Strain control of single crystal complex oxide epitaxial and freestanding films [electronic resource]
Keywords: Physics.

Many breakthroughs in condensed matter physics, including high Tc superconductivity, colossal magnetoresistance and multiferroics, originate from the study of transition metal perovskite oxides. Epitaxial strains on oxides can significantly alter these properties and realize novel functionalities, yet switching substrates not only changes the strain states of the film, but also affects the defect type and density in the film. In parallel with oxides, the recent development in exfoliated layered materials opens a new pathway of material manipulation, such as large strain or anisotropic strain application, due to their extreme geometry. However, creating exfoliated-material-like oxides has also proved challenging, limiting the manipulation capabilities of these oxides. This thesis is devoted to addressing these two limitations of epitaxial strain application for oxide films. In the first part, I will demonstrate that strain states and physical properties of oxide thin films can be controlled without affecting film quality by using Sr3Al2O6 epitaxial buffer layers. Sr3Al2O6 has lower elastic moduli than common oxides, which allows it to confine the mismatch dislocations to itself but not propagate to the overlaying films, preserving their quality. To show that the Sr3Al2O6 buffer layers are ideal for controlling physical properties of high quality oxide thin films, strain- and quality-sensitive Nd0.5Sr0.5MnO3 films are employed. By controlling the thickness of Sr3Al2O6 buffer layers, the (001)-oriented Nd0.5Sr0.5MnO3 films are tuned from a ferromagnetic metal to a insulator with low magnetization. By carefully tuning the strain state, a bulk-like ferromagnetic metal to charge-ordered insulator phase transition is also realized, which has previously proven to be difficult for (001)-oriented Nd0.5Sr0.5MnO3 films. In the second part, I will present a general method of fabricating oxide freestanding films, i.e. single crystal oxide thin films free from substrates. The key is to insert a sacrificial layer, again Sr3Al2O6, between an epitaxial oxide film and a substrate while keeping the epitaxial relation. The sacrificial layer was etched afterwards by room temperature water, and millimeter-sized high quality freestanding oxide films were obtained. As an example of demonstrating novel strain manipulation methods, I will also show how physical properties of SrTiO3 freestanding films can be changed by external strain application. The ground state of SrTiO3 is at the boundary of a paraelectric state and a ferroelectric state, and perturbation of its lattice can elevate the ferroelectric-paraelectric phase transition to non-zero temperature, where a maximum of dielectric constant is observed. By applying uniaxial stress, longitudinal strains of up to 1% can be generated in this system, the dielectric maxima were observed near 300 K, suggesting such a phase transition. Our studies establish Sr3Al2O6 buffer layers and oxide freestanding films as two general tools that can be used to broaden the strain manipulation methods on oxides, which has significant potential of drastically altering their physical properties.