Coupling and competition between ferroelectricity, magnetism, strain, and oxygen vacancies in AMnO3 perovskites

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, AMnO 3 (A = Ca 2+ , Sr). 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.

Nature of the octahedral tilting phase transitions in perovskites: A case study of CaMnO3

“…[41,42] report this mode to be imaginary at the equilibrium volume, Refs. [22,43] report no such instability at equilibrium volume and with the G-type AFM ordering. The presence of a weak FE instability in the cubic phase of perovskite oxides with AFD instabilities is quite common, and in many cases, such as the present one, the AFD instability is much stronger and completely dominates over the FE instability.…”

Strain-induced ferroelectricity and spin-lattice coupling in SrMnO3 thin films

“…This prediction was soon confirmed in bulk SMO by partially substituting strontium with barium, which induced negative chemical pressure and led to a polar ferroelectric state [16]. Although a rich phase diagram has been proposed recently for a strained manganite sample from theoretical aspects [17][18][19], the experimental studies in SMO thin films and other related manganite systems so far have been limited to small tensile strain cases (∼1.6%) [20][21][22][23][24][25], and the study for larger tensile strain remains as a great challenge due to the difficulty to maintain the strain state of the film and obtain correct oxygen stoichiometry in synthesizing these samples [26][27][28].…”

Defect Chemistry as a Crystal Structure Design Parameter: Intrinsic Point Defects and Ga Substitution in InMnO3

“…We find that, because of the energetic proximity of multiple competing ground states in InMnO 3 , the coupling between structure, defect chemistry and functionality is even stronger than in the case of the perovskite oxides, where it is also of tremendous current interest in the context of strained thin films. [24][25][26][27] Finally we discuss the enticing possibility of switching between the polar and non-polar phases through reversible modifications of the defect profile.…”