Ionically-controlled phase separation in cobaltite heterostructures*

Complex oxide heterostructures provide access to emergent functional and structural phases which are not present in the bulk constituent materials. Controlling ionic distribution and stoichiometry in complex oxide heterostructures has been utilized to significantly alter and tune the electronic, magnetic, and structural properties. Recently, deposition of a strong oxygen getter on top of an oxide thin film has emerged as a novel way to tailor oxygen stoichiometry and nanoscale functional properties. However, the role played by nanoscale morphology, i.e. phase separation and defects in these oxide thin films and heterostructures remains largely unknown. In this talk, I will focus on Gd/La_0.67Sr_0.33CoO₃ (LSCO) heterostructures due to the high oxygen ion conductivity, as well as the coupled magnetic and electronic properties of LSCO, which are strongly dependent on the oxygen stoichiometry. This combination of properties enable the ionic control of the functional properties of LSCO thin films through the presence of oxygen getter layers such as Gd. We utilize x-ray nanodiffraction to directly image the nanoscale morphology of LSCO thin films as they are progressively transformed from the equilibrium perovskite phase to the metastable brownmillerite (BM) phase with increasing Gd thickness. Our studies show the coexistence of perovskite and BM phases with a critical oxygen vacancy concentration threshold which leads to the formation of extended BM filaments. Strain maps reveal that the perovskite phase changes from compressive to tensile strain on opposite sides of a BM filament. Our studies provide an unprecedented nanoscale survey of the phase separation in the cobaltites and shed light on the formation of the metastable BM phase.