Abstract The work deals with the experimental study of ferroelectric (FE) performance scaling for nanometer-sized devices. In the emerging and advanced devices, it is desirable to couple FE performance with other functions. This requires integration of nanoscale FEs with other materials, which is especially promising in epitaxial heterostructures. Such heterostructures inevitably possess a large lattice mismatch, the effect of which on FE properties is unknown and is in the focus of the present work. In the study, heteroepitaxial thin and ultrathin films and superlattices of ABO3-type perovskite structure FEs were used, with A = Pb, Ba, Sr, K, and N, and B = Ti, Zr, Nb, and Ta. FE domains and local polarization switching were explored on the nanometer scale using piezoresponse force microscopy. The experiment was modified that allowed achieving images with high contrast and lateral resolution, and also allowed analysis of nanodomains in lateral capacitor configuration. Local properties were related to a macroscopic response. For this, the method of simultaneous on-wafer low-frequency impedance measurements was optimized allowing studies of thin and ultrathin (to 5 nm) films in a broad range of conditions and regimes. Experimental studies have reveled phenomena which cannot be explained in the frame of the existing theories. The observed new effects are important for applications such as multistate memory devices, storage capacitors, and FE tunnel junction devices.

Relaxation mechanisms in perpendicularly-magnetized ultrathin Fe/ 2 ML Ni(111)/ W(110) films, with thickness between 1.25 and 2.00 ML, have been studied using the ac magnetic susceptibility as a function of temperature and/or time. Different time scales were probed by varying the constant rate of temperature variation, R as the susceptibility was measured. After quenching the film from high temperature, the susceptibility curve was found to relax through a shift in the peak position along the temperature axis and through changes in shape, as a function of time. In general, two opposing behaviors were found; for small R (≤0.30 K/s) the susceptibility peak temperature decreases as R increases, for large R (≥ 0.30 K/s) the peak temperature increases with R. The first behavior is understood as a "dynamical observation" of a domain phase transformation. The density of topological defects in the quenched high temperature delocalized phase undergoes an activated relaxation as low temperature ordered stripe phase is established. The fundamental time scale (𝜏_0R) of this process is in the order of 1.0 s. These findings complement the results of numerical simulation [24, 26, 27] and quantify the important dynamical barriers involved in the geometrical rearrangement of domains in moving from a delocalized phase to the ordered stripe phase. The experiments at large R are sensitive to a much shorter time scale over which the domain density equilibrates when temperature is changed. This process causes an increase in the peak temperature with R that depends linearly on R over the range of values of R accessible in this study. / Thesis / Doctor of Philosophy (PhD)

Investigations of polarization switching over broad time and electric field domains, in various modified Pb-based perovskite ferroelectrics, were systematically performed by ferroelectric switching current transient and bipolar drive P-E responses. Studies were performed from E«Ec to E»Ec, where Ec is the coercive field These investigations have shown the presence of broad relaxation time distributions for the switching process, which can extend over several decades in order of magnitude in time, and where the distribution is strongly dependent on the applied electric field. By performing the study of domain dynamics and polarization switching over extremely broad time domains (10⁻⁸ t < 10² sec), more complete information has been obtained that allows for development of a better mechanistic understanding. Prior polarization kinetics studies have focused on relatively narrow time ranges, and were fit to the Avarami equation, which contains a single relaxation time. However, our broad band width polarization dynamics and frequency relaxation studies have been fit to multiple stretched exponential functions extending over decades of order of magnitude in the time domain. Stretched exponential functions for domain nuclei formation, and for domain variant growth have been found. For example, [001]c, [110]c, and [111]c oriented PZN-4.5%PT crystals, nucleation was found to be a volume process (n=3) rather than just a domain wall restricted process. Consequently, nucleation is heterogeneous. And, growth of a domain variant with reversed polarization was found to be a boundary process (n=2), involving diffuse or rough domain walls. We have extended these studies to various types of ferroelectrics including hard, soft and relaxor types. / Master of Science