The effects of spatial confinement and oxygen stoichiometry on complex metal oxides

Abstract

Complex metal oxides, such as transition-metal oxides, appearing with exotic properties and optimal functionalities, present formidable challenges in condensed matter physics, while at the same time, immense opportunities in materials science and engineering. The signature of these materials is the multitude of competing ground states that can be tuned or manipulated by doping, structural modification, strain induction, or the application of external stimulus, such as pressure, electric or magnetic fields, etc. The interest in these materials stems from the richness of their novel properties, the complexity of underlying physics, and the promise of technological applications. Novel phenomena emerge by spatial confinement. Stoichiometry can have more profound effect on spatially confined materials than on the bulk. The purpose of the thesis is to explore and understand the spatial confinement and oxygen stoichiometry effect on complex metal oxides to pave the way for oxide electronics. To achieve this goal, I first investigate the properties of the La2/3Sr1/3MnO3 (LSMO) thin films of different thicknesses with different oxygen deficient level. Reducing the thickness of the LSMO thin film drives the LSMO to behave far away from its bulk and leads to several new phenomena such as enhanced magnetoresistance at critical thickness, emergent of effect from substrate structure transition and interface coupling. Unlike the bulk, introduction of few oxygen vacancies can strongly affect the properties of LSMO ultrathin film. Furthermore, I also investigated the epitaxial columnar naoncomposite thin film to study nanoscale inhomogeneity effects on properties of thin films. It is revealed that percolation plays most important role in determining properties. Finally, I also studied the dynamic effects of the oxygen vacancies in oxide materials thin films under external electric field. I show that the field can control the oxygen stoichiometry via field induced oxygen migration and then control the properties of the complex metal oxide in real time. Due to the strong coupling of oxygen with the complex metal oxides, field induced oxygen migration will cause a colossal change of resistance. As a result, we can engineering complex metal oxides for the technological applications such as non-volatile memory devices.