Graduate Studies, UNL

 

Dissertations and Doctoral Documents from University of Nebraska-Lincoln, 2023–

First Advisor

Xia Hong

Degree Name

Doctor of Philosophy (Ph.D.)

Committee Members

Abdelghani Laraoui, Stephen Ducharme, Xiaoshan Xu

Department

Physics & Astronomy

Date of this Version

2025

Document Type

Dissertation

Citation

A dissertation presented to the faculty of the Graduate College of the University of Nebraska in partial fulfillment of requirements for the degree Doctor of Philosophy (Ph.D.)

Major: Physics

Under the supervision of Professor

Lincoln, Nebraska, December 2025

Comments

Copyright 2025, the author. Used by permission

Abstract

This dissertation presents a comprehensive study of the electrical, magnetic, and transport properties of epitaxial ferroic oxide thin films and free-standing membranes, including ferroelectric PbZr0.2Ti0.8O3 (PZT) and ferrimagnetic NiCo2O4 (NCO).

We evaluate the effects of electrostatic and mechanical boundary conditions on the ferroelectric properties of free-standing PZT membranes by transferring them on three types of base layers: the metallic Au, strongly correlated oxide La0.67Sr0.33MnO3 (LSMO), and 2D semiconductor MoS2. The coercive field and domain wall roughness depend sensitively on the metallicity and surface roughness of the bottom electrode. We also fabricate MoS2 field effect transistors top-gated by PZT membranes and achieve a nonvolatile current on/off ratio of 2.04 × 105 at room temperature by switching the polarization of PZT.

We combine monolayer MoS2 with PZT thin films or membranes and explore the effects of ferroelectric polarization and domain wall on the second harmonic generation (SHG) signal in the heterostructure. The SHG signal exhibits threefold symmetry at the polar domain regions. The PZT membrane/MoS2 stack allows in-operando tuning of the SHG response, which is promising for developing programmable nano-photonics.

We explore the unconventional magnetotransport behavior of epitaxial NCO thin films deposited on (001) MgAl2O4 (MAO) substrates in the ultrathin limit. The perpendicular magnetic anisotropy (PMA) revealed by anomalous Hall effect can be sustained in films down to 1.5 unit cell (1.2 nm). The sign change in the anomalous Hall conductivity and its scaling behavior with the longitudinal conductivity can be attributed to the competition between band intrinsic Berry phase effect and impurity scattering.

We also investigate the interplay between strain and defects in determining the metallicity, magnetoresistance, and magnetic anisotropy in NCO thin films and membranes. NCO films on lattice compatible MAO substrate exhibit metallic conduction, and two-fold sinusoidal anisotropic magnetoresistance (AMR). In contrast, NCO films on other perovskite substrates exhibit insulating conduction, and emergence of four-fold AMR component, which is attributed to strain-induced tetragonal magnetocrystalline anisotropy. The free-standing NCO membrane exhibits highly consistent magnetotransport properties with the unsuspended NCO film, suggesting the strain and disorder levels are mostly retained.

Advisor: Xia Hong

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