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Half-metallicity and magnetism in the Co2MnAl/CoMnVAl heterostructure
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
Univ Augsburg, Inst Phys, Ctr Elect Correlat & Magnetism, Theoret Phys 3, D-86135 Augsburg, Germany..
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
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 B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 3, article id 035105Article in journal (Refereed) Published
Abstract [en]

We present a study of the electronic structure and magnetism of Co2MnAl, CoMnVAl, and their heterostructure. We employ a combination of density-functional theory and dynamical mean-field theory (DFT+DMFT). We find that Co2MnAl is a half-metallic ferromagnet, whose electronic and magnetic properties are not drastically changed by strong electronic correlations, static or dynamic. Nonquasiparticle states are shown to appear in the minority spin gap without affecting the spin polarization at the Fermi level predicted by standard DFT. We find that CoMnVAl is a semiconductor or a semimetal, depending on the employed computational approach. We then focus on the electronic and magnetic properties of the Co2MnAl/CoMnVAl heterostructure, predicted by previous first-principle calculations as a possible candidate for spin-injecting devices. We find that two interfaces, Co-Co/V-Al and Co-Mn/Mn-Al, preserve the half-metallic character, with and without including electronic correlations. We also analyze the magnetic exchange interactions in the bulk and at the interfaces. At the Co-Mn/Mn-Al interface, competing magnetic interactions are likely to favor the formation of a noncollinear magnetic order, which is detrimental for the spin polarization.

Place, publisher, year, edition, pages
2018. Vol. 97, no 3, article id 035105
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:uu:diva-341501DOI: 10.1103/PhysRevB.97.035105ISI: 000419230600004OAI: oai:DiVA.org:uu-341501DiVA, id: diva2:1182476
Available from: 2018-02-13 Created: 2018-02-13 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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Di Marco, IgorKeshavarz, SamaraKvashnin, Yaroslav O

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