THERMAL CONTROL OF MAGNETIC PHASE TRANSFORMATIONS THROUGH ACTIVE INTERFACES

Oxide heterostructures possess a wide range of electrical and magnetic properties arising, in particular, via interactions across their interfaces. Iron oxides generally and Fe3O4 particularly have a multitude of electric and magnetic functionalities which makes them interesting candidates for magnetic applications and heterogeneous catalysis. Controlling the oxide-interfaces opens additional manufacturing possibilities for functional devices. Moreover, switching between the different functional phases of iron oxides may open up novel routes to control and tune magnetic states via thermal phase design. It is therefore, our primary goal to understand, control, and tune the interface properties of Fe3O4/SrTiO3 and Fe3O4/YSZ heterostructures. In this study, using hard X-ray photoelectron spectroscopy (HAXPES), we demonstrate phase transformations from Fe3O4 to either -Fe2O3 or FeO through active redox reactions across three relevant interfaces, i.e. (1) the outside atmosphere/FeO film interface, (2) the interface between phase-transformed FeO/FeO intralayers and (3) the FeO/oxide substrate interface. We find that the "active" oxide substrates (SrTiO3 or YSZ) play an important role as an additional oxygen supplier or scavenger. This leads to a clear alteration of the standard temperature-pressure phase diagram of iron oxides. Accordingly, we calculate the effective oxygen pressure through the interfaces and adjust the phase diagram. Moreover, using X-ray absorption spectroscopy (XAS) and X-ray magnetic circular and linear dichroism (XMCD and XMLD), we monitor the local distortion and the orbital character of the Fe sites in the iron oxides thin films. Our findings allow us not only to control the interfaces but more importantly, to tune their physical functionalities by a controlled thermal phase design, giving access to far from equilibrium phases