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Increased magnetocrystalline anisotropy in epitaxial Fe-Co-C thin films with spontaneous strain
IFW Dresden.
Demokritos National Center of Scientific Research, Athens.
IFW Dresden .
IFW Dresden .
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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.

Place, publisher, year, edition, pages
2014. Vol. 116, no 21, 213901- p.
Keyword [en]
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: urn:nbn:se:uu:diva-231846DOI: 10.1063/1.4901595ISI: 000346007400017OAI: oai:DiVA.org:uu-231846DiVA: diva2:745322
Funder
EU, European Research Council
Available from: 2014-09-10 Created: 2014-09-10 Last updated: 2017-12-05Bibliographically approved
In thesis
1. Theoretical Magnet Design: From the electronic structure of solid matter to new permanent magnets
Open this publication in new window or tab >>Theoretical Magnet Design: From the electronic structure of solid matter to new permanent magnets
2014 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

A good permanent magnet should possess a large saturation magnetisation (Ms), large mag- netocrystalline anisotropy energy (MAE) and a high Curie temperature (TC). A difficult but important challenge to overcome for a sustainable permanent magnet industry is to find novel magnetic materials, exhibiting a large MAE, without the use of scarcely available elements such as rare-earth metals. The purpose of this thesis is to apply computational methods, including density functional theory and Monte Carlo simulations, to assess the three above mentioned permanent magnet properties and in particular to discover new replacement materials with large MAE without the use of critical materials such as rare-earths.

One of the key results is the theoretical prediction of a tetragonal phase of Fe1−xCox-C with large Ms and significantly increased MAE which is later also experimentally confirmed. Furthermore, other potential materials are surveyed and in particular the properties of a number of binary alloys in the L10 structure, FeNi, CoNi, MnAl and MnGa, are thoroughly investigated and shown to posses the desired properties under certain conditions.

Place, publisher, year, edition, pages
Uppsala universitet, 2014. 50 p.
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-231810 (URN)
Presentation
2014-09-26, Ångströmlaboratoriet, Uppsala, 13:30 (English)
Opponent
Supervisors
Available from: 2014-09-23 Created: 2014-09-10 Last updated: 2014-09-23Bibliographically approved
2. Theoretical and Computational Studies on the Physics of Applied Magnetism: Magnetocrystalline Anisotropy of Transition Metal Magnets and Magnetic Effects in Elastic Electron Scattering
Open this publication in new window or tab >>Theoretical and Computational Studies on the Physics of Applied Magnetism: Magnetocrystalline Anisotropy of Transition Metal Magnets and Magnetic Effects in Elastic Electron Scattering
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
Magnetism, Magnetic anisotropy, DFT, Permanent magnets, Electron vortex beams, Electron microscopy, Electron scattering, Multislice methods, 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:nbn:se:uu:diva-304666 (URN)978-91-554-9753-8 (ISBN)
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: 2016-12-19Bibliographically approved

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Edström, AlexanderRusz, Jan

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