Interface Strain-Induced Physical Properties in Complex Oxides

Abstract

The interfaces become more important with the development of thin film growth engineering Besides the observation of intriguing properties at the interface the properties of the adjacent material can be tuned across the interface Among the lots of materials used in interfacial studies the complex oxides attract extensive attention due to rich properties and potential applications In this dissertation we divide the research into three parts to investigate the interface strain-induced physical properties in complex oxides In the first part we studied the ferroelectric retention behavior at the periodic heterointerfaces of tetragonal-like BiFeO3 (T-BFO)/rhombohedral-like BiFeO3 (R-BFO) in mixed-phase BFO film The enhancement of ferroelectric retention was observed and the retention time is longer than in the single-phase BFO Phase field simulation shows that the T/R mixed-phase region has lower elastic energy density than the T-BFO matrix which explains the experimental observation of better ferroelectric retention in the mixed-phase region Tracking the dynamic domain relaxation process reveals that T/R phase boundaries act as the pinning centers for the domain wall motion By taking the advantage of the in-plane periodic potential distribution in the mixed-phase region we demonstrated a simple way to create stable reversed domains with significantly enhanced long retention In the second part interface strain-affected structural and magnetic transformations at low temperature in mixed-phase BFO were investigated by atomic force microscopy (AFM) X-ray diffraction (XRD) and Raman spectroscopy We combined Raman spectroscopy and AFM to separate the Raman contribution of T-BFO and R-BFO by using the characteristic of electric field tunable T/R ratio Based on temperature-dependent XRD and resolved Raman spectra we observed two isostructural transitions at around 225 K and 150 K and they are strongly correlated with the magnetic ordering in the mixed-phase BFO film Meanwhile the transitions of T-BFO and R-BFO couple together by the mediation of interfacial strain Moreover through the effective spin-lattice coupling the evolution of the T/R polymorph is changed by the magnetic cooling process at low temperatures We provide a pathway to modulate phonon behaviors by magnetic fields in a highly strained system In the last part in order to know the effects caused by different kinds of heterointerfaces we studied the layer-by-layer CoFe2O4 (CFO) film and CoFe2O4-SrRuO3 (CFO-SRO) vertically aligned nanocomposite film XRD patterns show a strain-relaxed state in the layer-by-layer CFO film while the interfacial strain results in distorted structures in CFO-SRO film In addition the angle-resolved polarized Raman spectroscopy indicates there are different short-range octahedral-site orderings in relaxed and distorted CFO structures With the help of Raman polarization selection rules and theoretical results calculated by the first principle the relaxed CFO structure (c/a=1) with Imma symmetry and the distorted CFO (c/a>1) in CFO-SRO nanocomposite film with P4122 symmetry are obtained The interfacial strain plays an important role to manipulate the cation distribution in spinel ferrite

Date of Award	2017 Oct 16
Original language	English
Supervisor	Yi-Chun Chen (Supervisor)