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Insights into phase transitions and magnetism of MnBi crystals synthesized from self-flux
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry. Univ Sci & Technol Beijing, State Key Lab Adv Met & Mat, Beijing, Peoples R China.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
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2019 (English)In: Journal of Alloys and Compounds, ISSN 0925-8388, E-ISSN 1873-4669, Vol. 781, p. 308-314Article in journal (Refereed) Published
Abstract [en]

To effectively synthesize high purity ferromagnetic low temperature phase (LTP) MnBi with optimal microstructure is still a challenge that needs to be overcome for the system to reach its full potential. Here, the phase transitions and magnetic properties of MnBi crystals are reported. The phase transition between the low and high temperature structure of MnBi was systematically investigated at different heating/cooling rates using in situ synchrotron radiation X-ray diffraction. The material crystallizes in a layered hexagonal structure giving a platelike microstructure. The magnetic characterization of the crystals reveal that the saturation magnetization varies from 645 kA/m at 50 K to 546 kA/m at 300 K. Magnetization measurements also show that the sample upon heating becomes non-magnetic and transforms to the high temperature phase (HTP) at similar to 640 K, and that it regains ferromagnetic properties and transforms back to the LTP at similar to 610 K upon subsequent cooling.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE SA , 2019. Vol. 781, p. 308-314
Keywords [en]
In situ synchrotron radiation X-ray diffraction, Phase transitions, Rare earth free permanent magnet, MnBi, Single crystals
National Category
Materials Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-378612DOI: 10.1016/j.jallcom.2018.12.146ISI: 000457845900034OAI: oai:DiVA.org:uu-378612DiVA, id: diva2:1295201
Funder
Swedish Energy AgencySwedish Foundation for Strategic Research Available from: 2019-03-11 Created: 2019-03-11 Last updated: 2019-03-13Bibliographically approved
In thesis
1. Structural Studies of Mn-X (X=Al, Bi): Permanent Magnetic Materials without Rare Earth Metals
Open this publication in new window or tab >>Structural Studies of Mn-X (X=Al, Bi): Permanent Magnetic Materials without Rare Earth Metals
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

How to generate and use electricity in a more efficient way is a major challenge for humankind. In this context, permanent magnets play an important role within a very broad range of electric power applications. The strongest magnets used today are mainly based on alloys that contain rare-earth metals, which are neither economical nor sustainable. The search for new alternative alloys with satisfactory magnetic properties is the major motivation for the investigations summarized in this thesis. Interesting candidates for alternative rare-earth free alloys were selected with τ-MnAl as the basis. Theoretical studies suggest that such alloys may show good magnetic properties after chemical modifications to optimize them. Another compound with promising magnetic properties is MnBi, included in this study.

MnAl-Z (Z= C, B, Ga as doping elements) and MnBi compounds were synthesized through carefully devised high-temperature methods, followed by various milling and annealing steps. The structural phase analysis of the samples was based on X-ray and neutron diffraction. A systematic microstructural investigation was also performed for selected samples. The phase transitions of MnAl and MnBi during heating and cooling at different rates were studied by in situ X-ray diffraction from a synchrotron source. The magnetic properties were characterized by various methods.

By strict control of experimental parameters, the metastable τ-MnAl was found to be directly obtainable using a "drop synthesis” process. A cooling rate of 10 K/min yielded an almost pure ferromagnetic τ-MnAl phase. A microstructural characterization of similarly synthesized MnAl-C samples revealed the presence of phase segregation, a Mn-rich region and an Al-rich grain boundary phase.

A cryomilling process was employed which decreased the particle size of the MnAl-C sample. Neutron diffraction data disclosed accompanying amorphous features, related to changes in Mn and Al atom occupancies during the milling process. A flash heating procedure regenerated the structural ordering between Mn and Al in the structure, where the initial magnetic properties were recovered.

The MnBi compound was synthesized by a self-flux method in order to isolate single crystals. As for τ-MnAl, in situ diffraction studies were applied for following phase transitions and the magnetic properties were studied.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2019. p. 57
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1782
Keywords
Synthesis, Magnetism, Diffraction.
National Category
Inorganic Chemistry Materials Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-379177 (URN)978-91-513-0594-3 (ISBN)
Public defence
2019-05-03, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 14:15 (English)
Opponent
Supervisors
Available from: 2019-04-08 Created: 2019-03-13 Last updated: 2019-05-07

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Fang, HailiangShafeie, SamrandHedlund, DanielCedervall, JohanGómez, Cesar PaySvedlindh, PeterGunnarsson, KlasSahlberg, Martin

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Inorganic ChemistrySolid State PhysicsStructural Chemistry
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Journal of Alloys and Compounds
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