Hidden ferromagnetic phase and wrinkling-controlled magnetism in freestanding LaCoO3 membranes

Observation of hidden phases provides unique opportunities in revealing phase transition mechanisms. LaCoO3 thin film is a rare perovskite-oxide ferromagnetic insulator. LaCoO3 is paramagnetic in bulk, while the epitaxially strained films become ferromagnetic, whereas the origin of the emergent ferromagnetic order remains controversial. Here, we report the observation of an unexpected ferromagnetic order in freestanding LaCoO3 membranes, even though the strain has completely released in the membranes. Atomic-resolution microscopy revealed that the freestanding LaCoO3 membranes have maintained a large Co–O–Co bond angle after releasing from the substrates, giving rise to a hidden ferromagnetic phase as supported by density-functional theory calculations. Moreover, the magnetic state of the freestanding LaCoO3 membranes is controllable by wrinkle formation via transferring the membranes to wafers with distinct surface roughness. Our work proposes an alternative pathway for the creation of hidden phases in correlated oxides and demonstrates that strain is not a prerequisite of emergent ferromagnetism in LaCoO3 films.