Hans Boschker

Ph.D. thesis

Summary

Perovskite oxides are naturally suitable for heteroepitaxy. The new functionality of the heterostructures can be attributed to two fundamental effects in heteroepitaxy. At first the crystal structure of the layers is changed, due to the matching of the in-plane lattice constants to those of the substrate. The matching results in strain in the layers, the magnitude of which can be controlled with the use of an appropriate substrate. This is called strain engineering. Furthermore, the interfaces between different layers break symmetry and therefore new functionality can be expected at the interfaces. Heteroepitaxy allows for direct intervention, e.g. with dopant insertion, at the interface during growth, which is called interface engineering. In this thesis, an exploration of the possibilities with strain and interface engineering is made for both the LaAlO3/SrTiO3 (LAO/STO) interface and the fully spin polarized metal La0.67Sr0.33MnO3 (LSMO). At first, the conducting interface between the two band insulators LAO and STO is studied. The main experimental result is that the expected electrostatic potential buildup in the LAO layer is not observed. This indicates that the most widely used model to explain the conductivity at the interface, the electronic reconstruction due to the polar discontinuity, is not applicable to the experiments. An alternative model for the conductivity is proposed. In LSMO thin films, the crystal structure is determined by the strain from the underlying substrate. With the choice of a specific substrate surface, the magnetic properties of the LSMO layer can be controlled. A model was developed which predicts the magnetocrystalline anisotropy. The model was verified with measurements of the magnetic anisotropy of LSMO thin films grown on different surfaces of NdGaO3 single crystal substrates. The use of LSMO in spintronic devices requires spin polarized conductivity at the interface. Due to interface reconstructions, generally the spin polarized conductivity is reduced at the interface. Interface engineering was applied to study and improve the properties of the LSMO