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The effects of growth conditions on the chemistry, structure, electrical leakage, dielectric response, and ferroelectric behavior of Ba 1−x TiO y thin films are explored. Although single-phase, coherently-strained films are produced in all cases, small variations in the laser fluence during pulsed-laser deposition growth result in films with chemistries ranging from BaTiO 3 to Ba 0.93 TiO 2.87 . As the laser fluence increases, the films become more barium deficient and the out-of-plane lattice parameter expands (as much as 5.4% beyond the expected value for Ba 0.93 TiO 2.87 films). Stoichiometric BaTiO 3 films are found to be three orders of magnitude more conducting than Ba 0.93 TiO 2.87 films and the barium-deficient films exhibit smaller low-field permittivity, lower loss tangents, and higher dielectric maximum temperatures. Although large polarization is observed in all cases, large built-in potentials (shifted loops) and hysteresis-loop pinching are present in barium-deficient films – suggesting the presence of defect dipoles. The effects of these defect dipoles on ferroelectric hysteresis are studied using first-order reversal curves. Temperature-dependent current–voltage and deep-level transient spectroscopy studies reveal at least two defect states, which grow in concentration with increasing deficiency of both barium and oxygen, at ∼0.4 eV and ∼1.2 eV above the valence band edge, which are attributed to defect–dipole complexes and defect states, respectively. The defect states can also be removed via ex post facto processing. Such work to understand and control defects in this important material could provide a pathway to enable better control over its properties and highlight new avenues to manipulate functions in these complex materials. 

Armed with advances in our ability to synthesize, characterize, and model materials, it may be time to redefine the negative connotation surrounding defects in ceramic materials. But can defects really shine as the “good guys” in materials science? 

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 LaCoO₃thin 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 LaCoO₃thin 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 LaCoO₃thin films, but also identifies a promising strategy to design electronic states in complex oxides through cation‐stoichiometry engineering.