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Comparative study based on first principles to investigate unstrained and epitaxially strained Bi₂FeCrO₆ properties.

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Almesrati, Ali (2018). Comparative study based on first principles to investigate unstrained and epitaxially strained Bi₂FeCrO₆ properties. Mémoire. Québec, Université du Québec, Institut national de la recherche scientifique, Maîtrise en sciences de l'énergie et des matériaux, 102 p.

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Résumé

Using density functional theory, we have performed a comparative study between unstrained multiferroic Bi₂FeCrO₆ (BFCO) and BFCO epitaxially strained on SrTiO₃ (STO) substrate. We have compared the predicted structural, magnetic, and electric properties in order to understand the effect of epitaxial strain. In this work, all calculations were performed using the LSDA+U formalism implemented in the VASP code and used a 8×8×8 Monkhorst-Pack grid of k- points and a cutoff energy of 500 eV. We have first investigated the properties of the unstrained system that has a rhombohedral symmetry and belongs to the R3 space group, with lattice parameters arh=5.47Å and αrh=60.09°. The lattice parameters of the BFCO unit cell obtained through spin-polarized calculations and complete relaxation. The results revealed small differences with what has been reported in literature. The differences are chiefly in the contribution of Fe and Cr to the total magnetic moment, the energy of the fully optimized structure, as well as the position of some ions. The total magnetic moment per unit cell of the ferrimagnetic high spin (FiMHS) and ferromagnetic high spin (FMHS) configurations are 1.987 μB and 7.66 μB, respectively, while that of the ferrimagnetic low spin (FiMLS) and ferromagnetic low spin (FMLS) configurations results in values of 3.8 μB and 1.88 μB, respectively. For completeness, the ionic charges, as well as the density of state have been calculated. Despite slight differences, our results are in overall agreement with what has been reported in the literature. Secondly, we have investigated the effect of the epitaxial strain on the electronic and magnetic properties of the BFCO system. To this end, the lattice parameters of the rhombohedral BFCO unit cell have been fixed to the STO parameters in the plane of the BFCO/STO interface and the out-of-plane lattice parameter has been varied. We found that FiMHS remains the ground state configuration of the BFCO system under the epitaxial constraint of STO. The influence of the epitaxial deformation imposed by the STO substrate mainly manifests itself in the ionic contribution to spontaneous polarization by inducing a change of 7.64 μC cm⁻² for the the ground state (FiMHS), and producing a total polarization estimated at 90.24 μC cm⁻², as compared to 79.24 μC cm⁻² in the unstrained case. The magnetic moments of the constrained and unconstrained BFCO are very similar, the main difference being observed for the FiMLS phase, with 1.905 μB instead of 1.880 μB. Finally, we have calculated the density of states. Comparing with the case of the system without constraint, we found that the band gap remains practically unchanged for FiMHS, is reduced by 0.1 eV (-10%) for FMHS, and almost vanishes for FMLS and FiMLS.

Type de document: Thèse Mémoire
Directeur de mémoire/thèse: Pignolet, Alain
Co-directeurs de mémoire/thèse: Vidal, François
Mots-clés libres: unstrained multiferroic Bi2FeCrO6(BFCO); BFCO epitaxially strained; SrTiO3(STO) substrate; electronic and magnetic properties;
Centre: Centre Énergie Matériaux Télécommunications
Date de dépôt: 22 oct. 2018 21:21
Dernière modification: 22 oct. 2018 21:21
URI: http://espace.inrs.ca/id/eprint/7631

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