Ferroelectricity and Multiferroicity in Ultrathin Films of Unconventional Transition Metal Oxide

Authors

Xin Li, University of Nebraska-LincolnFollow

First Advisor

Xiaoshan Xu

Committee Members

Alexei Gruverman, Xia Hong, Jian Wang

Date of this Version

8-2025

Document Type

Dissertation

Citation

A dissertation presented to the faculty of the Graduate College at the University of Nebraska in partial fulfillment of requirements for the degree of Doctor of Philosophy

Major: Physics and Astronomy

Under the supervision of Professor Xiaoshan Xu

Lincoln, Nebraska, August 2025

Chapters 1 and 7 are partially based on:
Xin Li, Yu Yun, Xiaoshan Xu. Recent Progress on Multiferroic Hexagonal Rare-Earth Ferrites (h-RFeO₃, R = Y, Dy-Lu). Journal of Physics D: Applied Physics 58 073003 (2025)
Xin Li, Guodong Ren, Yu Yun, Arashdeep Singh Thind, Amit Kumar Shah, Abbey Bowers, Rohan Mishra, Xiaoshan Xu. Improper flexoelectricity in hexagonal rare-earth ferrites. arXiv preprint arXiv:2409.17022

Chapter 3 is based on:
Xin Li, Yu Yun, Xiaoshan Xu. Improper ferroelectricity in ultrathin hexagonal ferrites films. Applied Physics Letters 122, 182901 (2023)
Xin Li, Yu Yun, Guodong Ren, Arashdeep Singh Thind, Amit Kumar Shah, Rohan Mishra, Xiaoshan Xu. Strong tunability of epitaxial relationship and reconstruction at improper ferroelectric interface. arXiv preprint arXiv:2505.05265

Chapter 4 is based on:
Xin Li, Yu Yun, Pratyush Buragohain, Haidong Lu, Arashdeep Singh Thind, Donald A Walko, Detian Yang, Rohan Mishra, Alexei Gruverman, Xiaoshan Xu. Dual Mechanism for Transient Capacitance Anomaly in Improper Ferroelectrics. Physical Review Letters 133, 256801 (2024)
Yu Yun, Pratyush Buragohain, Arashdeep Singh Thind, Yuewei Yin, Xin Li, Xuanyuan Jiang, Rohan Mishra, Alexei Gruverman, Xiaoshan Xu. Spontaneous polarization in an ultrathin improper-ferroelectric/dielectric bilayer in a capacitor structure at cryogenic temperatures. Physical Review Applied 18, 034071 (2022)

Comments

Copyright 2025, Xin Li. Used by permission

Abstract

Ferroic orders of transition metal oxides, such as ferroelectricity and ferromagnetism, are determined by the intercoupling between spin, charge, lattice. Consequently, ferroic epitaxial thin films have attracted wide interest due to the profuse novel phenomena and the great application potentials for modern electronics. However, the mainstream of the study for transition metal oxides has focused on perovskite structure, limiting the understanding and discovery of novel phenomena associate with ferroic orders. In this thesis, the epitaxial films of hexagonal rare-earth ferrites (h-RFeO₃) and fluorite-structure hafnia oxide (HfO₂) are studied comprehensively, providing new experimental and theoretic insights for ferroic orders, especially the ferroelectricity in these novel systems.

Ferroelectricity in h-RFeO₃ is improper since the polarization originates from the non-polar structural distortion. In ultrathin range, the phenomenological model of interfacial clamping is developed, matching with thickness-dependent paraelectric-to-ferroelectric phase transition experimentally. By controlling the interfacial reconstruction, the interfacial clamping effect can be effectively removed. The multidimensional energy landscape of h-RFeO₃ enables more abrupt change of effective field than conventical ferroelectrics, which brings a substantial signature on transient negative capacitance effect in h-YbFeO₃ films. Moreover, h-RFeO₃ is multiferroic, it is found that the domain wall magnetoelectric (ME) coupling could induce the change of macroscopic magnetization, even if ME decoupling switching path is favored for bulk state.

The ferroelectricity of HfO₂ has been attributed to the metastable orthorhombic Pca2₁ phase, which can be transferred from the Pbca phase under electric field. The single-crystalline 10%La-doped HfO₂ (LHO) film with Pbca phase is thermodynamically stabilized on YSZ substrate. The antiferroelectric-like double hysteresis loop is obtained in LHO film even with one-unit-cell thickness. Moreover, Curie temperature, corresponding to tetragonal-to-orthorhombic phase transition increases when thickness decreases, reaches to 850 ^oC at two-dimensional limit. The orthorhombic-related distortion emerges in the initial monolayer of Hf with bulk-state value, corresponding to the absence of interfacial clamping effect. These results indicate the nanoscale stabilization of ferroelectric HfO₂ can be attributed to the antiferroelectric stable and ferroelectric stable state from Kittle model.

Advisor: Xiaoshan Xu