Gertjan Koster, University of Twente.
In complex oxide materials the occurrence of ferroelectric, ferromagnetic or other properties are for the most part determined by the detailed oxygen coordination of metal cations. More specifically, in the case of perovskite-type materials ABO3, where A and B are metal cations, by the BO6 octahedral orientations and rotations. At interfaces in epitaxial oxide hetero structures, for example magnetic junctions or capacitive structures, this oxygen sub-lattice is found to be different from its bulk counterpart.
Here we will present recent experiments revealing a relationship between precise oxygen ordering, electronic structure and transport properties using a system where photoemission as well as transport measurements can be performed under identical controlled conditions. Such a system is now available at the MESA+ laboratory in Twente also in collaboration with the university of Amsterdam.
Photoemission spectra, both core-level as well as valence-band spectra of in particular the 3d and 4d elements are very sensitive to their anionic surroundings, for example the Ru 3d peaks in SrRuO3 or Mn 2p peaks in LaSrMnO3, both important ferromagnetic metals. A technique related to photoemission is x-ray photoelectron diffraction is subsequently used to reveal the crystal structure near the model-interfaces.
Examples of oxygen sub-lattice engineering achieved by the insertion of oxide buffer layers that disrupt the perovskite-type BO6 sub-lattice are found in materials that exhibit a deviating oxygen sub-lattice from the six-fold oxygen coordination. The effects of such buffer layers are subsequently studied by previously mentioned characterization techniques and modeling. Often-encountered problems due to dead-layer effects, which normally hamper many ferromagnetic or ferroelectric functional devices, could be tackled this way. Besides improving the functionality of heterostructure devices one might expect to find surprising properties not found in the bulk, for example a new ferromagnetic insulating state, which has potential applications in spintronics