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Spin‐state transitions are an important research topic in complex oxides with the diverse magnetic states involved. In particular, the low‐spin to high‐spin transition in LaCoO3 thin films has drawn a wide range of attention due to the emergent ferromagnetic state. Although various mechanisms (e.g., structural distortion, oxygen‐vacancy formation, spin‐state ordering) have been proposed, an understanding of what really underlies the emergent ferromagnetism remains elusive. Here, the ferromagnetism in LaCoO3 thin films is systematically modulated by varying the oxygen pressure during thin‐film growth. Although the samples show dramatic different magnetization, their cobalt valence state and perovskite crystalline structure remain almost unchanged, ruling out the scenarios of both oxygen‐vacancy and spin‐ordering. This work provides compelling evidence that the tetragonal distortion due to the tensile strain significantly modifies the orbital occupancy, leading to a low‐spin to high‐spin transition with emergent ferromagnetism, while samples grown at reduced pressure demonstrate a pronounced lattice expansion due to cation‐off‐stoichiometry, which suppresses the tetragonal distortion and the consequent magnetization. This result not only provides important insight for the understanding of exotic ferromagnetism in LaCoO3 thin films, but also identifies a promising strategy to design electronic states in complex oxides through cation‐stoichiometry engineering. [ABSTRACT FROM AUTHOR]

Freestanding oxide nanomembranes have promising applications because of their novel electronic states and flexible crystalline structures. Several materials have been developed as sacrificial layers to exfoliate thin films from substrates via wet‐etching. However, these materials face great challenges in terms of either complicated crystalline structures or corrosive solutions. Here, a new sacrificial material, SrCoO2.5, is presented, which can be coherently grown with wide‐range strains and crystalline orientations and is also soluble in eco‐friendly solutions such as acetic acid, vinegar, and even carbonated drinks. With SrCoO2.5 as the sacrificial layer, high‐quality freestanding ferromagnetic SrRuO3 membranes are achieved from wide‐range epitaxial strains and different crystalline orientations. By investigating the evolution of the magnetic properties of these samples, it is discovered that epitaxial strain causes a distinct modification of the magnetic anisotropy of (001)pc‐oriented SrRuO3 samples, while its influence on the (110)pc and (111)pc samples is insignificant. This study not only demonstrates the freestanding SrRuO3 as a promising material for flexible spintronic devices, but also offers a great opportunity to engineer a wide range of strained and oriented complex oxides for novel freestanding electronics using this newly developed sacrificial material. [ABSTRACT FROM AUTHOR]

Strain engineering of transition metal oxides due to their desirable properties has long been a focal point in both physics and material sciences. Here, we investigate the strain dependence of electronic and optical properties of the high valence iron-based perovskite CaFeO3−δ. Using substrates with various lattice constants, we achieve a wide range of tunable epitaxial strain states in CaFeO3−δ thin films ranging from compressive −0.37% to tensile 3.58%. Electrical transport and optical absorption measurements demonstrate a distinct strain-dependent behavior, in which larger tensile strain leads to higher electrical resistivity and a larger optical bandgap. We attribute these modulations to tensile strain suppressed p-d hybridization in CaFeO3−δ, as evidenced by soft X-ray absorption spectra measurements. [ABSTRACT FROM AUTHOR]