Relaxation Mechanisms and Strain-Controlled Oxygen Vacancies in Epitaxial SrMnO₃ Films

SrMnO₃ has a rich epitaxial strain-dependent ferroic phase diagram, in which a variety of magnetic orderings, even ferroelectricity, and thus multiferroicity, are accessible by gradually modifying the strain. Different relaxation processes, though, including the presence of strain-induced oxygen vacancies, can severely curtail the possibility of stabilizing these ferroic phases. Here, we report on a thorough investigation of the strain relaxation mechanisms in SrMnO₃ films grown on several substrates imposing varying degrees of strain from slightly compressive (-0.39%) to largely tensile ≈+3.8%. First, we determine the strain dependency of the critical thickness (t _c) below which pseudomorphic growth is obtained. Second, the mechanisms of stress relaxation are elucidated, revealing that misfit dislocations and stacking faults accommodate the strain above t _c. Yet, even for films thicker than t _c, the atomic monolayers below t _c are proved to remain fully coherent. Therefore, multiferroicity may also emerge even in films that appear to be partially relaxed. Last, we demonstrate that fully coherent films with the same thickness present a lower oxygen content for increasing tensile mismatch with the substrate. This behavior proves the coupling between the formation of oxygen vacancies and epitaxial strain, in agreement with first-principles calculations, enabling the strain control of the Mn³⁺/Mn⁴⁺ ratio, which strongly affects the magnetic and electrical properties. However, the presence of oxygen vacancies/Mn³⁺ cations reduces the effective epitaxial strain in the SrMnO₃ films and, thus, the accessibility to the strain-induced multiferroic phase.