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Magnetic properties of Ruddlesden-Popper phases Sr3-&: A combined experimental and theoretical investigation
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
Stockholm Univ, Dept Mat & Environm Chem, S-10691 Stockholm, Sweden;Univ Warsaw, Fac Phys, Inst Expt Phys, Pasteura 5, PL-02093 Warsaw, Poland;Chalmers Univ Technol, Dept Chem & Chem Engn, SE-41296 Gothenburg, Sweden.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
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2018 (English)In: Physical Review Materials, ISSN 2475-9953, Vol. 2, no 4, article id 044005Article in journal (Refereed) Published
Abstract [en]

We present a comprehensive study of the magnetic properties of Sr3-xYx(Fe1.25Ni0.75)O-7(-delta )(0 <= x <= 0.75). Experimentally, the magnetic properties are investigated using superconducting quantum interference device (SQUID) magnetometry and neutron powder diffraction (NPD). This is complemented by a theoretical study based on density functional theory as well as the Heisenberg exchange parameters. Experimental results show an increase in the Ned temperature (T-N) with an increase of Y concentrations and O occupancy. The NPD data reveal that all samples are antiferromagnetically ordered at low temperatures, which has been confirmed by our theoretical simulations for the selected samples. Our first-principles calculations suggest that the three-dimensional magnetic order is stabilized due to finite interlayer exchange couplings. The latter give rise to finite interlayer spin-spin correlations, which disappear above T-N.

Place, publisher, year, edition, pages
2018. Vol. 2, no 4, article id 044005
National Category
Condensed Matter Physics Engineering and Technology
Identifiers
URN: urn:nbn:se:uu:diva-354116DOI: 10.1103/PhysRevMaterials.2.044005ISI: 000430385300001OAI: oai:DiVA.org:uu-354116DiVA, id: diva2:1220742
Funder
Swedish Research CouncilKnut and Alice Wallenberg FoundationAvailable from: 2018-06-19 Created: 2018-06-19 Last updated: 2018-08-24Bibliographically approved
In thesis
1. Magnetism in Transition Metal Systems: Interplay between structure, dimensionality and electron correlation
Open this publication in new window or tab >>Magnetism in Transition Metal Systems: Interplay between structure, dimensionality and electron correlation
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, an ab initio study of electronic structures and magnetic properties of transition metal systems has been presented, covering bulk, interface and surface geometries. Among them are Fe, Co, Ni, CaMnO3, Co2MnSi, a ferromagnetic Heusler alloy, as well as double-perovskites oxides such as Sr3(Fe1.25Ni0.75)O6 and Nd2NiMnO6.

Their electronic structures have been obtained within the framework of the density functional theory (DFT) in combination with Hubbard type interaction such as the static correction evaluated within the Hartree-Fock method (DFT+U) or within the more sophisticated method of dynamical mean-field theory (DFT+DMFT). Using many-body approaches enables us to treat the correlation effects such as non-quasiparitcle states above the Fermi level for Co2MnSi and in the half-metallic side of the Co2MnAl/CoMnVAl heterostructure.

Based on the converged electronic structure, the magnetic excitations were mapped onto the Heisenberg Hamiltonian. Among various ways to extract exchange parameters, in this Thesis the method of infinitesimal rotation of the spins has been applied in the framework of the local magnetic force approach. It is shown that the exchange interactions on the surface of transition metals can be substantially different from those in the corresponding bulk. At the same time, the dynamical correlations lead to a slight renormalization of the magnetic couplings. For CaMnO3, we demonstrated the crucial role of the atomic relaxations defining the magnetic order on the surface atoms. We were also able to extract the orbital decompositions, which helped identify the main contributions to the total exchange. For the double-perovskite systems, the extracted exchange parameters were then used to evaluate the ordering temperature using Monte-Carlo simulations, and the calculated critical temperatures were found to be in good agreement with our experimental measurements.

In a more technical investigation, the influence of the spin polarization of the DFT exchange-correlation functional on the extracted exchange parameters has been investigated. We found a very good correspondence between the computed total energies and the parametrized Heisenberg model for LDA+U calculations, but not for LSDA+U. This means that for the extraction of the exchange parameters based on total energy differences, LDA+U is more appropriate.

Finally, a systematic study of the emergence of the local minima in DFT+U calculations has been performed for the bulk of NiO, FeO, CoO and UO2. We extended the use of the occupation matrix control method to randomly generate density matrices which help better monitor the local minima and explore the energy landscape. The effect of the Hubbard U and the double-counting in introducing the local minima are discussed.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2018. p. 94
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1720
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-357525 (URN)978-91-513-0438-0 (ISBN)
Public defence
2018-10-26, Polhemsalen, 10134, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2018-10-05 Created: 2018-08-24 Last updated: 2018-10-05

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Keshavarz, SamaraKontos, SofiaKvashnin, YaroslavPereiro, ManuelPanda, Swarup K.Sanyal, BiplabEriksson, OlleGunnarsson, KlasSvedlindh, Peter

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Keshavarz, SamaraKontos, SofiaKvashnin, YaroslavPereiro, ManuelPanda, Swarup K.Sanyal, BiplabEriksson, OlleGunnarsson, KlasSvedlindh, Peter
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