Interface Functionalization of Magnetic Oxide Fe3O4/SrTiO3 Heterostructures

Oxide heterostructures possess a wide range of electrical and magnetic properties, mainly via interactions across their interfaces. The prospect of designing and controlling the magnetic properties at the atomic scale of oxide heterointerfaces is one of the major challenges. In this context, merging transition-metal oxides into heterostructures is very promising, owing to their many remarkable properties, such as emerging conductivities, magnetism or ferroelectricity. Furthermore, iron oxides including FeO, Fe₃O₄ and Fe₂O₃ polymorphs (α-Fe₂O₃, γ-Fe₂O₃...) with a multitude of electric and magnetic functionalities are interesting for many magnetic applications and heterogeneous catalysis. Controlling the oxide interfaces additionally strengthens the manufacturing of functional devices. Therefore, our primary goal is understanding, controlling and tuning the interface properties. For this purpose, we demonstrate the emergence and control of magnetic interfaces between magnetite Fe₃O₄, a ferrimagnetic half-metal, and SrTiO₃, a transparent non-magnetic insulator which is considered the bedrock of oxide-based electronics. The Verwey transition (TV ) is found to persist from bulk-like down to ultrathin Fe₃O₄ films, decreasing from 117 4 K (38 nm) to 25 4 K (2 nm), respectively. Element-selective electronic and magnetic properties of the ultrathin films and buried interfaces are studied by angle-dependent HAXPES and XMCD techniques. We prove that the SrTiO₃ substrates induce both strain and interface oxidation. The substrate-induced strain causes the easy axis to switch to 100. Furthermore, we observe a reduction of Fe2+ ions with decreas-ing film thickness, accompanied by an increase of Fe3+ ions in both tetrahedral and octahedral sites, and conclude on the formation of a magnetically active ferrimagnetic 2 u.c. γ-Fe₂O₃ intralayer. To manipulate the interfacial magnetic phase, a post-annealing process is conducted which causes the reduction of the γ-Fe₂O₃ that finally leads to stoichiometric and ferrimagnetic Fe₃O₄/SrTiO₃ (001) heterointerfaces. We demonstrate the thermally induced phase transformations between Fe₃O₄, and FeO ultrathin iron oxide films, which are part of all-oxide heterostructures γ-Fe₂O₃ es, and present a comprehensive thermodynamic analysis of the emerging interfacial redox processes through active redox reactions across three relevant interfaces, i.e. (1) the outside atmosphere/FeₓOy film interface, (2) the interface between FeₓOy/FeₓOy intralayers and (3) the FeₓOy/oxide substrate interface. 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. Our study goes beyond the current functionalization of oxide heterostructures and their phase transitions. This novel approach opens up the route towards reversible tuning of the physical functionalities, thus, a future integration of Fe₃O₄/SrTiO₃ heterostructures into resistive and magnetic switching devices.