Coupling and competition between ferroelectricity, magnetism, strain, and oxygen vacancies in AMnO 3 perovskites
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unctional Oxides Prospective Article
Coupling and competition between ferroelectricity, magnetism, strain, and oxygen vacancies in AMnO3 perovskites Astrid Marthinsen, Department of Materials Science and Engineering, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway† Carina Faber, Materials Theory, ETH Zürich, Wolfgang-Pauli Strasse 27, CH-8093 Zürich, Switzerland† Ulrich Aschauer, Materials Theory, ETH Zürich, Wolfgang-Pauli Strasse 27, CH-8093 Zürich, Switzerland; Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland Nicola A. Spaldin, Materials Theory, ETH Zürich, Wolfgang-Pauli Strasse 27, CH-8093 Zürich, Switzerland Sverre M. Selbach, Department of Materials Science and Engineering, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway Address all correspondence to Nicola A. Spaldin at [email protected] (Received 14 June 2016; accepted 3 August 2016)
Abstract We use first-principles calculations based on density functional theory to investigate the interplay between oxygen vacancies, A-site cation size/tolerance factor, epitaxial strain, ferroelectricity, and magnetism in the perovskite manganite series, AMnO3 (A = Ca2+, Sr2+, Ba2+). We find that, as expected, increasing the volume through either chemical pressure or tensile strain generally lowers the formation energy of neutral oxygen vacancies consistent with their established tendency to expand the lattice. Increased volume also favors polar distortions, both because competing rotations of the oxygen octahedra are suppressed and because Coulomb repulsion associated with cation off-centering is reduced. Interestingly, the presence of ferroelectric polarization favors ferromagnetic (FM) over antiferromagnetic (AFM) ordering due to suppressed AFM superexchange as the polar distortion bends the Mn–O–Mn bond angles away from the optimal 180°. Intriguingly, we find that polar distortions compete with the formation of oxygen vacancies, which have a higher formation energy in the polar phases; conversely the presence of oxygen vacancies suppresses the onset of polarization. In contrast, oxygen vacancy formation energies are lower for FM than AFM orderings of the same structure type. Our findings suggest a rich and complex phase diagram, in which defect chemistry, polarization, structure, and magnetism can be modified using chemical potential, stress or pressure, and electric or magnetic fields.
Introduction Strongly correlated oxides exhibit a vast range of functional properties owing to their intimate coupling between electronic, magnetic, and structural degrees of freedom.[1,2] The energetics of the various interactions tend to be similar in magnitude, and thus even small external perturbations, such as strain imposed on thin films through coherent heteroepitaxy, can have a large influence on the functionality.[3] An example is the generation of ferroelectric polarization in otherwise non-polar magnetic perovskite oxides such as EuTiO3[4] and SrMnO3[5], circumventing
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