. Moreover, the fash heated samples display high saturation magnetization as well as increased coercivity.

National Category
Materials Chemistry Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-341024 (URN)10.1038/s41598-018-20606-8 (DOI)000424189500012 ()29410462 (PubMedID)
Funder
Swedish Energy AgencySweGRIDS - Swedish Centre for Smart Grids and Energy StorageSwedish Research Council
Available from: 2018-02-06 Created: 2018-02-06 Last updated: 2019-03-13Bibliographically approved
Hedlund, D., Cedervall, J., Edström, A., Werwinski, M., Kontos, S., Eriksson, O., . . . Gunnarsson, K. (2017). Magnetic properties of the Fe5SiB2−Fe5PB2 system. Physical Review B, 96(9), Article ID 094433.
Open this publication in new window or tab >>Magnetic properties of the Fe5SiB2−Fe5PB2 system
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2017 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 9, article id 094433Article in journal (Refereed) Published
Abstract [en]

The magnetic properties of the compound Fe5Si1−xPxB2 have been studied, with a focus on the Curie temperature TC, saturation magnetization MS, and magnetocrystalline anisotropy. Field and temperature dependent magnetization measurements were used to determine TC(x) and MS(x). The saturation magnetization at 10 K (300 K) is found to monotonically decrease from 1.11MA/m (1.03MA/m) to 0.97MA/m (0.87MA/m), as x increases from 0 to 1. The Curie temperature is determined to be 810 and 615 K in Fe5SiB2 and Fe5PB2, respectively. The highest TC is observed for x=0.1, while it decreases monotonically for larger x. The Curie temperatures have also been theoretically determined to be 700 and 660 K for Fe5SiB2 and Fe5PB2, respectively, using a combination of density functional theory and Monte Carlo simulations. The magnitude of the effective magnetocrystalline anisotropy was extracted using the law of approach to saturation, revealing an increase with increasing phosphorus concentration. Low-field magnetization vs temperature results for x=0,0.1,0.2 indicate that there is a transition from easy-axis to easy-plane anisotropy with decreasing temperature.

Place, publisher, year, edition, pages
American Physical Society, 2017
Keywords
Magnetism, Ferromagnetism, First-principle calculations, Magnetic interactions, Magnetic order parameter, Magnetic phase transition
National Category
Condensed Matter Physics Engineering and Technology
Research subject
Physics
Identifiers
urn:nbn:se:uu:diva-330463 (URN)10.1103/PhysRevB.96.094433 (DOI)000411975700001 ()
Funder
Swedish Research CouncilGöran Gustafsson Foundation for Research in Natural Sciences and MedicineKnut and Alice Wallenberg Foundation, 2013.0020, 2012.0031EU, Horizon 2020
Available from: 2017-09-29 Created: 2017-09-29 Last updated: 2017-12-20Bibliographically approved
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