Phase Transitions and Domain Structures in Multiferroics

Open Access
- Author:
- Vlahos, Eftihia
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- February 18, 2011
- Committee Members:
- Venkatraman Gopalan, Dissertation Advisor/Co-Advisor
Venkatraman Gopalan, Committee Chair/Co-Chair
Clive A Randall, Committee Member
Long Qing Chen, Committee Member
Zhiwen Liu, Committee Member - Keywords:
- confocal microscopy
thin films
multiferroics
nonlinear optics
phase transitions
symmetry - Abstract:
- Thin film ferroelectrics and multiferroics are two important classes of materials interesting both from a scientific and a technological prospective. The volatility of lead and bismuth as well as environmental issues regarding the toxicity of lead are two disadvantages of the most commonly used ferroelectric random access memory (FeRAM) materials such as Pb(Zr,Ti)O<sub>3</sub> and SrBi<sub>2</sub>Ta<sub>2</sub>O<sub>9</sub>. Therefore lead-free thin film ferroelectrics are promising substitutes as long as (a) they can be grown on technologically important substrates such as silicon, and (b) their T<sub>c</sub> and P<sub>r</sub> become comparable to that of well established ferroelectrics. On the other hand, the development of functional room temperature ferroelectric ferromagnetic multiferroics could lead to very interesting phenomena such as control of magnetism with electric fields and control of electrical polarization with magnetic fields. This thesis focuses on the understanding of material structure-property relations using nonlinear optical spectroscopy. Nonlinear spectroscopy is an excellent tool for probing the onset of ferroelectricity, and domain dynamics in strained ferroelectrics and multiferroics. Second harmonic generation was used to detect ferroelectricity and the antiferrodistortive phase transition in thin film SrTiO<sub>3</sub>. Incipient ferroelectric CaTiO<sub>3</sub> has been shown to become ferroelectric when strained with a combination of SHG and dielectric measurements. The tensorial nature of the induced nonlinear polarization allows for probing of the BaTiO<sub>3</sub> and SrTiO<sub>3</sub> polarization contributions in nanoscale BaTiO<sub>3</sub>/SrTiO<sub>3</sub> superlattices. In addition, nonlinear optics was used to demonstrate ferroelectricity in multiferroic EuTiO<sub>3</sub>. Finally, confocal SHG and Raman microscopy were utilized to visualize polar domains in incipient ferroelectric and ferroelastic CaTiO<sub>3</sub>.