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Theoretical Magnet Design: From the electronic structure of solid matter to new permanent magnets
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.ORCID iD: 0000-0002-3326-7786
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: urn:nbn:se:uu:diva-231810OAI: oai:DiVA.org:uu-231810DiVA: diva2:745666
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
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: 2017-12-05
2. 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: 2017-12-05Bibliographically approved
3. 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

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