Charge and Strain Engineering of Ferroic and Correlated Oxides

Authors

Yifei Hao, University of Nebraska-LincolnFollow

First Advisor

Xia Hong

Department

Physics and Astronomy

Date of this Version

Spring 2024

Document Type

Dissertation

Comments

Copyright 2024, Yifei Hao. Used by permission

Abstract

This dissertation presents a comprehensive study of the charge and strain effects on the electronic and magnetic properties of epitaxial ferroic and correlated oxide thin films and heterostructures. We perform the planar Hall effect to quantify the magnetocrystalline anisotropy (MCA) in epitaxial La_1-xSr_xMnO₃ films (x = 0.2 and 0.33) deposited on SrTiO₃ substrates. The biaxial magnetic anisotropy energy of L_0.67Sr_0.33MnO₃ is about 17% larger than La_0.8Sr_0.2MnO₃. By comparing with the theoretically predicted values, we have decoupled the doping and strain effects on MCA. We examine the transition from the ferroelectric (FE) to dielectric (DE) state in PbZr_0.2Ti_0.8O₃/SrTiO₃ (PZT/STO) heterostructures via probing the polarization switching and transient negative capacitance (NC) effects. With decreasing FE-DE thickness ratio (r), the remanent polarization of the stack decreases monotonically and exhibits an abrupt drop at the critical ratio of r_c = 8-7, which is consistent with the Landau theory modeling of the free energy profile. Upon polarization switching, we observe the charge damping associated with the transient NC mode within 2 μs. The transient NC mode is quenched in the r = 6 sample, consistent with the suppressed ferroelectric order. We systematically study the ferroelectric field effect in RNiO₃ (R = Sm, Nd, La) and RNiO₃/La_0.67Sr_0.33MnO₃ (LSMO) channels gated by PZT. For single-layer channels, switching the PZT polarization induces a metal-insulator transition in the 1.5 nm LaNiO₃ channel. The resistance switching ratio ΔR/R_on peaks near the electrical dead layer thickness, with the highest ΔR/R_on of 1,619% at 300 K observed in 0.8 nm LaNiO₃. With the same channel thickness, devices with RNiO₃/LSMO composite channels exhibit up to three orders of magnitude higher ΔR/R_on. A record high ΔR/R_on of 38,540% is observed in the 1 nm LaNiO3/0.8 nm LSMO bilayer channel, which has been attributed to the interfacial charge transfer effect that reduces the net carrier density in RNiO₃ without compromising the screening of depolarization field in PZT. Our study addresses one of the critical material challenges that limit the application potential of epitaxial ferroelectric-gated Mott transistors.

Recommended Citation

Hao, Yifei, "Charge and Strain Engineering of Ferroic and Correlated Oxides" (2024). Dissertations and Doctoral Documents from University of Nebraska-Lincoln, 2023–. 78.
https://digitalcommons.unl.edu/dissunl/78