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Magnetization dynamics on the nanoscale: From first principles to atomistic spin dynamics
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. (Materials Theory)
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: urn:nbn:se:uu:diva-287415ISBN: 978-91-554-9598-5 (print)OAI: oai:DiVA.org:uu-287415DiVA: diva2:922992
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
List of papers
1. Thermally driven domain-wall motion in Fe on W(110)
Open this publication in new window or tab >>Thermally driven domain-wall motion in Fe on W(110)
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2014 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, no 1, 014434- p.Article in journal (Refereed) Published
Abstract [en]

It has recently been shown that domain walls (DWs) in ferromagnets can be moved in the presence of thermal gradients. In this work we study the motion of narrow domain walls in low-dimensional systems when subjected to thermal gradients. The system chosen is a monolayer of Fe on W(110) which is known to exhibit a large anisotropy while having a soft exchange, resulting in a very narrow domain wall. The study is performed by means of atomistic spin dynamics simulations coupled to first-principles calculations. By subjecting this system to thermal gradients we observe a temperature-dependent movement of the domain wall. The thermal gradient always makes the domain wall move towards the hotter region of the sample with a velocity proportional to the gradient. Our material specific study is complemented by model simulations to discern the interplay between the thermal gradient, magnetic anisotropy, and the exchange interaction and shows that the larger DW velocities are found for materials with broader domain-wall width. The relatively slow DW motion of the Fe/W(110) system is hence primarily caused by its narrow domain-wall width, which results from its large magnetic anisotropy and soft exchange.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-231296 (URN)10.1103/PhysRevB.90.014434 (DOI)000339990000005 ()
Available from: 2014-09-08 Created: 2014-09-07 Last updated: 2017-12-05
2. Electronic structure and magnetic properties of L1(0) binary alloys
Open this publication in new window or tab >>Electronic structure and magnetic properties of L1(0) binary alloys
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2014 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, no 1, 014402- p.Article in journal (Refereed) Published
Abstract [en]

We present a systematic study of the magnetic properties of L1(0) binary alloys FeNi, CoNi, MnAl, and MnGa via two different density functional theory approaches. Our calculations show large magnetocrystalline anisotropies in the order 1 MJ/m(3) or higher for CoNi, MnAl, and MnGa, while FeNi shows a somewhat lower value in the range 0.48-0.77 MJ/m(3). Saturation magnetization values of 1.3 MA/m, 1.0 MA/m, 0.8 MA/m, and 0.9 MA/m are obtained for FeNi, CoNi, MnAl, and MnGa, respectively. Curie temperatures are evaluated via Monte Carlo simulations and show T-C = 916 K and T-C = 1130 K for FeNi and CoNi, respectively. For Mn-based compounds Mn-rich off-stoichiometric compositions are found to be important for the stability of a ferro- or ferrimagnetic ground state with T-C greater than 600 K. The effect of substitutional disorder is studied and found to decrease both magnetocrystalline anisotropies and Curie temperatures in FeNi and CoNi.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-229718 (URN)10.1103/PhysRevB.90.014402 (DOI)000338649700003 ()
Available from: 2014-08-18 Created: 2014-08-12 Last updated: 2017-12-05
3. Topological excitations in a kagome magnet
Open this publication in new window or tab >>Topological excitations in a kagome magnet
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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.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-236091 (URN)10.1038/ncomms5815 (DOI)000342929000005 ()
Available from: 2014-11-14 Created: 2014-11-12 Last updated: 2017-12-05
4. Tunable damping, saturation magnetization, and exchange stiffness of half-Heusler NiMnSb thin films
Open this publication in new window or tab >>Tunable damping, saturation magnetization, and exchange stiffness of half-Heusler NiMnSb thin films
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2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 21, 214424Article in journal (Refereed) Published
Abstract [en]

The half-metallic half-Heusler alloy NiMnSb is a promising candidate for applications in spintronic devices due to its low magnetic damping and its rich anisotropies. Here we use ferromagnetic resonance (FMR) measurements and calculations from first principles to investigate how the composition of the epitaxially grown NiMnSb influences the magnetodynamic properties of saturation magnetization M-S, Gilbert damping alpha, and exchange stiffness A. M-S and A are shown to have a maximum for stoichiometric composition, while the Gilbert damping is minimum. We find excellent quantitative agreement between theory and experiment for M-S and alpha. The calculated A shows the same trend as the experimental data but has a larger magnitude. In addition to the unique in-plane anisotropy of the material, these tunabilities of the magnetodynamic properties can be taken advantage of when employing NiMnSb films in magnonic devices.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-274280 (URN)10.1103/PhysRevB.92.214424 (DOI)000366499500004 ()
Funder
Göran Gustafsson Foundation for promotion of scientific research at Uppala University and Royal Institute of TechnologySwedish Research CouncilSwedish Energy AgencyKnut and Alice Wallenberg FoundationCarl Tryggers foundation
Available from: 2016-02-25 Created: 2016-01-20 Last updated: 2017-11-30
5. First principle studies of Co/Ni/Co heterostructures on heavy metal substrates
Open this publication in new window or tab >>First principle studies of Co/Ni/Co heterostructures on heavy metal substrates
(English)Manuscript (preprint) (Other (popular science, discussion, etc.))
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-287426 (URN)
Available from: 2016-04-25 Created: 2016-04-25 Last updated: 2016-06-15
6. Helical spiral states in the MnZSn half-Heuslers with Z=(Tc, Ru, Rh, Os, Ir, Pt).A first principles study
Open this publication in new window or tab >>Helical spiral states in the MnZSn half-Heuslers with Z=(Tc, Ru, Rh, Os, Ir, Pt).A first principles study
(English)Manuscript (preprint) (Other (popular science, discussion, etc.))
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-287424 (URN)
Available from: 2016-04-25 Created: 2016-04-25 Last updated: 2016-06-15
7. Relativistic effects in domain wall dynamics in magnetic heterostrcutures
Open this publication in new window or tab >>Relativistic effects in domain wall dynamics in magnetic heterostrcutures
(English)Manuscript (preprint) (Other (popular science, discussion, etc.))
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-287428 (URN)
Available from: 2016-04-25 Created: 2016-04-25 Last updated: 2016-06-15

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