Interplay between oxygen vacancies, strain, and magnetism in SrMnO₃: a self-consistent site-dependent DFT+U study

Motivated by indications that strain and defects can stabilize a ferromagnetic ground state in normally antiferromagnetic SrMnO₃ thin films, we use self-consistent site-dependent (SCSD) DFT+U calculations to investigate the interplay between oxygen vacancies, strain, and magnetism in this material.
Already for the stoichiometric material, using a self-consistent U increases the accuracy of lattice parameter predictions, which is crucial to study strain-induced changes in properties and results in critical strain values for the magnetic phase transition in better agreement with experiment compared to previous theoretical studies.
Defect formation leads to changes in geometry and occupation on transition metal sites surrounding the defect. We find that taking into account the site-dependence of U around a defect has a strong impact on the computed formation energies and consequently on all related properties. As such U_SCSD prevents overestimating the stability of the ferromagnetic order thanks to a proper description of excess charge localization upon defect formation and helps in rationalizing different ordering of oxygen vacancies depending on the magnetic order in the epitaxial thin film.