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Topological excitations in a kagome magnet
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
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2014 (English)In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 5, 4815- p.Article in journal (Refereed) Published
Abstract [en]

Chirality-that is, left or right handedness-is present in many scientific areas, and particularly in condensed matter physics. Inversion symmetry breaking relates chirality with skyrmions, which are protected field configurations with particle-like and topological properties. Here we show that a kagome magnet, with Heisenberg and Dzyaloshinskii-Moriya interactions, causes non-trivial topological and chiral magnetic properties. We also find that under special circumstances, skyrmions emerge as excitations, having stability even at room temperature. Chiral magnonic edge states of a kagome magnet offer, in addition, a promising way to create, control and manipulate skyrmions. This has potential for applications in spintronics, that is, for information storage or as logic devices. Collisions between these particle-like excitations are found to be elastic at very low temperature in the skyrmion-skyrmion channel, albeit without mass-conservation. Skyrmion-antiskyrmion collisions are found to be more complex, where annihilation and creation of these objects have a distinct non-local nature.

Place, publisher, year, edition, pages
2014. Vol. 5, 4815- p.
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:uu:diva-236091DOI: 10.1038/ncomms5815ISI: 000342929000005OAI: oai:DiVA.org:uu-236091DiVA: diva2:763408
Available from: 2014-11-14 Created: 2014-11-12 Last updated: 2017-12-05
In thesis
1. Trends in Magnetism: From Strong Correlations to “-onics” Technology
Open this publication in new window or tab >>Trends in Magnetism: From Strong Correlations to “-onics” Technology
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Despite of enormous progress in experimental nanophysics theoretical studies of low-dimensional electron systems still remains a challenging task. Indeed, most of the structures are strongly correlated, so that an effective perturbative treatment is impossible due to the lack of a small parameter. The problem can be partly solved within the dynamical mean-field theory (DMFT) paradigm, nevertheless the correlations in physically relevant high-temperature superconductors are of purely non-local nature. The recently developed dual fermion approximation, combining field-theoretical diagram technique and numerical methods, allows for explicit account of spatial correlations. The approximation was shown to be of fastest convergence compared with standard DMFT extensions, and along with renormalization group is used here to study Fermi condensation on a triangular lattice near van Hove singularities. The still debated phenomenon of Fermi condensation is believed to be a precursor to strongly correlated low-temperature instability and is found in this thesis to be robust even at high temperature, making its experimental verification feasible. Unlike homogeneous ferromagnetic ordering a variety of non-collinear ground state configurations emerge as a result of competition among exchange, anisotropy, and dipole-dipole interaction. These particle-like states, e.g. magnetic soliton, skyrmion, domain wall, form a spatially localized clot of magnetic energy. Consistent study of spin, which essentially is a quantum mechanical entity, led to the emergence of spintronics (spin-based electronics) and magnonics (photonics with spin waves), in the meanwhile topologically protected magnetic solitons and skyrmions might potentially be applied for data processing and information storage in next generation of electronic technology (rapidly advancing solitonics and skyrmionics). An ability to easily create, address, and manipulate such structures is among the prerequisite forming a basis of "-onics" technology. It is shown here that spins on a kagome lattice, interacting via Heisenberg exchange and Dzyaloshinskii-Moriya coupling, allow the formation of topologically protected edge states through which a skyrmion can propagate. Not only can chemical methods be used to design novel functionality, but also geometric structuring. It is demonstrated that for graphene sandwiched between two insulating media external circularly-polarized light serves as an effective magnetic field. The direct practical implication permits to control light polarization and induce spin-waves propagating on the surface of e.g. a topological insulator. The newly discovered Dirac materials, graphene and three-dimensional topological insulators, are not easy to handle. In fact, the quasiparticle band function is gapless preventing them from being used in integrated circuits, nevertheless the problem is shown here to be partially relaxed by placing a vacancy on top of it. 

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2015. 109 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1214
Keyword
Strongly interacting electron systems, Spin dynamics, Topological matter
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-238177 (URN)978-91-554-9130-7 (ISBN)
Public defence
2015-02-13, Hall IX, Main University Building, Biskopsgatan 3, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2015-01-22 Created: 2014-12-10 Last updated: 2015-03-09
2. Magnetization dynamics on the nanoscale: From first principles to atomistic spin dynamics
Open this publication in new window or tab >>Magnetization dynamics on the nanoscale: From first principles to atomistic spin dynamics
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis first-principles methods, based on density functional theory, have been used to characterize a wide range of magnetic materials. Special emphasis has been put on pairwise magnetic interactions, such as Heisenberg exchange and Dzyaloshinskii-Moriya interactions, and also on in the Gilbert damping parameter. These parameters play a crucial role in determining the magnetization dynamics of the considered materials.

Magnetic interaction parameters, has been calculated for several materials based on Co/Ni/Co heterostructures deposited on non-magnetic heavy metals where. The aim was to clarify how the composition of the underlayers affect the magnetic properties, in particular the Dzyaloshinskii-Moriya interactions. The DMI was found to be strongly dependent on the material of the underlayer, which is consistent with previous theoretical works. Such behaviour can be traced back to the change of the spin-orbit coupling with the material of the underlayer, as well as with the hybridization of the d- states of the magnetic system with the d- state of the non-magnetic substrate.

First-principles calculations of the Gilbert damping parameter has been performed for several magnetic materials. Among them the full Heusler families, Co2FeZ, Co2MnZ with Z=(Al, Si, Ga, Ge). It was found that the first-principles methods, reproduce quite well the experimental trends, even though the obtained values are consistently smaller than the experimental measurements. A clear correlation between the Gilbert damping and the density of states at the Fermi energy was found, which is in agreement with previous works. In general as the density of states at the Fermi energy decreases, the damping decreases also.

The parameters from first principles methods, have been used in conjunction with atomistic spin dynamics simulations, in order to study ultra-narrow domain walls. The domain wall motion of a monolayer of Fe on W(110) has been studied for a situation when the domain wall is driven via thermally generated spin waves from a thermal gradient. It was found that the ultra-narrow domain walls have an unexpected behaviour compared to wide domain walls in the continuum limit. This behaviour have been explained by the fact that for ultra-narrow domain walls the reflection of spin waves is not negligible.

Furthermore, the dynamics of topologically protected structures, such as topological excitations in a kagome lattice and edge dislocations in FeGe has been studied. For the FeGe case, the description of the thermally driven dynamics of the edge dislocations, was found to be a possible explanation for the experimentally observed time dependence of the spiral wavelength. In the kagome lattice, it was also found that due to its topological properties, topological excitations can be created in it.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2016. 115 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1383
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-287415 (URN)978-91-554-9598-5 (ISBN)
Public defence
2016-06-14, Sieghbahnsalen, Ångströms laboratoriet Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2016-05-23 Created: 2016-04-25 Last updated: 2016-06-15

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Pereiro, ManuelYudin, DmitryChico, JonathanEtz, CorinaEriksson, OlleBergman, Anders

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