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Theoretical and Computational Studies on the Physics of Applied Magnetism: Magnetocrystalline Anisotropy of Transition Metal Magnets and Magnetic Effects in Elastic Electron Scattering
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.ORCID iD: 0000-0002-3326-7786
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, two selected topics in magnetism are studied using theoretical modelling and computational methods. The first of these is the magnetocrystalline anisotropy energy (MAE) of transition metal based magnets. In particular, ways of finding 3d transition metal based materials with large MAE are considered. This is motivated by the need for new permanent magnet materials, not containing rare-earth elements, but is also of interest for other technological applications, where the MAE is a key quantity. The mechanisms of the MAE in the relevant materials are reviewed and approaches to increasing this quantity are discussed. Computational methods, largely based on density functional theory (DFT), are applied to guide the search for relevant materials. The computational work suggests that the MAE of Fe1-xCox alloys can be significantly enhanced by introducing a tetragonality with interstitial B or C impurities. This is also experimentally corroborated. Alloying is considered as a method of tuning the electronic structure around the Fermi energy and thus also the MAE, for example in the tetragonal compound (Fe1-xCox)2B. Additionally, it is shown that small amounts (2.5-5 at.%) of various 5d dopants on the Fe/Co-site can enhance the MAE of this material with as much as 70%. The magnetic properties of several technologically interesting, chemically ordered, L10 structured binary compounds, tetragonal Fe5Si1-xPxB2 and Hexagonal Laves phase Fe2Ta1-xWx are also investigated. The second topic studied is that of magnetic effects on the elastic scattering of fast electrons, in the context of transmission electron microscopy (TEM). A multislice solution is implemented for a paraxial version of the Pauli equation. Simulations require the magnetic fields in the sample as input. A realistic description of magnetism in a solid, for this purpose, is derived in a scheme starting from a DFT calculation of the spin density or density matrix. Calculations are performed for electron vortex beams passing through magnetic solids and a magnetic signal, defined as a difference in intensity for opposite orbital angular momentum beams, integrated over a disk in the diffraction plane, is observed. For nanometer sized electron vortex beams carrying orbital angular momentum of a few tens of ħ, a relative magnetic signal of order 10-3 is found. This is considered realistic to be observed in experiments. In addition to electron vortex beams, spin polarised and phase aberrated electron beams are considered and also for these a magnetic signal, albeit weaker than that of the vortex beams, can be obtained.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2016. , 109 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1440
Keyword [en]
Magnetism, Magnetic anisotropy, DFT, Permanent magnets, Electron vortex beams, Electron microscopy, Electron scattering, Multislice methods
Keyword [sv]
Magnetism, magnetisk anisotropi, permanentmagneter, täthetsfunktionalteori, elektronmikroskopi, elektronvirvelstrålar, elektronspridningsteori
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
URN: urn:nbn:se:uu:diva-304666ISBN: 978-91-554-9753-8OAI: oai:DiVA.org:uu-304666DiVA: diva2:1033475
Public defence
2016-11-25, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Note

Felaktigt ISBN i den tryckta versionen: 9789155497149

Available from: 2016-11-02 Created: 2016-10-07 Last updated: 2016-11-23Bibliographically approved
List of papers
1. Stabilization of the tetragonal distortion of Fe chi Co1-chi alloys by C impurities: A potential new permanent magnet
Open this publication in new window or tab >>Stabilization of the tetragonal distortion of Fe chi Co1-chi alloys by C impurities: A potential new permanent magnet
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2014 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, no 14, 144403- p.Article in journal (Refereed) Published
Abstract [en]

We have analyzed by density functional theory calculations the structural and magnetic properties of Fe-Co alloys doped by carbon. In analogy with the formation of martensite in steels we predict that such a structure also forms for Fe-Co alloys in a wide range of concentrations. These alloys are predicted to have a stable tetragonal distortion, which in turn leads to an enhanced magnetocrystalline anisotropy energy of up to 0.75 MJ/m(3) and a saturated magnetization field of 1.9 T.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-224328 (URN)10.1103/PhysRevB.89.144403 (DOI)000333665500003 ()
Funder
Swedish National Infrastructure for Computing (SNIC), SNIC 001/12-198
Available from: 2014-05-14 Created: 2014-05-09 Last updated: 2016-10-07
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: 2016-10-07
3. Increased magnetocrystalline anisotropy in epitaxial Fe-Co-C thin films with spontaneous strain
Open this publication in new window or tab >>Increased magnetocrystalline anisotropy in epitaxial Fe-Co-C thin films with spontaneous strain
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2014 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 116, no 21, 213901- p.Article in journal (Other academic) Published
Abstract [en]

Rare earth free alloys are in focus of permanent magnet research since the accessibility of the elements needed for nowadays conventional magnets is limited. Tetragonally strained iron-cobalt (Fe-Co) has attracted large interest as promising candidate due to theoretical calculations. In experiments, however, the applied strain quickly relaxes with increasing film thickness and hampers stabilization of a strong magnetocrys- talline anisotropy. In our study we show that already 2 at% of carbon substantially reduce the lattice relaxation leading to the formation of a spontaneously strained phase with 3% tetragonal distortion. In these strained (Fe0.4Co0.6)0.98C0.02 films, a magnetocrystalline anisotropy above 0.4 MJ/m3 is observed while the large polarization of 2.1 T is maintained. Compared to binary Fe-Co this is a remarkable improve- ment of the intrinsic magnetic properties. In this paper, we relate our experimental work to theoretical studies of strained Fe-Co-C and find a very good agreement.

Keyword
Fe-Co, rare earth free permanent magnet, magnetocrystalline anisotropy, tetragonal strain, DFT, RHEED
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-231846 (URN)10.1063/1.4901595 (DOI)000346007400017 ()
Funder
EU, European Research Council
Available from: 2014-09-10 Created: 2014-09-10 Last updated: 2016-10-07Bibliographically approved
4. Toward Rare-Earth-Free Permanent Magnets: A Combinatorial Approach Exploiting the Possibilities of Modeling, Shape Anisotropy in Elongated Nanoparticles, and Combinatorial Thin-Film Approach
Open this publication in new window or tab >>Toward Rare-Earth-Free Permanent Magnets: A Combinatorial Approach Exploiting the Possibilities of Modeling, Shape Anisotropy in Elongated Nanoparticles, and Combinatorial Thin-Film Approach
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2015 (English)In: JOM: The Member Journal of TMS, ISSN 1047-4838, E-ISSN 1543-1851, Vol. 67, no 6, 1318-1328 p.Article in journal (Refereed) Published
Abstract [en]

The objective of the rare-earth free permanent magnets (REFREEPM) project is to develop a new generation of high-performance permanent magnets (PMs) without rare earths. Our approach is based on modeling using a combinatorial approach together with micromagnetic modeling and the realization of the modeled systems (I) by using a novel production of high-aspect-ratio (> 5) nanostructrures (nanowires, nanorods, and nanoflakes) by exploiting the magnetic shape anisotropy of the constituents that can be produced via chemical nanosynthesis polyol process or electrodeposition, which can be consolidated with novel processes for a new generation of rare-earth free PMs with energy product in the range of 60 kJ/m(3) < (BH)max < 160 kJ/m(3) at room temperature, and (II) by using a high-throughput thin-film synthesis and high-throughput characterization approach to identify promising candidate materials that can be stabilized in a tetragonal or hexagonal structure by epitaxial growth on selected substrates, under various conditions of pressure, stoichiometry, and temperature. In this article, we report the progress so far in selected phases.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-258032 (URN)10.1007/s11837-015-1431-7 (DOI)000355947400007 ()
Available from: 2015-07-13 Created: 2015-07-10 Last updated: 2016-10-07Bibliographically approved
5. From soft to hard magnetic Fe-Co-B by spontaneous strain: a combined first principles and thin film study
Open this publication in new window or tab >>From soft to hard magnetic Fe-Co-B by spontaneous strain: a combined first principles and thin film study
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2015 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 27, no 47, 476002Article in journal (Refereed) Published
Abstract [en]

In order to convert the well-known Fe-Co-B alloy from a soft to a hard magnet, we propose tetragonal strain by interstitial boron. Density functional theory reveals that when B atoms occupy octahedral interstitial sites, the bcc Fe-Co lattice is strained spontaneously. Such highly distorted Fe-Co is predicted to reach a strong magnetocrystalline anisotropy which may compete with shape anisotropy. To probe this theoretical suggestion experimentally, epitaxial films are examined. A spontaneous strain up to 5% lattice distortion is obtained for B content up to 4 at%, which leads to uniaxial anisotropy constants exceeding 0.5 MJ m(-3). However, a further addition of B results in a partial amorphisation, which degrades both anisotropy and magnetisation.

Keyword
Fe-Co, rare-earth free permanent magnet, magnetocrystalline anisotropy, tetragonal strain, DFT
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-270417 (URN)10.1088/0953-8984/27/47/476002 (DOI)000365346800013 ()26548574 (PubMedID)
Funder
Swedish National Infrastructure for Computing (SNIC), SNIC 001/12-198
Available from: 2016-01-19 Created: 2015-12-28 Last updated: 2016-10-07
6. Magnetic properties of (Fe1-xCox)(2)B alloys and the effect of doping by 5d elements
Open this publication in new window or tab >>Magnetic properties of (Fe1-xCox)(2)B alloys and the effect of doping by 5d elements
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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 17, 174413Article in journal (Refereed) Published
Abstract [en]

We have explored, computationally and experimentally, the magnetic properties of (Fe1-xCox)(2)B alloys. Calculations provide a good agreement with experiment in terms of the saturation magnetization and the magnetocrystalline anisotropy energy with some difficulty in describing Co2B, for which it is found that both full potential effects and electron correlations treated within dynamical mean field theory are of importance for a correct description. The material exhibits a uniaxial magnetic anisotropy for a range of cobalt concentrations between x = 0.1 and x = 0.5. A simple model for the temperature dependence of magnetic anisotropy suggests that the complicated nonmonotonic behavior is mainly due to variations in the band structure as the exchange splitting is reduced by temperature. Using density functional theory based calculations we have explored the effect of substitutionally doping the transition metal sublattice by the whole range of 5d transition metals and found that doping by Re or W elements should significantly enhance the magnetocrystalline anisotropy energy. Experimentally, W doping did not succeed in enhancing the magnetic anisotropy due to formation of other phases. On the other hand, doping by Ir and Re was successful and resulted in magnetic anisotropies that are in agreement with theoretical predictions. In particular, doping by 2.5 at.% of Re on the Fe/Co site shows a magnetocrystalline anisotropy energy which is increased by 50% compared to its parent (Fe0.7Co0.3)(2)B compound, making this system interesting, for example, in the context of permanent magnet replacement materials or in other areas where a large magnetic anisotropy is of importance.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-268398 (URN)10.1103/PhysRevB.92.174413 (DOI)000364402800001 ()
Funder
EU, European Research CouncilSwedish Research CouncilKnut and Alice Wallenberg FoundationStandUpeSSENCE - An eScience Collaboration
Available from: 2015-12-09 Created: 2015-12-04 Last updated: 2016-10-07Bibliographically approved
7. Magnetic properties of Fe5SiB2 and its alloys with P, S, and Co
Open this publication in new window or tab >>Magnetic properties of Fe5SiB2 and its alloys with P, S, and Co
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2016 (English)In: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 93, no 17, 174412Article in journal (Refereed) Published
Abstract [en]

Fe5SiB2 has been synthesized and magnetic measurements have been carried out, revealing that M-sat = 0.92 MA/mat T = 300 K. The M versus T curve shows a broad peak around T = 160 K. The anisotropy constant K-1, estimated at T = 300 K, is 0.25 MJ/m(3). Theoretical analysis of Fe5SiB2 system has been carried out and extended to the full range of Fe5Si1-xPxB2, Fe5P1-xSxB2, and (Fe1-xCox)(5)SiB2 compositions. The electronic band structures have been calculated using the full-potential local-orbital minimum-basis scheme (FPLO-14). The calculated total magnetic moments are 9.20, 9.15, 9.59, and 2.42 mu(B) per formula units of Fe5SiB2, Fe5PB2, Fe5SB2, and Co5SiB2, respectively. In agreement with experiment, magnetocrystalline anisotropy energies (MAE's) calculated for T = 0 K change from a negative (easy-plane) anisotropy -0.28 MJ/m(3) for Fe5SiB2 to the positive (easy-axis) anisotropy 0.35 MJ/m(3) for Fe5PB2. Further increase of the number of p electrons in Fe5P1-xSxB2 leads to an increase of MAE up to 0.77 MJ/m(3) for the hypothetical Fe5P0.4S0.6B2 composition. Volume variation and fixed spin moment calculations (FSM) performed for Fe5SiB2 show an inverse relation between MAE and magnetic moment in the region down to about 15% reduction of the spin moment. The alloying of Fe5SiB2 with Co is proposed as a practical realization of magnetic moment reduction, which ought to increase MAE. MAE calculated in virtual crystal approximation (VCA) for a full range of (Fe1-xCox)(5)SiB2 compositions reaches the maximum value of 1.16 MJ/m(3) at Co concentration x = 0.3, with the magnetic moment 7.75 mu(B) per formula unit. Thus, (Fe0.7Co0.3)(5)SiB2 is suggested as a candidate for a rare-earth free permanent magnet. For the stoichiometric Co5SiB2 there is an easy-plane magnetization, with the value of MAE = -0.15 MJ/m(3).

National Category
Physical Sciences Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-298092 (URN)10.1103/PhysRevB.93.174412 (DOI)000376244900004 ()
Funder
Göran Gustafsson Foundation for Research in Natural Sciences and MedicineSwedish Research CouncilEU, FP7, Seventh Framework Programme
Available from: 2016-06-29 Created: 2016-06-29 Last updated: 2016-10-07Bibliographically approved
8. Enhanced and Tunable Spin-Orbit Coupling in tetragonally Strained Fe-Co-B Films
Open this publication in new window or tab >>Enhanced and Tunable Spin-Orbit Coupling in tetragonally Strained Fe-Co-B Films
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Tetragonally strained interstitial Fe-Co-B alloys were synthesized as epitaxial films grown on Au0.55Cu0.45 buffer layer with the thickness of 20 nm. The strain axis is perpendicular to the film plane and the corresponding lattice constant c is enlarged with respect to the in-plane lattice parameter a. The tetragonal strain was stabilized in Fe-Co films with different c/a ratios of 1.013, 1.034 and 1.02 by adding interstitial boron with different concentrations 0, 4, and 10 at.%, respectively. Using ferromagnetic resonance (FMR) and x-ray magnetic circular dichroism (XMCD) we found that the total orbital magnetic moment significantly increases with increasing c/a ratio, indicating that reduced crystal symmetry and interstitial B leads to a noticeable enhancement of the effect of spin-orbit coupling (SOC) in the Fe-Co-B alloys. First-principles calculations reveal that the increase in orbital magnetic moment manly originates from B impurities in octahedral position and reduced symmetry around B atoms. These findings offers the possibility of tuning SOC phenomena - namely the magnetocrystalline anisotropy and the orbital moment - by stabilizing anisotropic strain through selection of B concentration, where B atoms preferentially occupy octahedral interstitial positions in the bct FeCo. We also discuss the influence of B doping on the Fe-Co film microstructure, their coercive field and magnetic relaxation. 

Keyword
magnetism, spin-orbit, magnetic anisotropy, FRM, XMCD, DFT
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-304518 (URN)
Available from: 2016-10-06 Created: 2016-10-06 Last updated: 2016-10-07
9. On the origin of perpendicular magnetic anisotropy in strained Fe-Co(-X) films
Open this publication in new window or tab >>On the origin of perpendicular magnetic anisotropy in strained Fe-Co(-X) films
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Very high magnetic anisotropies have been theoretically predicted for strained Fe-Co(-X) and indeed several experiments on epitaxial thin films seemed to confirm strain induced anisotropy enhancement. This study presents a critical analysis of the different contributions to perpendicular anisotropy: volume, interface and surface anisotropies. Tracing these contributions, thickness series of single layer films as well as multilayers with Au-Cu buffers/interlayers of different lattice parameters have been prepared. The analysis of their magnetic anisotropy reveals a negligible influence of the lattice parameter of the buffer. Electronic effects, originating from both, the Au-Cu interface and the film surface, outrange the elastic effects. Surface anisotropy, however, exceeds the interface anisotropy by more than a factor of three. A comparison with results from Density Functional Theory suggests, that the experimentally observed strong perpendicular surface anisotropy originates from a deviation from an ideal oxide-free surface. Accordingly, tailored Fe-Co-X/oxideinterfaces may open a route towards high anisotropy in rare-earth free materials.

National Category
Physical Sciences Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-304662 (URN)
Available from: 2016-10-06 Created: 2016-10-06 Last updated: 2016-10-07
10. Towards a magnetic phase diagram of the Fe5SiB2-Fe5PB2 alloy system
Open this publication in new window or tab >>Towards a magnetic phase diagram of the Fe5SiB2-Fe5PB2 alloy system
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(English)Manuscript (preprint) (Other academic)
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-304664 (URN)
Available from: 2016-10-06 Created: 2016-10-06 Last updated: 2016-10-07
11. Magnetocrystalline anisotropy of Laves phase Fe2Ta1−xWx from first principles - the effect of 3d-5d hybridisation
Open this publication in new window or tab >>Magnetocrystalline anisotropy of Laves phase Fe2Ta1−xWx from first principles - the effect of 3d-5d hybridisation
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The magnetic properties of Fe2Ta and Fe2W in the hexagonal Laves phase are computed using density functional theory in the generalised gradient approximation, with the full potential linearised augmented plane wave method. The alloy Fe2Ta1-xWx is studied using the virtual crystal approximation to treat disorder. Fe2W is found to be ferrimagnetic with a saturation magnetisation of µ0Ms = 0.35 T, while Fe2Ta is ferromagnetic with µ0Ms = 0.66. The transition from the ferri- to the ferromagnetic state occurs for x ≤ 0.1. The magnetocrystalline anisotropy energy (MAE) is calculated to 1.25 MJ/m3 for Fe2Ta and 0.87 MJ/m3 for Fe2W. It is found to be smaller for all values x in Fe2Ta1-xWx. The MAE is carefully analysed in terms of the electronic structure and discussed with respect to 3d-5d hybridisation.

Keyword
Magnetism, magnetic anisotropy, DFT, alloys
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-304665 (URN)
Available from: 2016-10-07 Created: 2016-10-07 Last updated: 2016-10-07
12. New permanent magnets; what to look for, and where
Open this publication in new window or tab >>New permanent magnets; what to look for, and where
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(English)Manuscript (preprint) (Other academic)
National Category
Physical Sciences
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-304516 (URN)
Available from: 2016-10-06 Created: 2016-10-06 Last updated: 2016-10-07
13. Prediction of a Larger Local Magnetic Anisotropy in Permalloy
Open this publication in new window or tab >>Prediction of a Larger Local Magnetic Anisotropy in Permalloy
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

By means of relativistic, first principles calculations, we investigate the microscopic origin of the vanishingly low magnetic anisotropy of Permalloy. Analyzing the local magnetic anisotropy of these alloys, we find that it can be several orders of magnitude larger than that of the bulk sample, and at least one order of magnitude larger than that of Fe or Ni. We discuss these results in terms of local symmetries of the alloy, and point out that an analysis based on perturbation theory has the spin-orbit coupling entering as a second-order term, as opposed to the naively expected fourth order contribution. The relevance for these findings in experiments, e.g. using pump-probe investigations, is discussed, as well as the implications for building effective spin-Hamiltonians of Permalloy. 

Keyword
magnetism, magnetic anisotropy
National Category
Physical Sciences
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-304517 (URN)
Available from: 2016-10-06 Created: 2016-10-06 Last updated: 2016-10-14
14. Elastic Scattering of Electron Vortex Beams in Magnetic Matter.
Open this publication in new window or tab >>Elastic Scattering of Electron Vortex Beams in Magnetic Matter.
2016 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 116, no 12, 127203Article in journal (Refereed) Published
Abstract [en]

Elastic scattering of electron vortex beams on magnetic materials leads to a weak magnetic contrast due to Zeeman interaction of orbital angular momentum of the beam with magnetic fields in the sample. The magnetic signal manifests itself as a redistribution of intensity in diffraction patterns due to a change of sign of the orbital angular momentum of the electron vortex beam. While in the atomic resolution regime the magnetic signal is most likely under the detection limits of present transmission electron microscopes, for electron probes with high orbital angular momenta, and correspondingly larger spatial extent, its detection is predicted to be feasible.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-283341 (URN)10.1103/PhysRevLett.116.127203 (DOI)000372729200017 ()27058098 (PubMedID)
Funder
Swedish Research CouncilGöran Gustafsson Foundation for promotion of scientific research at Uppala University and Royal Institute of TechnologyEU, FP7, Seventh Framework Programme, 312483-ESTEEM2
Available from: 2016-04-12 Created: 2016-04-12 Last updated: 2016-10-07Bibliographically approved
15. Magnetic Effects in the Paraxial Regime of Elastic Electron Scattering
Open this publication in new window or tab >>Magnetic Effects in the Paraxial Regime of Elastic Electron Scattering
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Based on a recent claim [Phys. Rev. Lett. 116, 127203 (2016)] that electron vortex can be used to image magnetism at the nanoscale in elastic scattering experiments, using transmission electron microscopy, a comprehensive computational study is performed to study magnetic effects in the paraxial regime of elastic electron scattering in magnetic solids. Magnetic interactions from electron vortex beams, spin polarized electron beams and beams with phase aberrations are considered, as they pass through ferromagnetic FePt or antiferromagnetic LaMnAsO. The magnetic signals are obtained by comparing the intensity over a disk in the diffraction plane for beams with opposite angular momentum or aberrations. The strongest magnetic signals are obtained from vortex beams with large orbital angular momentum, where relative magnetic signals above 10−3 are indicated for 10ℏ orbital angular momentum, meaning that relative signals of one percent could be expected with the even larger orbital angular momenta, which have been produced in experimental setups. All results indicate that beams with low acceleration voltage and small convergence angles yield stronger magnetic signals, which is unfortunately problematic for the possibility of high spatial resolution imaging. Nevertheless, under atomic resolution conditions, relative magnetic signals in the order of 10−4 are demonstrated, corresponding to an increase with one order of magnitude compared to previous work.

Keyword
Vortex beams, magnetism, electron microscopy, scattering theory
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-304515 (URN)
Available from: 2016-10-06 Created: 2016-10-06 Last updated: 2016-10-07

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