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Scanning Probe Microscopy Studies of Interface Phenomena in Ferroic Thin Films and Heterostructures
This dissertation presents the scanning probe microscopy studies of nanoscale ferroelectrics and magnetic materials, including ferroelectric PbZr1-xTixO3 and ferrimagnetic NiCo2O4 thin films and van der Waals (vdW) materials interfaced with PbZr0.2Ti0.8O3 and ferroelectric copolymer P(VDF-TrFE). Piezoresponse force microscopy (PFM) studies of epitaxial PbZr0.2Ti0.8O3 thin films on different correlated oxide electrodes reveal the competing effects of disorder pinning and thermal roughening on ferroelectric domain walls (DWs). We extract the disorder pinning energy and discuss possible scenarios for the correlated oxide electrodes in modifying the ferroelectric energy landscape for DW motion. Compared with the diffusive DW written in vdW ferroelectric CuInP2S6 on doped Si and Au, PFM studies reveal precise domain control and enhanced Curie temperature above 200 °C in CuInP2S6 thin flakes prepared on prepatterned domains of PbZr0.2Ti0.8O3. The piezoelectric coefficient for ultrathin CuInP2S6 on PbZr0.2Ti0.8O3 changes sign and increases with decreasing flake thickness. Theoretical modeling points to the critical role of interface-mediated structure distortion in tunning the ferroelectric quadruple-well energy in CuInP2S6. We observe the self-assembly of highly ordered, close-packed single-crystalline P(VDF-TrFE) nanowires (P-NWs) on vdW material 1T’-ReS2, which can be attributed to the epitaxial interfacial coupling. PFM studies show the polar axis in P-NW is out-of-plane. The robust polar switching hysteresis and ultralow coercive field of P-NW film make it promising for developing low-power wearable nanoelectronics and optoelectronics. We also perform vector PFM and c-AFM studies on polycrystalline PbZr0.35Ti0.65O3 films at different bias voltages. The DW density in these films can be well correlated with the steep slope switching behavior observed in PbZr0.35Ti0.65O3-gated MoS2 transistors, yielding strong support for DW-enabled negative capacitance effect. We also study the magnetic switching behaviors of epitaxial NiCo2O4 thin films deposited on (100), (110), and (111) MgAl2O4 substrates. Using magnetic force microscopy, we map out the evolution of domain structures with a magnetic field in (110) NiCo2O4 films, which can be correlated with the topological Hall effect, suggesting the emergence of chiral spin textures.
Wang, Kun, "Scanning Probe Microscopy Studies of Interface Phenomena in Ferroic Thin Films and Heterostructures" (2023). ETD collection for University of Nebraska - Lincoln. AAI30488647.