La_0.6Sr_0.4CoO_3- Films Under Deoxygenation: Magnetic And Electronic Transitions Are Apart from The Structural Phase Transition

Topotactic phase transitions induced by changes in the oxygen vacancy concentration can largely alter the physical properties of complex oxides, including electronic and magnetic phases, while maintaining the structural integrity of the crystal lattice. An oxygen-vacancy-induced topotactic phase transition from perovskite (PV) to brownmillerite (BM) is achieved in epitaxial La0.6Sr0.4CoO3-delta (LSCO) thin films. Two novel intermediate states with different oxygen content are identified by X-ray diffraction, which involves a single-phase reduced PV state and a mixed state of co-existing PV and BM. The combination of depth-sensitive polarized neutron reflectometry (PNR) and Rutherford backscattering (RBS) allows a quantitative determination of magnetization and the mean oxygen content in all states, revealing a continuous transition from La0.6Sr0.4CoO2.97 to La0.6Sr0.4CoO2.5. BM formation is observed for an LSCO layer with an oxygen content of 2.67, while the magnetic and electronic transition already occurs for a layer with a higher oxygen content of 2.77 (and above) and in the absence of a BM signature. These results demonstrate that the physics of electronic metal-to-insulator transition (MIT), magneticferromagnet-to-non-ferromagnet transition (FM-to-non-FM), and structural PV-to-BM phase transition should be considered within the framework of separate but interrelated processes.