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Bessarab, P. F., Mueller, G. P., Lobanov, I. S., Rybakov, F. N., Kiselev, N. S., Jonsson, H., . . . Delin, A. (2018). Lifetime of racetrack skyrmions. Scientific Reports, 8, Article ID 3433.
Open this publication in new window or tab >>Lifetime of racetrack skyrmions
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2018 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 8, article id 3433Article in journal (Refereed) Published
Abstract [en]

The skyrmion racetrack is a promising concept for future information technology. There, binary bits are carried by nanoscale spin swirls-skyrmions-driven along magnetic strips. Stability of the skyrmions is a critical issue for realising this technology. Here we demonstrate that the racetrack skyrmion lifetime can be calculated from first principles as a function of temperature, magnetic field and track width. Our method combines harmonic transition state theory extended to include Goldstone modes, with an atomistic spin Hamiltonian parametrized from density functional theory calculations. We demonstrate that two annihilation mechanisms contribute to the skyrmion stability: At low external magnetic field, escape through the track boundary prevails, but a crossover field exists, above which the collapse in the interior becomes dominant. Considering a Pd/Fe bilayer on an Ir(111) substrate as a well-established model system, the calculated skyrmion lifetime is found to be consistent with reported experimental measurements. Our simulations also show that the Arrhenius pre-exponential factor of escape depends only weakly on the external magnetic field, whereas the pre-exponential factor for collapse is strongly field dependent. Our results open the door for predictive simulations, free from empirical parameters, to aid the design of skyrmion-based information technology.

Place, publisher, year, edition, pages
NATURE PUBLISHING GROUP, 2018
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-224023 (URN)10.1038/s41598-018-21623-3 (DOI)000425590600057 ()29467438 (PubMedID)2-s2.0-85042361199 (Scopus ID)
Note

QC 20180323

Available from: 2018-03-23 Created: 2018-03-23 Last updated: 2018-03-23Bibliographically approved
Bergqvist, L. & Bergman, A. (2018). Realistic finite temperature simulations of magnetic systems using quantum statistics. Physical Review Materials, 2(1), Article ID 013802.
Open this publication in new window or tab >>Realistic finite temperature simulations of magnetic systems using quantum statistics
2018 (English)In: Physical Review Materials, ISSN 2475-9953, Vol. 2, no 1, article id 013802Article in journal (Refereed) Published
Abstract [en]

We have performed realistic atomistic simulations at finite temperatures using Monte Carlo and atomistic spin dynamics simulations incorporating quantum (Bose-Einstein) statistics. The description is much improved at low temperatures compared to classical (Boltzmann) statistics normally used in these kind of simulations, while at higher temperatures the classical statistics are recovered. This corrected low-temperature description is reflected in both magnetization and the magnetic specific heat, the latter allowing for improved modeling of the magnetic contribution to free energies. A central property in the method is the magnon density of states at finite temperatures, and we have compared several different implementations for obtaining it. The method has no restrictions regarding chemical and magnetic order of the considered materials. This is demonstrated by applying the method to elemental ferromagnetic systems, including Fe and Ni, as well as Fe-Co random alloys and the ferrimagnetic system GdFe3.

Place, publisher, year, edition, pages
American Physical Society, 2018
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-221348 (URN)10.1103/PhysRevMaterials.2.013802 (DOI)000419236100001 ()
Funder
Swedish Research Council, 2017-03763Swedish e‐Science Research CenterEU, FP7, Seventh Framework Programme, 600382
Note

QC 20180117

Available from: 2018-01-17 Created: 2018-01-17 Last updated: 2018-01-29Bibliographically 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.

Place, publisher, year, edition, pages
American Physical Society, 2017
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-211618 (URN)10.1103/PhysRevB.95.184432 (DOI)000405203000011 ()2-s2.0-85024367724 (Scopus ID)
Funder
Swedish Research Council, VR 2015-04608Swedish Research Council, VR 2016-05980Swedish e‐Science Research CentereSSENCE - An eScience CollaborationEU, FP7, Seventh Framework Programme, 600382Swedish Energy Agency, STEM P40147-1
Note

QC 20170810

Available from: 2017-08-10 Created: 2017-08-10 Last updated: 2017-11-29Bibliographically approved
Lizarraga, R., Pan, F., Bergqvist, L., Holmstrom, E., Gercsi, Z. & Vitos, L. (2017). First Principles Theory of the hcp-fcc Phase Transition in Cobalt. Scientific Reports, 7, Article ID 3778.
Open this publication in new window or tab >>First Principles Theory of the hcp-fcc Phase Transition in Cobalt
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2017 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 7, article id 3778Article in journal (Refereed) Published
Abstract [en]

Identifying the forces that drive a phase transition is always challenging. The hcp-fcc phase transition that occurs in cobalt at similar to 700 K has not yet been fully understood, although early theoretical studies have suggested that magnetism plays a main role in the stabilization of the fcc phase at high temperatures. Here, we perform a first principles study of the free energies of these two phases, which we break into contributions arising from the vibration of the lattice, electronic and magnetic systems and volume expansion. Our analysis of the energy of the phases shows that magnetic effects alone cannot drive the fcc-hcp transition in Co and that the largest contribution to the stabilization of the fcc phase comes from the vibration of the ionic lattice. By including all the contributions to the free energy considered here we obtain a theoretical transition temperature of 825 K.

Place, publisher, year, edition, pages
NATURE PUBLISHING GROUP, 2017
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-211395 (URN)10.1038/s41598-017-03877-5 (DOI)000403650300021 ()28630476 (PubMedID)2-s2.0-85021051213 (Scopus ID)
Note

QC 20170808

Available from: 2017-08-08 Created: 2017-08-08 Last updated: 2017-11-10Bibliographically approved
Smith, A. D., Elgammal, K., Fan, X., Lemme, M. C., Delin, A., Råsander, M., . . . Östling, M. (2017). Graphene-based CO2 sensing and its cross-sensitivity with humidity. RSC Advances, 7, 22329-22339
Open this publication in new window or tab >>Graphene-based CO2 sensing and its cross-sensitivity with humidity
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2017 (English)In: RSC Advances, ISSN 2046-2069, E-ISSN 2046-2069, Vol. 7, p. 22329-22339Article in journal (Refereed) Published
Abstract [en]

We present graphene-based CO2 sensing and analyze its cross-sensitivity with humidity. In order to assess the selectivity of graphene-based gas sensing to various gases, measurements are performed in argon (Ar), nitrogen (N2), oxygen (O2), carbon dioxide (CO2), and air by selectively venting the desired gas from compressed gas bottles into an evacuated vacuum chamber. The sensors provide a direct electrical readout in response to changes in high concentrations, from these bottles, of CO2, O2, nitrogen and argon, as well as changes in humidity from venting atmospheric air. From the signal response to each gas species, the relative graphene sensitivity to each gas is extracted as a relationship between the percentage-change in graphene's resistance response to changes in vacuum chamber pressure. Although there is virtually no response from O2, N2 and Ar, there is a sizeable cross-sensitivity between CO2 and humidity occurring at high CO2 concentrations. However, under atmospheric concentrations of CO2, this cross-sensitivity effect is negligible – allowing for the use of graphene-based humidity sensing in atmospheric environments. Finally, charge density difference calculations, computed using density functional theory (DFT) are presented in order to illustrate the bonding of CO2 and water molecules on graphene and the alterations of the graphene electronic structure due to the interactions with the substrate and the molecules.

Place, publisher, year, edition, pages
Royal Society of Chemistry, 2017
National Category
Nano Technology
Identifiers
urn:nbn:se:kth:diva-206164 (URN)10.1039/C7RA02821K (DOI)
Note

QC 20170517

Available from: 2017-04-27 Created: 2017-04-27 Last updated: 2018-07-26
Kumar, A., Pan, F., Husain, S., Akansel, S., Brucas, R., Bergqvist, L., . . . Svedlindh, P. (2017). Temperature-dependent Gilbert damping of Co2FeAl thin films with different degree of atomic order. Physical Review B, 96(22), Article ID 224425.
Open this publication in new window or tab >>Temperature-dependent Gilbert damping of Co2FeAl thin films with different degree of atomic order
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2017 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 22, article id 224425Article in journal (Refereed) Published
Abstract [en]

Half-metallicity and low magnetic damping are perpetually sought for spintronics materials, and full Heusler compounds in this respect provide outstanding properties. However, it is challenging to obtain the well-ordered half-metallic phase in as-deposited full Heusler compound thin films, and theory has struggled to establish a fundamental understanding of the temperature-dependent Gilbert damping in these systems. Here we present a study of the temperature-dependent Gilbert damping of differently ordered as-deposited Co2FeAl full Heusler compound thin films. The sum of inter-and intraband electron scattering in conjunction with the finite electron lifetime in Bloch states governs the Gilbert damping for the well-ordered phase, in contrast to the damping of partially ordered and disordered phases which is governed by interband electronic scattering alone. These results, especially the ultralow room-temperature intrinsic damping observed for the well-ordered phase, provide fundamental insights into the physical origin of the Gilbert damping in full Heusler compound thin films.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2017
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-220823 (URN)10.1103/PhysRevB.96.224425 (DOI)000418572700007 ()2-s2.0-85039444027 (Scopus ID)
Note

QC 20180111

Available from: 2018-01-11 Created: 2018-01-11 Last updated: 2018-03-07Bibliographically approved
Cedervall, J., Andersson, M. S., Sarkar, T., Delczeg-Czirjak, E. K., Bergqvist, L., Hansen, T. C., . . . Sahlberg, M. (2016). Magnetic structure of the magnetocaloric compound AlFe2B2. Journal of Alloys and Compounds, 664, 784-791
Open this publication in new window or tab >>Magnetic structure of the magnetocaloric compound AlFe2B2
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2016 (English)In: Journal of Alloys and Compounds, ISSN 0925-8388, E-ISSN 1873-4669, Vol. 664, p. 784-791Article in journal (Refereed) Published
Abstract [en]

The crystal and magnetic structures of AlFe2B2 have been studied with a combination of X-ray and neutron diffraction and electronic structure calculations. The magnetic and magnetocaloric properties have been investigated by magnetisation measurements. The samples have been produced using high temperature synthesis and subsequent heat treatments. The compound crystallises in the orthorhombic crystal system Cmmm and it orders ferromagnetically at 285 K through a second order phase transition. At temperatures below the magnetic transition the magnetic moments align along the crystallographic a-axis. The magnetic entropy change from 0 to 800 kA/m was found to be - 1.3 J/K kg at the magnetic transition temperature.

Place, publisher, year, edition, pages
Elsevier, 2016
Keywords
X-ray diffraction, Neutron diffraction, Magnetic structure, Magnetocaloric effect
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-183302 (URN)10.1016/j.jallcom.2015.12.111 (DOI)000369061700101 ()2-s2.0-84954450006 (Scopus ID)
Note

QC 20160308

Available from: 2016-03-08 Created: 2016-03-07 Last updated: 2017-11-30Bibliographically approved
Huang, S., Li, W., Li, X., Schönecker, S., Bergqvist, L., Holmström, E., . . . Vitos, L. (2016). Mechanism of magnetic transition in FeCrCoNi-based high entropy alloys. Materials & design, 103, 71-74
Open this publication in new window or tab >>Mechanism of magnetic transition in FeCrCoNi-based high entropy alloys
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2016 (English)In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 103, p. 71-74Article in journal (Refereed) Published
Abstract [en]

First-principles alloy theory and Monte-Carlo simulations are performed to investigate the magnetic properties of FeCrCoNiAlx high entropy alloys. Results show that face-centered-cubic (fcc) and body-centered-cubic (bcc) structures possess significantly different magnetic behaviors uncovering that the alloy's Curie temperature is controlled by the stability of the Al-induced single phase or fcc-bcc dual-phase. We show that the appearance of the bcc phase with increasing Al content brings about the observed transition from the paramagnetic state for FeCrCoNi to the ferromagnetic state for FeCrCoNiAl at room-temperature. Similar mechanism is predicted to give rise to room-temperature ferromagnetism in FeCrCoNiGa high entropy alloy.

Place, publisher, year, edition, pages
Elsevier, 2016
Keywords
First-principles calculation, High-entropy alloy, Magnetic transition, Monte-Carlo simulation, Aluminum, Calculations, Crystal structure, Entropy, Ferromagnetism, Intelligent systems, Magnetism, Stainless steel, Body-centered cubic (bcc) structure, Face-centered cubic, Ferromagnetic state, High entropy alloys, Magnetic transitions, Paramagnetic state, Room temperature ferromagnetism, Monte Carlo methods
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-186897 (URN)10.1016/j.matdes.2016.04.053 (DOI)000376892300008 ()2-s2.0-84964558121 (Scopus ID)
Note

QC 20160518

Available from: 2016-05-18 Created: 2016-05-16 Last updated: 2018-06-01Bibliographically 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
Etz, C., Bergqvist, L., Bergman, A., Taroni, A. & Eriksson, O. (2015). Atomistic spin dynamics and surface magnons. Journal of Physics: Condensed Matter, 27(24), Article ID 243202.
Open this publication in new window or tab >>Atomistic spin dynamics and surface magnons
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2015 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 27, no 24, article id 243202Article, review/survey (Refereed) Published
Abstract [en]

Atomistic spin dynamics simulations have evolved to become a powerful and versatile tool for simulating dynamic properties of magnetic materials. It has a wide range of applications, for instance switching of magnetic states in bulk and nano-magnets, dynamics of topological magnets, such as skyrmions and vortices and domain wall motion. In this review, after a brief summary of the existing investigation tools for the study of magnons, we focus on calculations of spin-wave excitations in low-dimensional magnets and the effect of relativistic and temperature effects in such structures. In general, we find a good agreement between our results and the experimental values. For material specific studies, the atomistic spin dynamics is combined with electronic structure calculations within the density functional theory from which the required parameters are calculated, such as magnetic exchange interactions, magnetocrystalline anisotropy, and Dzyaloshinskii-Moriya vectors.

Keywords
magnetism, spin-wave excitations, atomistic spin dynamics simulations
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-169950 (URN)10.1088/0953-8984/27/24/243202 (DOI)000355834800003 ()26030259 (PubMedID)2-s2.0-84930960291 (Scopus ID)
Funder
EU, European Research CouncilSwedish Research CouncilKnut and Alice Wallenberg FoundationCarl Tryggers foundation
Note

QC 20150626

Available from: 2015-06-26 Created: 2015-06-25 Last updated: 2017-12-04Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0003-4341-5663

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