Strain-Induced Magnetic Transitions in SrMO_2.5 (M = Mn, Fe) Thin Films with Ordered Oxygen Vacancies

We examine the epitaxial-strain-induced phase transitions in thin films of perovskite-derived SrMnO_2.5 and SrFeO_2.5 exhibiting ordered oxygen vacancies (OOVs). We find that SrMnO_2.5 hosts multiple magnetic transitions to other ordered states, including antiferromagnetic (AFM) and ferromagnetic (FM) orders of E*-AFM, C-AFM, and FM types depending on the compressive or tensile strain state. In contrast, no magnetic transitions occur in thin-film SrFeO_2.5 (G-AFM to FM), whereas its bulk phase exhibits a hydrostatic pressure-induced AFM-to-FM transition. We explain the origin of these dependencies on the transition-metal configuration, that is, d⁴ Mn versus d⁵ Fe, and the relative orientation of the OOVs in the Ca₂Mn₂O₅-type structure with respect to the epitaxial interface. We find that the magnetic phase stability can be predicted by using exchange striction arguments, with FM (AFM) spin interactions preferring longer (shorter) Mn-O bonds in the square pyramidal MnO₅ unit comprising SrMnO_2.5. Because the Mn-O bond lengths directly shrink or elongate to accommodate the applied stress without considerable polyhedral rotations, we show that compressive and tensile strain tune the unit cell structure to favor different combinations of exchange interactions that stabilize the various magnetic spin orders. Our study shows that the strong coupling between the OOV structure and spin orders with epitaxial strain is a promising route to achieve picoscale control of functional electronic and magnetic responses in complex oxide thin films.