Thermal phase design of ultrathin magnetic iron oxide films: from Fe3O4 to γ-Fe2O3 and FeO

We demonstrate the thermally induced phase transformations between Fe3O4, γ-Fe2O3 and FeO ultrathin iron oxide films, which are part of all-oxide heterostructures, and present a comprehensive thermodynamic analysis of the emerging interfacial redox processes. We thereby reveal the essential – but mostly underrated – role of oxide substrates, which can completely alter the standard FexOy temperature–pressure phase diagram as an additional oxygen supplier or scavenger. We introduce an adjusted phase diagram specifically for FexOy/Nb:SrTiO3 and FexOy/YSZ heterostructures based on a total effective oxygen activity. This novel approach opens up the route towards tuning physical functionalities in all-oxide heterostructures by a controlled thermal phase design.