# COUPLED PHENOMENA IN DOMAINS AND DOMAIN WALLS IN COMPLEX POLAR OXIDES

Restricted (Penn State Only)

- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- September 18, 2017
- Committee Members:
- Venkatraman Gopalan, Dissertation Advisor
- Venkatraman Gopalan, Committee Chair
- Wenwu Cao, Committee Member
- Long-Qing Chen, Committee Member
- Moses H. W. Chan, Outside Member
- Thomas E. Mallouk, Committee Member
- Zhiqiang Mao, Special Member

- Keywords:
- domains
- domain walls
- complex oxides
- polar
- ferroelectrics
- polar metals
- magnetoelectric coupling

- Abstract:
- Complex oxides represent a family of solid state materials that possesses a wide spectrum of properties, such as ferroelectricity, ferromagnetism, superconductivity, colossal magnetoresistance, large nonlinear optical response and metal-to-insulator transition. One type of such materials in the focus of the current work is polar materials, where the charge centers of the anions and cations are shifted relative to each other. The criteria whether the polar-axis exists or not thus divides all materials into two type — polar or nonpolar. From the symmetry point of view, the appearance of polar axis or inversion symmetry breaking naturally leads to a crystal lattice with lower symmetry and the introduction of a certain order, just like the phase transformation from water to ice. The appearance of domain microstructure is a direct consequence when such symmetry lowering process happens. It is the existence of domain structure, its tunability, and its ability to couple with other order parameters that gives rise to a plethora of interesting coupled phenomena and make polar materials extremely useful for a wide range of applications. The rich coupled phenomena and their mechanisms are the center focus of this work. In Chapter 2, a systematic finite element modeling (FEM) approach was applied in understanding the quantitative piezoelectric origin of piezoresponse force microscopy (PFM) response across a 180° domain walls. Using FEM simulations, the coupled PFM response from each individual piezoelectric coefficient is conveniently decoupled, thus building a bridge between the measured PFM response and intrinsic piezoelectric properties of a ferroelectric material. In order to understand the coupled piezoresponse across domain walls, complete three-component piezoresponse line-profiles are calculated, allowing a direct comparison with experiments. While excellent agreement is achieved on vertical PFM (VPFM) response between experiments and theory, less satisfactory agreement is found in the lateral PFM (LPFM) response. Additionally, a new component of piezoresponse is predicted near the wall. A new approach to evaluate the tip-sample contact radius is also proposed and verified experimentally. In Chapter 3, based on the domain wall symmetry argument, a decoupling treatment of VPFM response is proposed and experimentally demonstrated to isolate the in-plane and out-of-plane piezoresponse contribution, thus allowing the achievement of three-dimensional piezoresponse from a single imaging geometry. The experimental VPFM and LPFM line-profiles after decoupling treatment show good agreements with the predicted ones from FEM simulations. In this Chapter, the background subtraction process is also demonstrated to be critical in reproducing the intrinsic piezoresponse phase contrast across the antiparallel domain wall. In Chapter 4, a first direct observation of ferroelectric-like polar domains in a polar metal is provided. The quasi-two-dimensional polar domain has large aspect ratio of ~103-104. This is beyond the domain geometry described by the general Kittel-Mitsui-Furuichi law, which gives the aspect ratio of ~10. The appearance of polar domains in a polar metal is found to be related to the geometric effect rather than the electrostatic effect from DFT calculations. The large aspect ratio of polar domains is found to be related to the reduced electrostatic energy due to the effective electronic screening. This suggests the role of conducting carriers in stabilizing the quasi-two-dimensional geometry of polar domains. Importantly, a first-order quasi-phase-matching effect is demonstrated on thin polar domains (thickness ~10 nm), paving a route to mirror-less optical parametric oscillators using the first-order quasi-phase-matching effect. In Chapter 5, results of detailed second harmonic generation, resistivity, specific heat, magnetization and neutron diffraction studies are provided to give a comprehensive picture on the magnetic phase diagrams for a polar material Ca3(Ru0.95Fe0.05)2O7. Although the polar mechanism is driven by the hybrid improper mechanism and the structure is also isostructural to the prototype material Ca3Mn2O7, the magnetoelastic coupling mechanism shows distinctively different behaviors from that in Ca3Mn2O7. The coexistence of weak ferromagnetism and magnetoelastic coupling in the polar material Ca3(Ru0.95Fe0.05)2O7 suggests a new route for ME coupling: an improper polar lattice distortion plus the metastable incommensurate weak ferromagnetic order In Chapter 6, I propose two future research topics based on some preliminary results that can be further pursued for better understanding on the domains/domain walls involved coupling phenomena. Two material systems are proposed for these future works, one is a tetragonal ferroelectric PbTiO3 and the other one is a polar metal Ca3Ru2O7. Theoretical studies such as phase-field simulations and DFT calculations are proposed as well to provide guidance on practical experiment design and lend theoretical support for the experimental observations.