Abstract
In this thesis, we investigated the correlations between structural variation and physical properties of the some multi-phases materials, such as multiferroic or nano-structured composite thin films. It will be discussed in two parts: one is the study of phase transition and corresponding ferroelectric response in multiferroic BiFeO3 thin films; and the other is the study of coupling effect in self-assembled nano-structured thin films composed of photostrictive perovskite SrRuO3 (SRO) and magnetostrictive spinel CoFe2O4 (CFO). First, recent researches have shown that the multiferroic bismuth ferrite (BiFeO3, BFO) thin films exhibit high correlation between coupled electric and magnetic orders and abundant structure variation. Especially for those grown on the substrate with large lattice misfit such as LaAlO3 (LAO) or YAlO3 (YAO), it is found that BFO becomes tetragonal-like structure instead of original rhombohedral structure due to the large in-plane compressive strain, and its ferroelectric polarization also changes with the structural variation. In this thesis, we studied a series of the strain evolution on the structural nature of BFO thin films by X-ray reciprocal space mapping (RSM) and transmission electron microscopy (TEM). A strain-driven phase boundaries occur at medium thickness with coexistence of tetragonal-like and rhombohedral-like phases because of strain relaxation. The detailed structures reveals that two extra tilted monoclinic phases form to accommodate the large lattice mismatch from tetragonal-like structure to rhombohedral-like structure, resulting in the feature of periodic strips presented in topography. Besides, the BFO thin films grown on YAO substrate even show a discrepancy of arrayed stripe morphology due to the anisotropic strain in different direction of YAO substrate. We then focused on the intermediate phase at the boundaries between the tetragonal-like and rhombohedral-like phases, whose content and structure are strongly dependent on temperature. We also found a monoclinic phase transition at temperature around 100~200℃ in ambient condition. The observed transition is between an MC symmetry and an MA symmetry. Studies of the ferroelectric domains of the MC and MA phases clearly show that their ferroelectric polarizations rotate when the phase transition occurs. Piezoelectric response in the BiFeO3 thin films displays a substantial enhancement at the MC−MA transition temperature. These findings directly unveil the close correlations between structural changes, polarization rotation and high piezoelectricity in ferroelectrics. On the other hand, self-assembled vertical nanostructures have also attracted extensive attentions recently due to their advantage of high interface-to-volume ratio. They could be used to design new functionalities by choosing proper combination of constituents. While most of the studies up to date have emphasized the functional controllability of the nanostructures using external electric or magnetic fields, we try to demonstrate a new coupling mechanism: the photo-magnetic coupling effect, which describes light (or photons) as the external control parameter in this study. We have successfully synthesized oxide nanostructures with CoFe2O4 (CFO) nanopillars embedded in SrRuO3 (SRO) matrix on (001) SrTiO3 substrate and confirmed that all constitutes still keep their original properties form measurements of ultra-fast laser, conducting atomic force microscopy (CAFM), and Superconducting Quantum Interference Device (SQUID). Combination of the photostrictive SRO and magnetostrictive CFO in the intimately assembled nanostructures can leads to a light-induced, ultrafast change in magnetization of the CFO nanopillars. Our work demonstrates a new concept on oxide nanostructure design and engineering and opens a pathway alternative to the traditional routes for the explorations of new fuctionalities. At the end of this work, we can simply conclude that whether in the single-constitute materials or composite materials, the multi-phase systems always have more complicated structural characteristic and abundant properties than single-phase systems due to the high interface-to-volume ratio, so studying these kinds of system can provide us more new un-discovered physical phenomena.