Topotactic Transformations in Cobaltite Thin Films

The development of next generation memory, logic, and energy conversion devices motivates the search for materials where their physical properties can be controlled through the migration of charged ions. Among the candidate materials, perovskite oxides ABO₃ have received considerable interests because of their multitude of related phases such as the Grenier (ABO_2.7), brownmillerite (BM, ABO_2.5), square planar (ABO₂), and Ruddlesden-Popper (RP, A_n+1B_nO_3n+1 where n = integer) phases. In this work, a series of topotactic transformations between these perovskite-related phases has been observed in La_0.7Sr_0.3CoO_3-d thin films upon annealing under highly reducing conditions. At intermediate annealing conditions, a composite phase consisting of ferromagnetic Co nanoparticles embedded in a RP matrix was observed. We show that each structure possesses its own distinct set of physical properties including ferromagnetic/metallic, anti-ferromagnetic/insulating, and ferromagnetic/insulating phases. These results highlight the potential for harnessing these topotactic transformations in next-generation spintronic, neuromorphic, and energy conversion devices based on the movement of oxygen anions.