Switchable polar properties of ferroelectric and multiferroic nanostructures are ideal to further diversify applications of mainstream semiconductors. Recent breakthroughs in Scanning Probe Microscopy (SPM) have enabled tailoring of polar domain structures at the nanoscale, which is critical to fabricate polarization-based devices. However, highly inhomogeneous electric fields of biased SPM-tips complicate polarization physics in ferroelectrics and multiferroics. Also, typical diffused phase transition in relaxor bulks originates from coupled inhomogeneities of intrinsic polar nanoregions (PNRs). In this thesis, anisotropic and time-dependent mechanisms were developed to study SPM-tip poled polarization switching in ferroelectric and multiferroic thinfilms. Moreover, frequency-related PNR thermodynamics and its effect on electrocaloric effect of locally disordered relaxors were modeled.
Firstly, a three dimensional model was established to clarify tip-poling effect on ferroelectric domain nucleation and growth. The concept of “domain shape invariance” was confirmed through constant aspect ratio obtained for conic ferroelectric nucleus. This domain aspect ratio was found to abruptly decrease under the depolarization effect, saturating domain radius. Further increasing tipvoltage could drive longitudinal breakdown of already reverted domains throughout film thickness.
Subsequently, tip-activated evolution of domain wall width in ferroelectric and multiferroic thinfilms was studied via extended Kittle’s law, which included anisotropic and dynamic effects arising from tip-fields. Our calculation results showed that wall width in LiNbO3 varied slightly in an initial stage, followed by a drastic change. This wall variation corresponded to three varying regions of coercive field. Besides, we highlighted three polarization switching modes in BaTiO3 - absence, activation and nonactivation mode. Importantly, distinct switching modes, i.e., breakdown mode of 71° domain switching and activation mode of 180°/109° switching, were revealed to fundamentally control filmorientation dependent multipolarization switching sequence in BiFeO3.
Thirdly, Pauli’s mater theory was utilized to bridge microscopic evolution of PNRs and characteristic properties of Pb(Mg1/3Nb2/3)O3 (PMN) relaxors. Temperature dispersion and frequency dependence of PMN dielectric susceptibility were related to nonlinear PNR dynamics over a broad temperature interval. We could not validate PNR-volume predictions of percolation theory above the freezing temperature, but suggest a gradual saturation of PNR volume at lower temperatures. Besides, observed deviations of relaxor permittivity from the Curie-Weiss law were attributed to thermal effects on PNR dynamics and resultant polarization rotations. Furthermore, time-dependent PNR dynamics was proposed to study strong frequency dependence of typical relaxor behaviors. It was implied that frequency effect on PNR coercive field was governed by classic Merz’s-switching, leading to suitability of Vogel-Fulcher law for relaxors bulks.
Last but not least, above-mentioned framework for PMN relaxors was incorporated with Landau-Ginzburg-Devonshire thermodynamics and Maxwell relation to better understand recently observed giant electrocaloric (EC) effect of relaxor thinfilms, which is promising for solid-state refrigeration. Three contributions were found to dominate relaxor EC response: temperature-dependent dielectric dispersion, inverse pyroelectric effect and thermally enhanced dielectric stiffness. We emphasized that the EC material with larger dielectric stiffness and smaller correlation length could extend its enormous EC response above Curie temperature. Finally, potential approaches, e.g., by manipulating shape, volume and density of PNRs, were suggested to engineer the EC enhancement in relaxor nanostructures. / published_or_final_version / Mechanical Engineering / Master / Master of Philosophy
| Identifer | oai:union.ndltd.org:HKU/oai:hub.hku.hk:10722/180977 |
| Date | January 2012 |
| Creators | Shi, Yuping, 史玉平 |
| Contributors | Soh, AK |
| Publisher | The University of Hong Kong (Pokfulam, Hong Kong) |
| Source Sets | Hong Kong University Theses |
| Language | English |
| Detected Language | English |
| Type | PG_Thesis |
| Source | http://hub.hku.hk/bib/B49617989 |
| Rights | The author retains all proprietary rights, (such as patent rights) and the right to use in future works., Creative Commons: Attribution 3.0 Hong Kong License |
| Relation | HKU Theses Online (HKUTO) |
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