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Publications (6 of 6) Show all publications
Hellsvik, J., Thonig, D., Modin, K., Iusan, D., Bergman, A., Eriksson, O., . . . Delin, A. (2019). General method for atomistic spin-lattice dynamics with first-principles accuracy. Physical Review B, 99(10), Article ID 104302.
Open this publication in new window or tab >>General method for atomistic spin-lattice dynamics with first-principles accuracy
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2019 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 10, article id 104302Article in journal (Refereed) Published
Abstract [en]

We present a computationally efficient and general first-principles based method for spin-lattice simulations for solids and clusters. The method is based on a coupling of atomistic spin dynamics and molecular dynamics simulations, expressed through a spin-lattice Hamiltonian, where the bilinear magnetic term is expanded up to second order in displacement. The effect of first-order spin-lattice coupling on the magnon and phonon dispersion in bcc Fe is reported as an example, and we observe good agreement with previous simulations. We also illustrate the coupled spin-lattice dynamics method on a more conceptual level, by exploring dissipation-free spin and lattice motion of small magnetic clusters (a dimer, trimer, and tetramer). The method discussed here opens the door for a quantitative description and understanding of the microscopic origin of many fundamental phenomena of contemporary interest, such as ultrafast demagnetization, magnetocalorics, and spincaloritronics.

Place, publisher, year, edition, pages
American Physical Society, 2019
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-248337 (URN)10.1103/PhysRevB.99.104302 (DOI)000461953800003 ()2-s2.0-85063194644 (Scopus ID)
Note

QC 20190410

Available from: 2019-04-10 Created: 2019-04-10 Last updated: 2019-04-10Bibliographically approved
Shirinyan, A. A., Kozin, V. K., Hellsvik, J., Pereiro, M., Eriksson, O. & Yudin, D. (2019). Self-organizing maps as a method for detecting phase transitions and phase identification. Physical Review B, 99(4), Article ID 041108.
Open this publication in new window or tab >>Self-organizing maps as a method for detecting phase transitions and phase identification
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2019 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 4, article id 041108Article in journal (Refereed) Published
Abstract [en]

Originating from image recognition, methods of machine learning allow for effective feature extraction and dimensionality reduction in multidimensional datasets, thereby providing an extraordinary tool to deal with classical and quantum models in many-body physics. In this study, we employ a specific unsupervised machine learning technique-self-organizing maps-to create a low-dimensional representation of microscopic states, relevant for macroscopic phase identification and detecting phase transitions. We explore the properties of spin Hamiltonians of two archetype model systems: a two-dimensional Heisenberg ferromagnet and a three-dimensional crystal, Fe in the body-centered-cubic structure. The method of self-organizing maps, which is known to conserve connectivity of the initial dataset, is compared to the cumulant method theory and is shown to be as accurate while being computationally more efficient in determining a phase transition temperature. We argue that the method proposed here can be applied to explore a broad class of second-order phase-transition systems, not only magnetic systems but also, for example, order-disorder transitions in alloys.

Place, publisher, year, edition, pages
American Physical Society, 2019
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-242980 (URN)10.1103/PhysRevB.99.041108 (DOI)000455825600002 ()2-s2.0-85059904004 (Scopus ID)
Funder
EU, Horizon 2020Swedish Research CouncilKnut and Alice Wallenberg FoundationSwedish Foundation for Strategic Research Swedish Energy AgencyeSSENCE - An eScience CollaborationStandUp
Note

QC 20190204

Available from: 2019-02-04 Created: 2019-02-04 Last updated: 2019-02-04Bibliographically approved
Kadas, K., Iusan, D., Hellsvik, J., Cedervall, J., Berastegui, P., Sahlberg, M., . . . Eriksson, O. (2017). AlM2B2 (M = Cr, Mn, Fe, Co, Ni): a group of nanolaminated materials. Journal of Physics: Condensed Matter, 29(15), Article ID 155402.
Open this publication in new window or tab >>AlM2B2 (M = Cr, Mn, Fe, Co, Ni): a group of nanolaminated materials
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2017 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 29, no 15, article id 155402Article in journal (Refereed) Published
Abstract [en]

Combining theory with experiments, we study the phase stability, elastic properties, electronic structure and hardness of layered ternary borides AlCr2B2, AlMn2B2, AlFe2B2, AlCo2B2, and AlNi2B2. We find that the first three borides of this series are stable phases, while AlCo2B2 and AlNi2B2 are metastable. We show that the elasticity increases in the boride series, and predict that AlCr2B2, AlMn2B2, and AlFe2B2 are more brittle, while AlCo2B2 and AlNi2B2 are more ductile. We propose that the elasticity of AlFe2B2 can be improved by alloying it with cobalt or nickel, or a combination of them. We present evidence that these ternary borides represent nanolaminated systems. Based on SEM measurements, we demonstrate that they exhibit the delamination phenomena, which leads to a reduced hardness compared to transition metal mono-and diborides. We discuss the background of delamination by analyzing chemical bonding and theoretical work of separation in these borides.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2017
Keywords
nanolaminated ternary borides, phase stability, elastic constants, hardness, scanning electron microscopy
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-205430 (URN)10.1088/1361-648X/aa602a (DOI)000397921600002 ()28192279 (PubMedID)2-s2.0-85015870039 (Scopus ID)
Note

QC 20170522

Available from: 2017-05-22 Created: 2017-05-22 Last updated: 2017-05-22Bibliographically approved
Fransson, J., Thonig, D., Bessarab, P. F., Bhattacharjee, S., Hellsvik, J. & Nordström, L. (2017). Microscopic theory for coupled atomistic magnetization and lattice dynamics. Physical Review Materials, 1(7), Article ID 074404.
Open this publication in new window or tab >>Microscopic theory for coupled atomistic magnetization and lattice dynamics
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2017 (English)In: Physical Review Materials, ISSN 2475-9953, Vol. 1, no 7, article id 074404Article in journal (Refereed) Published
Abstract [en]

A coupled atomistic spin and lattice dynamics approach is developed which merges the dynamics of these two degrees of freedom into a single set of coupled equations of motion. The underlying microscopic model comprises local exchange interactions between the electron spin and magnetic moment and the local couplings between the electronic charge and lattice displacements. An effective action for the spin and lattice variables is constructed in which the interactions among the spin and lattice components are determined by the underlying electronic structure. In this way, expressions are obtained for the electronically mediated couplings between the spin and lattice degrees of freedom, besides the well known interatomic force constants and spin-spin interactions. These former susceptibilities provide an atomistic ab initio description for the coupled spin and lattice dynamics. It is important to notice that this theory is strictly bilinear in the spin and lattice variables and provides a minimal model for the coupled dynamics of these subsystems and that the two subsystems are treated on the same footing. Questions concerning time-reversal and inversion symmetry are rigorously addressed and it is shown how these aspects are absorbed in the tensor structure of the interaction fields. By means of these results regarding the spin-lattice coupling, simple explanations of ionic dimerization in double-antiferromagnetic materials, as well as charge density waves induced by a nonuniform spin structure, are given. In the final parts, coupled equations of motion for the combined spin and lattice dynamics are constructed, which subsequently can be reduced to a form which is analogous to the Landau-Lifshitz-Gilbert equations for spin dynamics and a damped driven mechanical oscillator for the ionic motion. It is important to notice, however, that these equations comprise contributions that couple these descriptions into one unified formulation. Finally, Kubo-like expressions for the discussed exchanges in terms of integrals over the electronic structure and, moreover, analogous expressions for the damping within and between the subsystems are provided. The proposed formalism and types of couplings enable a step forward in the microscopic first principles modeling of coupled spin and lattice quantities in a consistent format.

Place, publisher, year, edition, pages
American Physical Society, 2017
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-221013 (URN)10.1103/PhysRevMaterials.1.074404 (DOI)000418772500005 ()
Funder
Swedish Research Council, 2016-06955The Wenner-Gren Foundation
Note

QC 20180112

Available from: 2018-01-12 Created: 2018-01-12 Last updated: 2018-01-12Bibliographically approved
Bergman, A., Hellsvik, J., Bessarab, P. F. & Delin, A. (2016). Spin relaxation signature of colossal magnetic anisotropy in platinum atomic chains. Scientific Reports, 6, Article ID 36872.
Open this publication in new window or tab >>Spin relaxation signature of colossal magnetic anisotropy in platinum atomic chains
2016 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 6, article id 36872Article in journal (Refereed) Published
Abstract [en]

Recent experimental data demonstrate emerging magnetic order in platinum atomically thin nanowires. Furthermore, an unusual form of magnetic anisotropy-colossal magnetic anisotropy (CMA)-was earlier predicted to exist in atomically thin platinum nanowires. Using spin dynamics simulations based on first-principles calculations, we here explore the spin dynamics of atomically thin platinum wires to reveal the spin relaxation signature of colossal magnetic anisotropy, comparing it with other types of anisotropy such as uniaxial magnetic anisotropy (UMA). We find that the CMA alters the spin relaxation process distinctly and, most importantly, causes a large speed-up of the magnetic relaxation compared to uniaxial magnetic anisotropy. The magnetic behavior of the nanowire exhibiting CMA should be possible to identify experimentally at the nanosecond time scale for temperatures below 5 K. This time-scale is accessible in e.g., soft x-ray free electron laser experiments.

Place, publisher, year, edition, pages
Nature Publishing Group, 2016
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-198960 (URN)10.1038/srep36872 (DOI)000387564100001 ()27841287 (PubMedID)2-s2.0-84995389822 (Scopus ID)
Funder
Swedish e‐Science Research CenterSwedish Research CouncilThe Royal Swedish Academy of SciencesKnut and Alice Wallenberg FoundationSwedish Energy AgencySwedish Foundation for Strategic Research Carl Tryggers foundation eSSENCE - An eScience CollaborationGöran Gustafsson Foundation for promotion of scientific research at Uppala University and Royal Institute of Technology
Note

QC 20170116

Available from: 2017-01-16 Created: 2016-12-22 Last updated: 2017-11-29Bibliographically approved
Pan, F., Chico, J., Hellsvik, J., Delin, A., Bergman, A. & Bergqvist, L. (2016). Systematic study of magnetodynamic properties at finite temperatures in doped permalloy from first-principles calculations. Physical Review B, 94(21), Article ID 214410.
Open this publication in new window or tab >>Systematic study of magnetodynamic properties at finite temperatures in doped permalloy from first-principles calculations
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2016 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 21, article id 214410Article in journal (Refereed) Published
Abstract [en]

By means of first-principles calculations, we have systematically investigated how the magnetodynamic properties Gilbert damping, magnetization, and exchange stiffness are affected when permalloy (Py) (Fe0.19Ni0.81) is doped with 4d or 5d transition metal impurities. We find that the trends in the Gilbert damping can be understood from relatively few basic parameters such as the density of states at the Fermi level, the spin-orbit coupling, and the impurity concentration. The temperature dependence of the Gilbert damping is found to be very weak which we relate to the lack of intraband transitions in alloys. Doping with 4d elements has no major impact on the studied Gilbert damping, apart from diluting the host. However, the 5d elements have a profound effect on the damping and allow it to be tuned over a large interval while maintaining the magnetization and exchange stiffness. As regards the spin stiffness, doping with early transition metals results in considerable softening, whereas late transition metals have a minor impact. Our result agree well with earlier calculations where available. In comparison to experiments, the computed Gilbert damping appears slightly underestimated, whereas the spin stiffness shows a general good agreement.

Place, publisher, year, edition, pages
American Physical Society, 2016
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-199481 (URN)10.1103/PhysRevB.94.214410 (DOI)000389573700002 ()2-s2.0-85006340172 (Scopus ID)
Note

QC 20170120

Available from: 2017-01-20 Created: 2017-01-09 Last updated: 2017-11-29Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0003-0210-4340

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