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Bergman, Anders
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Publications (10 of 70) Show all publications
Koumpouras, K., Yudin, D., Adelmann, C., Bergman, A., Eriksson, O. & Pereiro, M. (2018). A majority gate with chiral magnetic solitons. Journal of Physics: Condensed Matter, 30(37), Article ID 375801.
Open this publication in new window or tab >>A majority gate with chiral magnetic solitons
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2018 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 30, no 37, article id 375801Article in journal (Refereed) Published
Abstract [en]

In magnetic materials, nontrivial spin textures may emerge due to the competition among different types of magnetic interactions. Among such spin textures, chiral magnetic solitons represent topologically protected spin configurations with particle-like properties. Based on atomistic spin dynamics simulations, we demonstrate that these chiral magnetic solitons are ideal to use for logical operations, and we demonstrate the functionality of a three- input majority gate, in which the input states can be controlled by applying an external electromagnetic field or spin-polarized currents. One of the main advantages of the proposed device is that the input and output signals are encoded in the chirality of solitons, that may be moved, allowing to perform logical operations using only minute electric currents. As an example we illustrate how the three input majority gate can be used to perform logical relations, such as Boolean AND and OR.

Place, publisher, year, edition, pages
IOP PUBLISHING LTD, 2018
Keywords
magnonics, majority gate, solitons, spin dynamics, magnetic nanodevices
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-362096 (URN)10.1088/1361-648X/aad82f (DOI)000442630600001 ()30079893 (PubMedID)
Funder
Knut and Alice Wallenberg Foundation, 2013.0020Knut and Alice Wallenberg Foundation, 2012.0031Swedish Research CouncileSSENCE - An eScience Collaboration
Available from: 2018-10-03 Created: 2018-10-03 Last updated: 2018-10-03Bibliographically approved
Yadav, R., Pereiro, M., Bogdanov, N. A., Nishimoto, S., Bergman, A., Eriksson, O., . . . Hozoi, L. (2018). Heavy-mass magnetic modes in pyrochlore iridates due to dominant Dzyaloshinskii-Moriya interaction. PHYSICAL REVIEW MATERIALS, 2(7), Article ID 074408.
Open this publication in new window or tab >>Heavy-mass magnetic modes in pyrochlore iridates due to dominant Dzyaloshinskii-Moriya interaction
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2018 (English)In: PHYSICAL REVIEW MATERIALS, ISSN 2475-9953, Vol. 2, no 7, article id 074408Article in journal (Refereed) Published
Abstract [en]

Materials with strong spin-orbit interactions are presently a main target in the search for systems with novel magnetic properties. Magnetic anisotropies can be very large in such compounds, ranging from strongly frustrated Kitaev exchange and the associated spin-liquid states in honeycomb iridates to robust antisymmetric couplings in square-lattice Sr2IrO4. Here we predict from ab initio quantum chemistry calculations that another highly unusual regime is realized in pyrochlore iridium oxides: the isotropic nearest-neighbor Heisenberg term can vanish while the antisymmetric Dzyaloshinskii-Moriya interaction reaches values as large as 5 meV, a result which challenges common notions and existing phenomenological models of magnetic superexchange. The resulting spin-excitation spectra reveal a very flat magnon dispersion in the Nd- and Tb-based pyrochlore iridates, suggesting the possibility of using these modes to store magnetic information. Indeed, the magnetization dynamics indicates that these modes are unable to propagate out of the excitation region. Although most of the results presented here are predictions of exotic magnetic states based on first-principles theory, we make connections to observations and establish the accuracy of our approach by reproducing experimental data for Sm2Ir2O4.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-361999 (URN)10.1103/PhysRevMaterials.2.074408 (DOI)000439987600002 ()
Funder
Knut and Alice Wallenberg Foundation, 2013.0020Knut and Alice Wallenberg Foundation, 2012.0031Swedish Research CouncileSSENCE - An eScience CollaborationGerman Research Foundation (DFG), SFB-1143 HO-4427/2
Available from: 2018-10-08 Created: 2018-10-08 Last updated: 2018-10-08Bibliographically approved
Pan, F., Chico, J., Delin, A., Bergman, A. & Bergqvist, L. (2017). Extended spin model in atomistic simulations of alloys. Physical Review B, 95(18), Article ID 184432.
Open this publication in new window or tab >>Extended spin model in atomistic simulations of alloys
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2017 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 18, article id 184432Article in journal (Refereed) Published
Abstract [en]

An extended atomistic spin model allowing for studies of the finite-temperature magnetic properties of alloys is proposed. The model is obtained by extending the Heisenberg Hamiltonian via a parametrization from a first-principles basis, interpolating from both the low-temperature ferromagnetic and the high-temperature paramagnetic reference states. This allows us to treat magnetic systems with varying degree of itinerant character within the model. Satisfactory agreement with both previous theoretical studies and experiments are obtained in terms of Curie temperatures and paramagnetic properties. The proposed model is not restricted to elements but is also applied to binary alloys, such as the technologically important material permalloy, where significant differences in the finite magnetic properties of Fe and Ni magnetic moments are found. The proposed model strives to find the right compromise between accuracy and computational feasibility for accurate modeling, even for complex magnetic alloys and compounds.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-331944 (URN)10.1103/PhysRevB.95.184432 (DOI)000405203000011 ()
Funder
Swedish Research Council, VR 2015-04608, VR 2016-05980Swedish Energy Agency, STEM P40147-1Göran Gustafsson Foundation for promotion of scientific research at Uppala University and Royal Institute of Technology
Available from: 2017-10-24 Created: 2017-10-24 Last updated: 2017-11-29Bibliographically approved
de Melo Rodrigues, D. C., Pereiro, M., Bergman, A., Eriksson, O. & Klautau, A. B. (2017). First-principles theory of electronic structure and magnetism of Cr nano-islands on Pd(111). Journal of Physics: Condensed Matter, 29(2), Article ID 025807.
Open this publication in new window or tab >>First-principles theory of electronic structure and magnetism of Cr nano-islands on Pd(111)
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2017 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 29, no 2, article id 025807Article in journal (Refereed) Published
Abstract [en]

We report on the electronic structure, magnetic moments and exchange interactions of one-and two-dimensional Cr clusters on a Pd(1 1 1) substrate, using a real-space method based on density functional theory in the local spin density approximation. We find in general that for the investigated clusters, the magnetic moments are sizeable and almost entirely of spin-character. We demonstrate that the interactions in general are dominated by nearest-neighbor antiferromagnetic Heisenberg form, which implies that Cr on Pd(1 1 1) forms an ideal model system, in which clusters of almost any shape and size can be investigated from a Heisenberg Hamiltonian, using a nearest-neighbor exchange model. We have also found that complex magnetic structures can be realized for linear chains of Cr, due to a competition between exchange interaction and a weaker Dzyaloshinskii-Moriya interaction.

Keywords
nanomagnetism, electronic structure, metallic nanowires
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-311166 (URN)10.1088/0953-8984/29/2/025807 (DOI)000388615900007 ()
Funder
Knut and Alice Wallenberg Foundation, 2013.0020 2012.0031Swedish Research Council
Available from: 2016-12-22 Created: 2016-12-22 Last updated: 2017-11-29Bibliographically approved
Bondarenko, N., Kvashnin, Y., Chico, J., Bergman, A., Eriksson, O. & Skorodumova, N. V. (2017). Spin-polaron formation and magnetic state diagram in La-doped CaMnO3. Physical Review B, 95(22), Article ID 220401.
Open this publication in new window or tab >>Spin-polaron formation and magnetic state diagram in La-doped CaMnO3
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2017 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 22, article id 220401Article in journal (Refereed) Published
Abstract [en]

LaxCa1-xMnO3 (LCMO) has been studied in the framework of density functional theory (DFT) using Hubbard-U correction. We show that the formation of spin polarons of different configurations is possible in the G-type antiferromagnetic phase. We also show that the spin-polaron (SP) solutions are stabilized due to an interplay of magnetic and lattice effects at lower La concentrations and mostly due to the lattice contribution at larger concentrations. Our results indicate that the development of SPs is unfavorable in the C- and A-type antiferromagnetic phases. The theoretically obtained magnetic state diagram is in good agreement with previously reported experimental results.

Place, publisher, year, edition, pages
American Physical Society, 2017
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-327218 (URN)10.1103/PhysRevB.95.220401 (DOI)000402799700002 ()
Available from: 2017-08-17 Created: 2017-08-17 Last updated: 2018-02-05Bibliographically approved
Cardias, R., Szilva, A., Bergman, A., Di Marco, I., Katsnelson, M. I., Lichtenstein, A. I., . . . Kvashnin, Y. O. (2017). The Bethe-Slater curve revisited; new insights from electronic structure theory. Scientific Reports, 7, Article ID 4058.
Open this publication in new window or tab >>The Bethe-Slater curve revisited; new insights from electronic structure theory
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2017 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 7, article id 4058Article in journal (Refereed) Published
Abstract [en]

The Bethe-Slater (BS) curve describes the relation between the exchange coupling and interatomic distance. Based on a simple argument of orbital overlaps, it successfully predicts the transition from antiferromagnetism to ferromagnetism, when traversing the 3d series. In a previous article [Phys. Rev. Lett. 116, 217202 (2016)] we reported that the dominant nearestneighbour (NN) interaction for 3d metals in the bcc structure indeed follows the BS curve, but the trends through the series showed a richer underlying physics than was initially assumed. The orbital decomposition of the inter-site exchange couplings revealed that various orbitals contribute to the exchange interactions in a highly non-trivial and sometimes competitive way. In this communication we perform a deeper analysis by comparing 3d metals in the bcc and fcc structures. We find that there is no coupling between the E-g orbitals of one atom and T-2g orbitals of its NNs, for both cubic phases. We demonstrate that these couplings are forbidden by symmetry and formulate a general rule allowing to predict when a similar situation is going to happen. In gamma-Fe, as in alpha-Fe, we find a strong competition in the symmetry-resolved orbital contributions and analyse the differences between the high-spin and low-spin solutions.

Place, publisher, year, edition, pages
NATURE PUBLISHING GROUP, 2017
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-330728 (URN)10.1038/s41598-017-04427-9 (DOI)000403874900041 ()28642615 (PubMedID)
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note

Erratum

doi:10.1038/s41598-017-09611-5

In the original version of this Article, Y. O. Kvashnin was incorrectly affiliated with ‘Faculdade de Fisica, Universidade Federal do Para, Belem, PA, Brazil’. The correct affiliation is listed below.

Department of Physics and Astronomy, Division of Materials Theory, Uppsala University, Box 516, SE-75120, Uppsala, Sweden.

This error has now been corrected in the PDF and HTML versions of the Article.

Available from: 2017-10-10 Created: 2017-10-10 Last updated: 2018-04-07Bibliographically approved
Arnalds, U. B., Chico, J., Stopfel, H., Kapaklis, V., Bärenbold, O., Verschuuren, M. A., . . . Hjörvarsson, B. (2016). A new look on the two-dimensional Ising model: thermal artificial spins. New Journal of Physics, 18, Article ID 023008.
Open this publication in new window or tab >>A new look on the two-dimensional Ising model: thermal artificial spins
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2016 (English)In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 18, article id 023008Article in journal (Refereed) Published
Abstract [en]

We present a direct experimental investigation of the thermal ordering in an artificial analogue of an asymmetric two-dimensional Ising system composed of a rectangular array of nano-fabricated magnetostatically interacting islands. During fabrication and below a critical thickness of the magnetic material the islands are thermally fluctuating and thus the system is able to explore its phase space. Above the critical thickness the islands freeze-in resulting in an arrested thermalized state for the array. Determining the magnetic state we demonstrate a genuine artificial two-dimensional Ising system which can be analyzed in the context of nearest neighbor interactions.

Keywords
magnetic ordering, artificial spins, Ising model
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-288634 (URN)10.1088/1367-2630/18/2/023008 (DOI)000372453700002 ()
Funder
Knut and Alice Wallenberg FoundationSwedish Research CouncilThe Swedish Foundation for International Cooperation in Research and Higher Education (STINT)
Available from: 2016-05-04 Created: 2016-04-28 Last updated: 2017-11-30Bibliographically approved
Koumpouras, K., Bergman, A., Eriksson, O. & Yudin, D. (2016). A spin dynamics approach to solitonics. Scientific Reports, 6, Article ID 25685.
Open this publication in new window or tab >>A spin dynamics approach to solitonics
2016 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 6, article id 25685Article in journal (Refereed) Published
Abstract [en]

In magnetic materials a variety of non-collinear ground state configurations may emerge as a result of competition among exchange, anisotropy, and dipole-dipole interaction, yielding magnetic states far more complex than those of homogenous ferromagnets. Of particular interest in this study are particle-like configurations. These particle-like states, e.g., magnetic solitons, skyrmions, or domain walls, form a spatially localised clot of magnetic energy. In this paper we address topologically protected magnetic solitons and explore concepts that potentially might be relevant for logical operations and/or information storage in the rapidly advancing filed of solitonics (and skyrmionics). An ability to easily create, address, and manipulate such structures is among the prerequisite forming a basis of "-onics technology", and is investigated in detail here using numerical and analytical tools.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-297269 (URN)10.1038/srep25685 (DOI)000375436700001 ()27156906 (PubMedID)
Funder
Knut and Alice Wallenberg Foundation, KAW 2013.0020, KAW 2012.0031eSSENCE - An eScience CollaborationSwedish Research Council
Available from: 2016-06-23 Created: 2016-06-22 Last updated: 2017-11-28Bibliographically approved
Rodrigues, D. C. M., Szilva, A., Klautau, A. B., Bergman, A., Eriksson, O. & Etz, C. (2016). Finite-temperature interatomic exchange and magnon softening in Fe overlayers on Ir(001). PHYSICAL REVIEW B, 94(1), Article ID 014413.
Open this publication in new window or tab >>Finite-temperature interatomic exchange and magnon softening in Fe overlayers on Ir(001)
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2016 (English)In: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 94, no 1, article id 014413Article in journal (Refereed) Published
Abstract [en]

We evaluate how thermal effects soften the magnon dispersion in 6 layers of Fe(001) on top of Ir(001). We perform a systematic study considering noncollinear spin arrangement and calculate configuration-dependent exchange parameters J(ij)(nc) following the methodology described by Szilva et al. [Phys. Rev. Lett. 111, 127204 (2013)]. In addition, Monte Carlo simulations were performed in order to estimate the noncollinear spin arrangement as a function of temperature. Hence the J(ij)(nc)'s related to these configurations were calculated and used in an atomistic spin dynamics approach to evaluate the magnon spectra. Our results show good agreement with recent room-temperature measurements, and highlights how thermal effects produce magnon softening in this, and similar, systems.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-300459 (URN)10.1103/PhysRevB.94.014413 (DOI)000379497400007 ()
Funder
Swedish Research Council, 2013.0020; 2012.0031
Available from: 2016-08-09 Created: 2016-08-09 Last updated: 2017-04-12Bibliographically approved
Chico, J., Keshavarz, S., Kvashnin, Y., Pereiro, M., Di Marco, I., Etz, C., . . . Bergqvist, L. (2016). First principles studies of the Gilbert damping and exchange interactions for half-metallic Heuslers alloys. Physical Review B. Condensed Matter and Materials Physics, 93(21), Article ID 214439.
Open this publication in new window or tab >>First principles studies of the Gilbert damping and exchange interactions for half-metallic Heuslers alloys
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2016 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 93, no 21, article id 214439Article in journal (Refereed) Published
Abstract [en]

Heusler alloys have been intensively studied due to the wide variety of properties that they exhibit. One of these properties is of particular interest for technological applications, i.e., the fact that some Heusler alloys are half-metallic. In the following, a systematic study of the magnetic properties of three different Heusler families Co(2)MnZ, Co(2)FeZ, and Mn(2)VZ with Z = (Al, Si, Ga, Ge) is performed. A key aspect is the determination of the Gilbert damping from first-principles calculations, with special focus on the role played by different approximations, the effect that substitutional disorder and temperature effects. Heisenberg exchange interactions and critical temperature for the alloys are also calculated as well as magnon dispersion relations for representative systems, the ferromagnetic Co2FeSi and the ferrimagnetic Mn2VAl. Correlation effects beyond standard density-functional theory are treated using both the local spin density approximation including the Hubbard U and the local spin density approximation plus dynamical mean field theory approximation, which allows one to determine if dynamical self-energy corrections can remedy some of the inconsistencies which were previously reported for these alloys.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-291375 (URN)10.1103/PhysRevB.93.214439 (DOI)000378809800001 ()
Funder
Swedish Research CouncilGöran Gustafsson Foundation for promotion of scientific research at Uppala University and Royal Institute of TechnologyKnut and Alice Wallenberg Foundation, 2013.0020; 2012.0031
Available from: 2016-05-02 Created: 2016-05-02 Last updated: 2018-08-24Bibliographically approved
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