Impact of Off-Stoichiometry on Defect Structure and Magnetism in LaCoO₃ Thin Films

By growing LaCoO3 (LCO) in a thin film geometry and imposing tensile strain via the substrate, LCO at low temperatures is transformed from a low-spin diamagnet to a ferromagnet. The origin of this magnetic state has been intensely studied, but no consensus has been achieved. One striking feature observed in most TEM studies of this material is a periodic array of dark stripes. The composition of the dark stripes and their relationship to magnetism has been investigated, but no clear connection has been established. By using low-pressure, off-axis rf sputtering, we are able to grow highly stoichiometric LCO thin films that are still ferromagnetic despite possessing no dark stripes. The magnetism in our films is weaker than what has been previously reported, suggesting that defect structure has a strong impact on the magnetization. We also fabricate La deficient samples that show an enhancement of magnetic properties after annealing in air, further demonstrating the effect of stoichiometry and defect structure on the magnetism.