Yifei Hao will present his thesis topic, “Charge and Strain Engineering of Ferroic and Correlated Oxides” in person.

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 La1-xSrxMnO3 films (x = 0.2 and 0.33) deposited on SrTiO3 substrates. The biaxial magnetic anisotropy energy of L0.67Sr0.33MnO3 is about 17% larger than La0.8Sr0.2MnO3. 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 PbZr0.2Ti0.8O3/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 rc = 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 ?s. 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/La0.67Sr0.33MnO3 (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 ?R/Ron peaks near the electrical dead layer thickness, with the highest ?R/Ron 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 ?R/Ron. A record high ?R/Ron 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.