Oxygen vacancy driven interfacial phenomena in oxide heterostructures

Resumen

The quest for novel functionalities in strongly correlated oxide systems has often been determined by reducing dimensionality and designing nanostructured systems that exploit new physical effects. Any physical phenomena occurring within nanometric length scales are often ruled by the properties of small active regions, such as interfaces, point or extended defects, etc. In the growth process of oxide based heterostructures, the oxygen content constitutes a degree of freedom that can strongly affect the physical properties. Thus, the study of interfacial effects driven by oxygen vacancies is of great interest in order to pursue new functionalities aimed at designing future devices. In this thesis we have grown and characterized a number of systems comprised of complex oxide heterostructures where the O stoichiometry is tuned to produce physical behaviors not present in bulk. The local atomic structure and chemical properties of our samples have been probed with aberration corrected scanning transmission electron microscopy (STEM) combined with electron energy-loss spectroscopy (EELS). By means of spatially resolved analyses of slight fluctuations of the local crystalline structure, composition or electronic environment imposed by the presence of O point defects, we have been able to explain a number of physical effects in oxide interfaces of interest in spintronics or energy materials. First, we have grown and characterized high quality ferromagnetic/insulating La0.7Sr0.3MnO3/LaCoO3 (LSMO/LCO) interfaces, with the objective of fabricating novel multiferroic tunnel junctions (MTJs) with LaCoO3 as insulating barrier. For this purpose, the La1-xSrxCoO3 system with different Sr doping levels has been studied in order to establish the sample quality and also obtain internal calibrations for our STEM-EELS studies. We have explored the system conductance and magnetic properties to find that tunneling magnetoresistance (TMR) values were unusually low due to changes in the magnetic anisotropy of LSMO. We have synthetized LCO/LSMO (LSMO/LCO) bilayers where LSMO(LCO) is grown on top of LCO(LSMO), to find structural distortions arising in LaCoO3 buffer due to the presence of inhomogeneous densities of O vacancies layers, which suppress the strain-induced biaxial magnetic anisotropy of the top LSMO. Second, we have also fabricated a number of asymmetric LSMO/BaTiO3/La0.86Sr0.14CuO3/LSMO MTJs with a main purpose: breaking the symmetry of the ferromagnetic/ ferroelectric/ ferromagnetic LSMO/BaTiO3/LSMO system by introducing an ultrathin film of an oxygen deficient La0.86Sr0.14CuO3 cuprate between the BaTiO3 barrier and the top LSMO electrode. We have found that the introduction of a high density of oxygen vacancies in the system contributes to the enhancement of the tunnel electroresistance by 104 %. The ferroelectric polarization state along with the location of oxygen vacancies control the size of the depletion region. We have also detected a significant electron doping at the other side of BaTiO3 interface and a change of the sign of the TMR, which we interpret that charge transfer as a spin filter effect. In summary, we show that the mechanisms of ferroelectric polarization and oxygen vacancy migration through the barrier are coupled. The last objective has been the turn to the study of materials systems for energy storage. Electric conduction properties in ionic conductors are determined by presence of point defects, and also by lattice strains in thin films. This is the case of Y2O3:ZrO2 (YSZ), where the carriers are oxygen vacancies themselves. We have grown a series of epitaxial YSZ(111) (8% mol) thin films on YAlO3 substrates. Reciprocal space maps reveal that the samples present a domain matching epitaxy despite the lattice mismatch, promoting high quality growth. However, the dissimilar symmetries at the interface strongly affect the conduction properties of our which can be determining to understand the conduction mechanism.